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Lekking in the ocre-bellied flycatcher, Mionectes oleagineus Westcott, David Andrew 1995

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LEKKING IN THE OCI{RE-BELLIED FLYCATCHER, MIONECTES OLEAGINEUSbyDAVID ANDREW WESTCOTTB.Sc. (Hons.), Australian National University, 1987M.Sc.,The University of British Columbia, 1991A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FORTHE DEGREE OF DOCTOR OF PHILOSOPHYinTHE FACULTY OF GRADUATE STUDIESDEPARTMENT OF ZOOLOGYWe accept this thesis as conformingto the required standardTHE UNIVERSITY OF BRITISH COLUMBIAMay 1995© David Andrew Westcott 1995In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission.(Signature)Department of________________The University of British ColumbiaVancouver, CanadaDateDE-6 (2188)iiABSTRACTIn this thesis, I examine the social and ecological pressures that have resulted in theevolution and maintenance of lekking in the ochre-bellied flycatcher, Mionectes oleagineus. I dothis by asking two general questions: i) where should leks occur, and ii) how many males shouldsettle at a site.I begin by considering the influence of female distributions on male settlementpatterns. The “hotspot” hypothesis suggests that population level patterns of female movementand/or dispersion determine male settlement patterns. Leks occur due to marked heterogeneitiesin these distributions. I confirmed three novel, interspecific predictions of the hotspothypothesis: 1) leks of ecologically similar, sympatric species are clustered; 2) the degree ofclustering is related to the degree of similarity in diet between the species; and, 3) species withsimilar diets show correlated changes in the sizes of neighbouring leks.If, as suggested by this interspecific comparison, hotspot mechanisms are involved inlek evolution, then leks should be associated with some aspect of female distribution. Usingradio-telemetry I found that females use identifiable and discrete routes when travelling throughtheir environment. Lek location was strongly associated with these routes and tended to beassociated with areas where the most females could be encountered.I also examined the consequences of variation in lek size. Females visited and matedat both leks and solitary territories. While female visitation rates increased with lek size, they didso in proportion to the number of males present. Thus not only did females not show apreference for larger leks, but there were no identifiable benefits to males of display at largerleks. There was, however, a cost. At leks, 28% of displays to females were interrupted byintruders, such interruption did not occur at solitary display sites. These results contradict thepredictions of “preferencet’hypotheses and at least one version of the hotshot hypothesis.Finally, I investigated the hypothesis that males attempt to limit lek size due to thecosts associated with display at larger leks. This was confirmed in a playback experiment. Thisexperiment also demonstrated that males can distinguish neighbours from strangers using song.iii-TABLE OF CONTENTSAbstract iiTable of Contents iiiList of Tables vList of Figures viAcknowledgents viiChapter One: Introduction 1What are lek mating systems? 1More variations on the lek theme 2The taxononmic distribution of lekking 3Aims of this thesis 4Chapter Two: Leks of leks: a role for hotspots in lek evolution? 5Abstract 5Introduction 6The hotspot hypothesis: the interspecific case 7Methods 8Results 10Discussion 11Chapter Three: The relationship between lek locations and patterns of femalemovement and distribution in a Neotropical frugivorous bird. 20Abstract 20Introduction 21Methods 23Study species 23Study site 24Telemetry 25Results 27Discussion 29The influence of males on female movement 29Female distributions and the hotspot hypothesis 29Previous tests of the hotspot hypothesis 32Summary 33Chapter Four: Consequences of variation in lek size in the ochre-belliedflycatcher, Mionectes oleagineus. 44ivAbstract 44Introduction 45Methods 47Study species 47Data collection 48Data analysis 49Results 49Discussion 51Small Sample sizes and non-significant correlations 51Do females prefer leks? 52The costs and benefits of lekking for males 52Hotshots and lek sizes 54Female visitation, lek size and hotspots 55Summary 57Chapter Five: Neighbours, strangers and male-male aggression as adeterminant of lek size. 65Abstract 65Introduction 66Methods 68Study site 68Playbacks 69Statistical analysis 70Results 71Lek type and male responses 71Playbacks at leks: effects on song 72Playbacks at leks: the effects on distance from speaker 72Discussion 73Distinguishing between neighbours and strangers 73Male-male interactions and lek evolution 75Chapter Six: Conclusions 84Community level patterns of lekking 84Female movements and distributions 84Is lekking a better strategy 85Despots and lek size 85The evolution of lekking in Mionectes oleagineus 86Lekking on an evolutionary time frame 87Bibliography 89VLIST OF TABLESpageTable 2.1 Summary of statistics relating to lek aggregation. 14Table 2.2 Summary of lek size changes 15Table 4.1 A summary of the number of males and display sites that werepresent on the study site in each year of the study 58vi.LIST OF FIGURESpageFigure 2.1 A map of the locations of the centres of the leks of eachof the four species in each year from 1989 to 1992. 17Figure 2.2 Mean distances (in m, ± S.E.) from the leks of each species totheir nearest heterospecific lek in metres, for each of the four years 19Figure 3.1 Distribution of use of the study site in terms of time spent ineach 25 m2 cell by females 35Figure 3.2 Female distribution in terms of the number of individual femalesrecorded in each 25 m2 cell. 37Figure 3.3 Distribution of travel routes over the study site 39Figure 3.4 Travel routes in relation to topography. 41Figure 3.5 Examples of the travel routes of three females that used thesame part of the study site 43Figure 4.1 Female visitation rates 60Figure 4.2 Song rates 62Figure 4.3 Mean intrusion rates 64Figure 5.1 Proportion of the total amount of song that was sung duringthe playback, ± S.E. 79Figure 5.2 Change in mean distance, ± S.E., from the playback speaker inthe two phases of the experiment 81Figure 5.3 Comparison of the proportion of song sung by males at the threecategories of display site 83viiACKNOWLEDGMENTSThroughout my time as his graduate student, Jamie Smith has given me the academic freedom toexplore questions that I found exciting and enticing. His generosity with funding ensured that,with a little creativity, I was able to translate my ideas and excitement into data. Jamie also gavefreely his time, advice and editorial skills. Thus my most heartfelt thanks must go to the weeladdie, McSmith, for making such a fine time possible.None of the data presented in this thesis would have been collected if someone hadn’toccasionally roused themselves from their hammock, put down their daiquiri and strolled off intothe forest. Arne Mooers, Christine Chesson, Rodolfo Quieroz F., and Janet Winbourne wereabsolute heroes in this regard. They endured poor food, poor pay, poor living conditions, hardwork, long hours and me. Thanks guys. En Sirena, los guardaparques fueron buen amigos.Paulino Valverde y Gilberto Fonseca me ayudieron mucho y ademas la hicieron bien divertida lavida en La Republica Federale. Les agradezco al Servicio de Parques Nacionales de Costa Ricael permiso para trabajar en Corcovado.Producing this thesis was made much easier by the efforts of a number of people. Special thanksmust go to Alistair Blachford for his time and skills. The radio-telemetry study presented inChapter 2 would have been far less elegant and informative without his assistance in producingcomputer maps and graphics. Many people discussed ideas with me or commented on andimproved parts of the thesis. My thanks in this regard go to; L. Barrett-Lennard, A. Cassidy, S.Heard, W. Hochachka, J. Hoglund, E. Korpimacki, F. Kroon, A. Lotem, A. Mooers, G. Muir, J.McKinnon, S. Pruett-Jones, L. Rowe, L. Thomas, D. Ward and two anonymous reviewers. L.Gass, D. Schiuter, and J.N.M. Smith all battled through and commented on the entire thesis.Thanks.This work was supported by a N.S.E.R.C. grant to Jamie Smith, and by grants from the AmericanMuseum of Natural History and the Sigma Xi Foundation. I recieved personal support in theform of N.S.E.R.C. Post-Graduate Scholarships, U.B.C. Teaching Assistantships and a MacleanFraser Summer Fellowship.1CHAPTER ONE.INTRODUCTION.What are lek mating systems?Leka is a Swedish word meaning to play. The term lek was first used in reference toaggregations of displaying male ruff, Philomachus pugnax, by Edward Selous (1906). Generallythe term has been used to describe species in which males cluster together to display to attractmates. Bradbury (1977) set out four defining characteristics of a lek mating system:1) there is no male parental care; males contribute only sperm to a reproductive attempt.2) the display territories of males are spatially aggregated and fill only a small part of theavailable habitat.3) the display territories contain no significant resources required by females other thanthe males themselves.4) females visiting the lek can choose their mates (Bradbury 1977; 1981).Lek mating systems are known from a very small number of taxa; they represent lessthan 0.5% of known avian, and fewer than 0.2% of known mammalian mating systems (Davies1991). Despite their rarity, lek systems have inspired a rich literature that dates back to thebeginning of the century (Selous 1906, 1927; Bradbury 1981; Bradbury and Gibson 1983; Wiley1991). The fascination that lek mating systems hold for behavioral and evolutionary ecologistsstems from three questions raised by the mating system. First, what factors determine maledisplay dispersion, in particular, what mechanisms drive clustered male display? Second, giventhat females of lekking species receive only sperm from their mates, how do females choose theirmates and do they benefit from exerting that choice? Third, what factors are involved in theevolution of the elaborate secondary sexual traits and displays of many lekking species.Within the scope of Bradbury’s four characteristics, lek mating systems show a greatdeal of variation on several levels. Perhaps the best documented aspect of this variation is that2encountered in male display dispersion. Contrary to popular belief, lek mating systems are not adiscrete category consisting only of large, tightly clustered groups of males. Rather, the matingsystem includes a continuum of male display dispersions, ranging from only slightly to highlyclustered aggregations.For convenience, two categories of leks are referred to: exploded and classical leks.The most dispersed lek systems are “exploded leks” (Bradbury 1981). Exploded leks differ fromthe closest non-lek mating systems, in which males are dispersed uniformly (Beehler and PruettJones 1983; Beehler and Foster 1988), in that display territories fill only a small part of theavailable habitat and are spatially clustered. On exploded leks, the territories of neighbouringmales do not share contiguous boundaries, and males are usually in vocal rather than visualcontact, e.g. the epomorphorine bat Epopomops frangueti (Bradbury 1977), the king bird ofparadise, Ciccinurus regius (Beehier and Pruett-Jones 1983), capercaillie, Tetrao urogallus(Lumsden 1961), and McConnell’s flycatcher, Mionectes macconnelli (Willis et al. 1978). Insome species the clustered nature of male display dispersion may only be apparent at apopulation level. For example in the bearded beilbird Procnias averano (Snow B.K. 1970) thedisplay territories of males at an exploded lek are separated by as much as several hundredmetres.Classical leks, where males are clustered tightly together on small, contiguous displayterritories, have been the focus of most lek research. These leks may consist of as few as 2displaying males, as in Lawe’s parotia, Parotia lawesii (Pruett-Jones and Pruett-Jones 1990), orup to several hundred as in sage grouse, Centrocercus urophasianus (Scott 1942).More Variations on the Lek ThemeWhile individual species can be categorized as falling into one of the above groups,variation in lekking can also be found within a species, or even within a population. In the buffbreasted sandpiper, Trvngites subruficollis, males shift from display at leks to solitary display asthe breeding season progresses (Pruett-Jones 1988). In other species while some males display at3leks others display solitarily, e.g. Lawe’s parotia (Pruett-Jones and Pruett-Jones 1990), fallowdeer, Dama dama, (Apollonio et al. 1992), white-bearded manakins, Manaucus manaucus (Liii1974), and capercaillie, Tetrao urogalius (Wegge and Roistad 1986). In a number of ungulatespecies iekking can occur simultaneously in the same population with resource-defenseterritoriality (Clutton-Brock et al. 1988; Gosling et al. 1987; Balmford and Turyaho 1992).Variation in lek behaviour can also be genetically determined. In the ruff, in addition to anumber of behavioural strategies (Lank and Smith 1987), there are two genetically distinct typesof males: dark coloured territory owners and light coloured males that display on the territories ofother males (van Rhijn 1983).Finally, though males at leks are engaged in the seemingly competitive activity ofattracting mates, males of a few species cooperate in their display to females, e.g. manakins ofthe genera Chiroxiphia (Foster 1977; 1981) and Pipra (Robbins 1985; Schwartz and Snow 1975).Chiroxiphia lineata males display cooperatively on leks which are functionally a single territory.Though multiple males are required both to attract females to the territory and for effectivedisplay, only the alpha male ever mates (McDonald 1989a). Subordinate males must wait fortheir turn at the top, a wait that typically lasts 8 years (McDonald 1989b).The Taxonomic Distribution of LekkingDespite its overall rarity, the taxonomic distribution of lekking is as varied as itsforms. In birds, the mating system is known from at least 12 families. In some avian familieslekking occurs only in a single species, e.g. among the Psittacidae only the kakapo, Strigopshabrotilus (Merton et al. 1975) leks, while in others it is the rule, e.g. Pipridae (Sick 1967; Prum1994). Amongst mammals lekking is known from the marsupial mouse Antechinus stuartii(Lazenby-Cohen and Cockburn 1988), from old world ungulates (Clutton-Brock 1989), and batsBradbury (1977). To date, only one species of frog, Ololygon rubra. has been shown to have alek mating system (Bourne 1992). Amongst the insects lekking is known from a wide variety ofHymenopterans, Dipterans and from the Lepidoptera (Campanella and Wolf 1974; Alexander41975; Parsons 1977; Alcock 1987).Aims of this thesisIn this thesis, I address the question of what determines male display dispersion in thelek mating system of the ochre-bellied flycatcher, Mionectes oleagineus (Aves:Tyrannidae). Thisspecies has a lek mating system in which some males defend solitary display territories whileothers defend territories at small leks. A large proportion of males do not hold territories at all(Westcott and Smith 1994). I focus on two major questions: i) what determines where leksoccur?, and ii) what determines the size of leks? Under the umbrella of these two questions Ihave addressed a number of related issues.I begin in Chapter 2 (Westcott 1994) by comparing the locations and sizes of leks ofthe understory lek breeding species that occur on my study site. My aim here is to document thecommunity level patterns of lekking and to provide a test of predictions of the hotspot hypothesisusing inter-specific comparisons.In Chapter 3, I again examine the hotspot hypothesis but in this instance I shift fromthe community level to focus specifically on M oleagineus. The aim of this chapter is toexamine whether lek locations are associated with some aspect of female distributions.In Chapter 4, I address the question of what determines lek size. I use data collectedduring behaviour observations over three breeding seasons to address two further hypotheses forlek evolution: the preference hypotheses and the hotshot hypothesis.In Chapter 5, I use a song playback experiment to determine whether male-maleinteractions influence male settlement patterns, in particular, how many males settle at a site andwhether males are capable of distinguishing between neighbours and strangers on the basis ofsong.5CHAPTER TWOLEKS OF LEKS: A ROLE FOR HOTSPOTS IN LEK EVOLUTION?ABSTRACTA lek is a cluster of males on display territories. Despite the attention that lek mating systemshave received, the factors involved in lek evolution are still poorly understood. The “hotspot’hypothesis suggests that population level patterns of female movement and/or dispersiondetermine male settlement patterns. Males, it is argued, should settle where females are mostlikely to be encountered or where female densities are greatest. Leks occur due to markedheterogeneities in female distributions. I tested three novel interspecific predictions of thehotspot hypothesis: 1) the leks of ecologically similar, sympatric species should be clustered; 2)the degree of this clustering should be related to the degree of similarity in diet between thespecies; and, 3) species with similar diets should show correlated changes in the sizes ofneighbouring leks. I documented the size and locations of leks of four Neotropical bird speciesover four years. All three of the predictions were confirmed. The interspecific aggregation ofleks strongly suggests a role of hotspot mechanisms in the evolution of lekking in these species.6INTRODUCTIONIn lek mating systems, males gather into aggregations, called leks, and display toprospective mates. Within a lek each male defends a display territory. Display territories containno significant resources and it is assumed that females visit leks solely to assess potential matesand for copulation (Bradbury 1981; Wiley 1991). Because females at leks can choose their matesrelatively free of coercion by males, much interest in lek mating systems has focused on femalemate choice (Baimford 1991). Recently attention has again focused on the pressures leadingmales to aggregate (e.g., Alatalo et al. 1992; Stillman et al. 1993; Théry 1992). Despite this, themechanisms responsible for the evolution of leks themselves are still unclear (Clutton-Brock1989; Balmford et al. 1993).Hypotheses for the evolution of leks fall into three general categories. 1) The‘preference’ hypotheses propose that males andlor females prefer mating at leks because of theadvantages they gain compared to mating at solitary sites. Leks might be preferred because: i)they reduce the risk of predation (Koivisto 1965; Hjorth 1970; Wittenberger 1978; Gosling1986); ii) they reduce mate search costs (Alexander 1975); iii) they allow a more efficientcomparison of more males (Emlen and Oring 1977; Parker 1978); iv) lek size acts as an arbitraryFisherian trait (Queller 1987); or, v) receptive females at leks are better able to avoid harassmentby males and males are better able to attract and retain receptive females (Clutton-Brock et al.1993; Stiliman et al. 1993). 2) The ‘hotshot’ and ‘attractiveness’ hypotheses suggest that leks formas a result of high variance in male mating success. Males that obtain few copulations clusteraround those that are successful (Arak 1983; Beehler and Foster 1988; Hoglund and Robertson1990a). 3) The ‘hotspot’ hypothesis (Liii 1976; Bradbury and Gibson 1983; Bradbury et al. 1986)suggests that patterns of female movement and/or dispersion determine where males settle: leksshould form where female densities are highest or where females are most likely to beencountered.Of these three ideas, the hotspot hypothesis is appealing simply because its7underlying mechanism, that male dispersion is a function of female distributions, is fundamentalto our ideas about mating systems evolution in general (Vehrencamp and Bradbury 1984; Davies1991). Because males in lek mating systems males provide only sperm in a reproductive attempt,they are freed from the constraints of parental care and are able to pursue those options thatmaximize their reproductive success. Consequently, it is in lek mating systems that the effects ofhotspots should be most apparent. It is, therefore, surprising that with a few notable exceptions(see, for example, Pruett-Jones 1988; Théry 1992; Schroeder and White 1993) there has beenlittle empirical support for the role of hotspots in lek evolution. In this study, I assess theplausibility of the hotspot hypothesis in explaining the evolution of leks in a group of Neotropicalbirds. I develop and test three predictions concerning the interspecific patterns of lek distributionof sympatric species that might be expected when hotspot mechanisms underlie lek evolution.The Hotspot Hypothesis: the Interspecific Case.To extend the hotspot hypothesis to several species, I begin by considering a suite ofsympatric, lekking bird species. These species are similar in size and use similarly distributedresources. If ranging patterns are determined primarily by resources (Emlen and Oring 1977;Davies and Lundberg 1984; Wimberger 1988; Hews 1993) the females of these species shouldexhibit similar ranging patterns. Furthermore, if these species have similar environmentalconstraints on their movements, both the routes they use to travel to resources and the points atwhich their movements are concentrated should be in roughly the same locations.Males should then settle where their exposure to the greatest number of potentialmates is maximized. This is usually thought to be where female densities are highest (Hoglundand Robertson 1990b; Théry 1992; Balmford et al. 1993). However, female home range use isoften heterogeneous because the locations of resources vary over the breeding season (Hoglundand Robertson 1990b; Bradbury et al. 1989a; Worthington 1982). Males may therefore do bestby settling at sites where females’ movements are channelled by environmental features. Malescan then be certain that females will pass near these points as they move between resource8patches, irrespective of the patches’ exact locations.Given the above scenario, three predictions follow if hotspot mechanisms areinvolved in lek evolution. First, because females of all the species face similar constraints onmovement, leks of the different species should be clustered. Second, because resources determineranging patterns, the strength of this clustering should be related to the degree of diet similaritybetween the species. Finally, the hotspot hypothesis suggests that the number of males in an areashould change to track the number of females using that area. Consequently, species with similardiets should exhibit correlated changes in the sizes (number of males) of neighbouring leks.METHODS.This study was conducted at Estación Sirena (lat. 80 29’ N, long. 83° 36’ W) inCorcovado National Park, Costa Rica, from 1989 through 1992. The study area is 1.15 km2 andis divided between low rugged hills (to approximately 140 m in height) and an alluvial plain.The area receives a mean annual rainfall of 5,305 ± 301 mm, n = 10 years (Servicio de ParquesNacionales de Costa Rica) with a distinct wet season between May and November. Thevegetation is tropical, pre-montane, wet forest (Tosi 1969) and approximately 80% is primaryforest with a canopy height of approximately 35 m. The remainder is 15 year-old second growthwith a variable canopy height, roughly between 15 - 25m.On the study site, four bird species, the ochre-bellied flycatcher (Mionectesoleagineus), the red-capped manakin (Pipra mentalis), the blue-crowned manakin (Pipracoronata), and the long-tailed hermit hummingbird (Phaethornis superciliosus), form leks in theforest understory. The first three of these birds are 11- 15 g frugivores that show broad overlapin resource use (Leck 1972; Flemming 1979; Skutch 1980; Loiselle and Blake 1990; pers. obs.).Local abundance of these three species is known to change in response to changes in fruitavailability (Levey 1988; Blake et al. 1990). The fourth species is a 6 g, trap-lining nectarivore.9All four species move large distances (as much as 1 km, pers. obs.) within the forest understorybetween dispersed resource patches and thus probably face similar environmental constraints ontheir movements.Each year I conducted exhaustive searches of the study site. During 1989 all areaswere visited repeatedly to map the study site, make trails and survey vegetation. I checked thetrunk of every canopy tree once every two weeks in 1990 and once every month in 1991 and1992 in searches for nests and displaying males. Additional visits to all areas of the study sitewere made in the course of concurrent work in all years.During searches I determined lek locations, the number of displaying males and thelocation of each lek centre. Lek centres were then plotted onto a map of the study site and aDonnelly modification of the Clarke and Evans test (Krebs 1989) was used to describe lekdistribution quantitatively. This technique tests for deviations from a random distribution bycomparing observed and expected nearest-neighbour distances in populations whose densities areknown. An index of aggregation of R = 1.0 indicates a random distribution. Scores lower thanand greater than R = 1.0 indicate deviations towards clumped and uniform distributionsrespectively.I also used the Clarke and Evans test to identify any clustering of leks aroundtopographical features that might be expected to channel movements. Such topographicalfeatures included: i) confluences of drainages, ii) saddles in ridgelines, iii) the ends of ridgelines,and, iv) in flat areas, watercourses. Using the 1992 data, I identified areas where more than onespecies lekked and determined the centre of the combined leks in each of these areas. Thedistance to the nearest topographical feature was then determined from the study site map.RESULTSFigure 2.1 summarizes the numbers of leks and males as well as lek locations for the10four species in each year of the study. Because i superciliosus were not individually marked, nodata on numbers of males were recorded for this species.In all four years, the leks of the four species were significantly clumped (Figure 2.1,Table 2.1 i). At some sites, this clumping was so extreme that all four species could beencountered displaying simultaneously on partially overlapping leks. More commonly, however,leks were located short distances from each other (Figure 2.2). Leks were also significantlyaggregated with topographic features (Clarke and Evans Test, R=O.77, n=38, z=-3.15, P<O.05).While the leks of all four species were clustered, the nectarivore leks showed the leastclustering (Table 2.1 i, Figure 2.2). Significant differences across all species in mean distance tonearest heterospecific lek were found in only one year (Table 2.1 ii). In each year, however,nectarivore leks had larger mean distances to the nearest heterospecific lek than did thefrugivores (Table 2.1 iii). There were no significant differences in mean distance to the nearestheterospecific lek among the three frugivores in any year (Table 2.1 iv).Monitoring changes in lek sizes from one breeding season to the next showed thatneighbouring frugivore leks tended to change size in the same direction (Table 2.2). Because ofmovements of males among leks within or between years, lek sizes are not independent of eachother. Consequently, simple correlations between the sizes of neighbouring leks were notcalculated. At sites where more than one frugivore species leldced, I scored each possible speciespair at the site as follows: + 1 if their lek sizes changed in the same direction (both increase orboth decrease); -1 if they changed in opposite directions (one increases while the otherdecreases); and, 0 if one changed size while the other remained constant. If neither lek showedany change in size they were both dropped from the analysis. The mean of these scores was thencalculated. A positive mean score indicates that average lek sizes changed in the same direction,a negative mean score that they changed in opposite directions. Though the mean scores werelow in individual comparisons between years , the results were consistent across the threecomparisons. The mean score over the three years, 0.118, is significantly greater than zero (onesample, one-tailed t-test t=15.72, d.f.=2, P<0.05).11DISCUSSIONThe clustering of leks documented here argues for the action of some common factoron all species. Under my starting assumptions, the effects of environmental features, e.g.topography and vegetation, on movement are likely candidates. This scenario suggested that thesame environmental features will act to constrain the movements of all four species at the samelocations because they all move long distances and do so through the forest understory. Thisprocess should result in interspecific foci of female movements. According to the hotspothypothesis, males should settle at these sites, thus producing the observed clusters of leks.My suggestion that the interspecific clustering of leks is due to the shared influenceof environmental features on female movements is supported by several observations. First, leksare significantly associated with topographical features that would channel movements through aparticular area. Other environmental features, such as boundaries between different vegetationtypes, might also have an effect. In addition, radio-telemetry data from M oleagineus indicatesthat topographical influences on movement are strong and that features like those listed above doindeed channel female movements (Chapter 3).Can the clustering of leks be explained through other mechanisms? One alternativemight be that the four species share preferences for a particular display habitat of limitedavailability. This is unlikely. M oleagineus males do show preferences for,a particular displayhabitat. However, the distribution of the preferred display habitat does not appear to influencemale distribution and much similar habitat away from current leks is vacant, 11% of that sampled(Westcott 1993). In addition, though all four species often occur in one area, within that areathey choose different levels of the understory for their displays. This results in markeddifferences across the species in the microhabitat types selected.Alternatively, lek clustering might be due to decreased predation risk associated with12the interspecific clusters. Potential predators are common on the study site. However, in over500 hours of observations at leks, only two predation attempts, both unsuccessful, were observed(unpubl. data). In both instances, neither conspecifics nor heterospecifics in adjacent territoriesappeared to notice the incidents and display continued uninterrupted. Alarm calls elicitinggeneral responses to predators were not recorded at the leks of any of the species. In addition,predators were not more or less common at leks than elsewhere on the study site (unpubi. data).These observations cannot rule out an influence of predation on lekking. However, they dosuggest that predation is probably not the primary force in the production and maintenance ofleks (Lill 1976; Beehier and Pruett-Jones 1983; Deutsch and Weeks 1992; Baimford and Turyaho1992).Another alternative is that lek sites are chosen because they are very close to, or evenwithin, resource patches. Several observations argue against resource patch location as the majordeterminant of lek location. First, resource patches are relatively short lived and their exactlocations vary both within and between years. In contrast, lek locations remain constant within,and with some exceptions, between years (Figure 2.1). There is little indication for any of thesespecies that males shift lek locations within years to track changes in resource distributions.Second, if leks are located near or within resource patches both males and females should feedclose to leks. Two M oleagineus males radio-tagged in 1990 foraged large distances from theirleks, even going to foraging sites beyond other conspecific leks. Recaptures of foraging maleslong distances from their display territory were also not unusual. Finally, radio-tagged female 4.oleagineus do most of their foraging at sites that are no more or less associated with leks thanexpected by chance (Chapter 3). This is not to say that resources cannot sometimes be found onleks. Insectivory by lek attending males of all four species was common. However, frugivory atleks was rarer (Westcott and Smith 1994) and nectar feeding was not observed.As predicted, the sizes of the leks of the frugivores at a given site tended to change inthe same direction. Although the overall mean change score was positive, correlated changes inlek size were by no means the rule. This suggests that mechanisms other than hotspots probably13also influence lek size (Bradbury et al. 1986; Beehier and Foster 1988; Alatalo et al. 1992;Baimford et al. 1993). If this is the case, then the best interpretation of the observationspresented here is that hotspots provide the initial impetus for male aggregation and determine leklocation. Additional factors then modify lek size.This report of “leks of leks” suggests that the hotspot hypothesis is a plausibleexplanation for lek evolution. My results raise several interesting points. First, a criticism of thehotspot hypothesis has been that areas of high female density are unlikely to be small enough toproduce the clusters of males that characterize many lek systems (Bradbury et al. 1986; Alatalo etal. 1992). If environmental constraints on movement in given locations are severe, however,females’ movements may be sufficiently channeled to produce tightly clustered leks throughhotspot mechanisms alone. Second, the questions of why lek?, where should a lek form?, andhow many males should settle at a lek? may all require separate consideration.14Table 2.1. Summary of the statistics relating to i) the aggregation of leks and ii -iv) distancesbetween heterospecific leks. In part i), R is the Index of Aggregation from the Clarke and Evanstest, see text, n is the number of leks included in the analysis, and z is the test statistic.YEAR TESTIndex of Aggregation of leksR=0 . 57R=0 . 59R=0 .58R= 0.57n= 44n= 42n= 42n=48z=—4 . 55z=—4 . 19z=—4 .22z=—4. 69P<0 .05P<0 .05P<0 .05P<0 .051ii111ivOne-way ANOVA with prior comparisons:across all speciesa. f . =3d. f . =3a. f . =3a. f . =31989199019911992198919901991199219891990199119921989199019911992P=0 .032P=0 .062P=0. 083P=0 .21between nectarivore and thegrouped frugivoresF=3 .249F=2 . 658F=2 .406F=1.557F=5 .200F=5 .818F=5 .254F=7 .29F=0 .197F=0 .829F=1 .928F=0 .538a. f . =1a. f . =1a. f . =1a. f . =1P<0 . 05P<0 . 025P<0 .05P<0 . 025within the frugivoresa.f.=2a. f. =2d. f . =2a. t . =2P>0 .05P>0 .05P>0 .05P>0 .05Table2.2:Summaryofleksizechanges(#ofdisplayterritories)foreachof thespeciespairs.Themeansarepresentedforeachspeciespairwithinayear, forallpairswithinayear, andforallpairsinallyears.SeeTextforexplanation.SpeciespairScoreMean10-1withinallgrandpairspairsmean89/90M.oleagineus/P.mentalis2520.0M.oleagineus/P.coronata1510.00.115P.coronata/P.mentalis3700.390/91M.oleagineus/P.mentalis3710.18M.oleagineus/P.coronata2420.00.1070.118P.coronata/P.mentalis2610.1191/92M.oleagineus/P.mentalis254-0.18M.oleagineus/P.coronata3420.110.133P.coronata/P.mentalis7120.5V.P16Figure 2.1:A map of the locations of the centres of the leks of each the four species in each year from 1989to 1992: diamond = M oleagineus; square = E mentalis; triangle = i coronata; circle =superciliosus. The dotted lines are creeks. The number of leks, number of territorial males andterritorial males/ha for each species in each year are as follows: oleagineus - 1989: 15, 26,0.22; 1990: 13, 22, 0.19; 1991: 12, 20, 0.17; 1992: 15, 29,0.25; mentalis - 1989:9,28,0.24;1990: 11, 24, 0.21; 1991: 10, 22, 0.19; 1992: 14, 25, 0.22; coronata- 1989: 12, 35, 0.3; 1990:11,30,0.26; 1991: 12, 27, 0.23; 1992: 12, 30, 0.26; E superciliosus (number of leks only) -1989: 8; 1990: 7; 1991: 8; 1992: 7.00ILIISLJJ_J.0.000A‘IrcjL/C.I’,0v.4-\j .V 1D4.C0.V—I‘9G3NVLI18Figure 2.2:Mean distances (in m, ± S.E.) from the leks of each of the species to their nearest heterospecificneighbor lek in meters, for each of the four years.19100 10080 80w 60 w 6040 4020 200 0_ac.sG—.a çi’SPECIES100 199280w60402001989 1990++eScc’3 çjr’SPECIES199110080w8O40200c0 !c*S pe cØc*SSPECIESSPECIES20CHAPTER THREE.THE RELATIONShIP BETWEEN LEK LOCATIONS AND PATTERNS OF FEMALEMOVEMENT AND DISTRIBUTION IN A NEOTROPICAL FRUGIVOROUS BIRD.ABSTRACTThe different versions of the hotspot hypothesis for the evolution of leks variously suggest thatleks should form either in areas 1) where there is a high degree of overlap of female home ranges,2) where females spend most of their time, 3) where the most females can be encountered, 4) orin areas where there is high female traffic. The notion underlying these predictions, that malesadjust their behaviour according to that of females, is basic to how we think about the evolutionof mating systems in general. I monitored female movements using radio-telemetry to test thevarious predictions of the hotspot hypothesis in the lek mating system of the ochre-belliedflycatcher, Mionectes oleagineus (Ayes: Tyrannidae). I found that females use identifiable routeswhen travelling through their environment. Each route was used by several females. Leklocations were strongly associated with these routes, and tended to be in areas where the mostfemales could be encountered. Leks were not located in areas where females spent the mosttime. These results support the female traffic version of the hotspot hypothesis for the evolutionof leks in this species.21INTRODUCTIONThe most recognizable aspects of lek mating systems are the leks themselves. Leksare clusters of displaying males attempting to attract potential mates. Determining the nature ofthe pressures that result in the formation of leks has been a primary focus of research into lekmating systems. Hypotheses for lek evolution can be divided into three general categories. Inthe ‘preference’ hypotheses, males and/or females prefer mating at leks because of the advantagesthey gain, compared to mating in other male display dispersions. Leks might be preferredbecause: i) they reduce the risk of predation (Koivisto 1965; Hjorth 1970; Wittenberger 1978;Gosling 1986); ii) they reduce mate search costs (Alexander 1975); iii) they allow a moreefficient comparison of more males (Emlen and Oring 1977; Parker 1978); iv) lek size acts as anarbitrary Fisherian trait (Queller 1987); or, v) receptive females at leks are better able to avoidharassment by males, and males are better able to attract and retain receptive females (CluttonBrock et al. 1993; Stiliman et al. 1993). In the second category are the ‘hotshot’ hypotheses.These suggest that leks form as a result of high variance in male mating success. Males thatobtain few copulations cluster around those that are more successful (Arak 1983; Beebler andFoster 1988; Hoglund and Robertson 1990a). Finally, the ‘hotspot’ hypothesis (Alexander 1975;Bradbury 1981; Bradbury et al. 1986) suggests that patterns of female movement and/ordispersion determine where males settle: leks form where female densities are highest or wherefemales are most likely to be encountered.The appeal of the hotspot hypothesis lies in the fact that its underlying mechanism,that male dispersion is a function of female distributions, is fundamental to our ideas about theevolution of mating systems in general (Crook 1965; Emlen and Oring 1977; Vehrencamp andBradbury 1984; Davies 1991). Males in lek mating systems are freed from the constraints ofparental care and, therefore, are able to pursue other options to maximize their reproductivesuccess. These options include moving to display at sites where their exposure to potential matesis maximized. Consequently, it is reasonable to expect that hotspot effects should be apparent in22lek mating systems, and indeed several studies have provided some support for the hypothesis(Pruett-Jones 1988; Théry 1992; Schroeder and White 1993; Westcott 1994, Chapter 2).The hotspot hypothesis has been proposed in a number of different formats.Following from Kruijt et al.’s (1972) observations of the formation of a black grouse lek, Lull(1976) suggested that leks form when males “solve’ the problem of meeting receptive hens.. .byusing a fixed display site at a location favoured by feeding females” (p. 72). Bradbury (1981)suggested that leks should form at locations where females were most likely to pass, i.e. athotspots in female traffic. The first formal version of the hotspot hypothesis was provided byBradbury and Gibson (1983) and developed later by Bradbury et al. (1986). They proposed thatmales settle at those sites where female densities are the greatest. In their models, these authorsused as their measure of female density the extent of female home range overlap. Mostsubsequent tests of the hotspot hypothesis have been based on the results of Bradbury et al.’s(1986) simulations (Bradbury et al. 1989a; Hoglund and Robertson 1990b; Théry 1992;Schroeder and White 1993; Balmford et al. 1993).Empirical tests of the hotspot hypothesis have had mixed results. Leks can be locatedat sites with apparently high female traffic, e.g. along a route travelled by females as part of theirdaily movements (Appolonio et al. 1990), and in locations with high female densities (CluttonBrock et al. 1988; Pruett-Jones 1988; Apollonio 1989; Gosling and Petrie 1990), though in sagegrouse, Centrocercus urophasianus, this is only apparent at a coarse grain (Bradbury et al.1989a). Similarly, Schroeder and White (1993) found that in greater prairie chicken,Tympanuchus cupido, males settled in areas where their “breeding potential” was maximised.The hotspot prediction, that female home range size and the degree of male clustering should bepositively correlated, has been supported in studies of both mammals (Bradbury 1981; Baimfordet al. 1993) and birds (Bradbury et al. 1986; Théry 1992). However, this prediction is also sharedwith other hypotheses for lek evolution (Emlen and Oring 1977; Bradbury 1981; Beehler andFoster 1988). Finally, Westcott (1994; Chapter 2) found that the leks of sympatric lekking birdspecies were clustered, as would be predicted from the hotspot hypothesis if we assume similar23movements and similar constraints on movement within and across species.In contrast, several studies have failed to confirm the hypothesis’ predictions. Therewas no association between lek location and areas of greatest female densities in three species ofungulate (Balmford et al. 1993), or in the small heath butterfly, Coenonympha pamphilus(Wickman et al. 1995). Nor were leks located in the areas of the highest female home rangeoverlap in capercaillie (Wegge and Roistad 1986) or great snipe, Gallinago media (Hoglund andRobertson 1 990b). In addition, inter-lek distances in these species appear to be greater thanpredicted by the hypothesis, i.e. less than the average female’s home range diameter (Wegge andRolstad 1986; Hoglund and Robertson 1990b). However, this prediction is also shared withfemale preference models (Bradbury et al. 1986; Gibson et al. 1990).In the work I report here I provide a direct test of the hotspot hypothesis’ most basicprediction, that the distributions of leks is related to some aspect of female distribution ormovement patterns. I monitored movements of female ochre-bellied flycatchers, Mionectesoleagineus (Ayes: Tyrannidae), by radio-telemetry to examine three different formulations of thehotspot hypothesis:1) Males settle where females spend the most time, e.g. at resting and/or feeding sites,the time/resource model (Kruijt et al. 1972; Lill 1976; Gosling and Petrie 1990).2) Males settle in those areas which are used by the greatest number of females,i.e. have the highest female densities.3) Males settle at hotspots in female traffic.METHODSStudy SpeciesMionectes oleagineus is a resident of the understory of lowland, wet foreststhroughout the Neotropics. It is a small bird (11-12 g) and the sexes are monomorphic, being24dull, olive green in colour with a slight ochre wash on the belly. M. oleagineus is a frugivore thattakes a variety of small arils, fruits etc. (Sherry 1983) and supplements this diet with arthropods(pers. obs.).From roughly late January to August, male M oleagineus defend small displayterritories in the forest understory (median size 477 m2,Westcott and Smith 1994). Territorialmales attempt to attract mates and defend their territory using song (Westcott 1992).Approximately 30% of territorial males hold solitary display territories while the remainder arefound in small leks of 2 - 6 individuals. Only half the male population is territorial, however, andthe remainder behave either as floaters or as satellites on the territories of other males (Westcottand Smith 1994).Females build nests and care for the young alone. Eggs are incubated for about 21days and the nestling stage lasts approximately 18 days. Eighty six percent of nests fail prior tofledging, usually due to predation (Westcott, unpubi. data). Because the breeding season can lastup to 8 months in a good year, females have the opportunity for renesting and may rear more thanone brood per year. I have recorded renesting within a breeding season occurring only after nestfailure, though it is possible that renesting might also occur after successful attempts. Femalesare rare visitors to leks (Westcott and Smith 1994).Study SiteThis study was conducted between May 1991 and December 1992 at Estacion Sirenain Corcovado National Park, Costa Rica (lat. 8° 29 N, long. 83° 36’ W). The study area is 1.151cm2 and is divided between low rugged hills (to approximately 140 m in height) and an alluvialplain. The area receives a mean annual rainfall of 5,305 mm, ± 301, n = 10 years (Servicio deParques Nacionales de Costa Rica). The majority of this rain falls between late June andNovember. The vegetation is tropical, pre-montane, wet forest (Tosi 1969) and approximately80% is primary forest, the remainder being 17 year-old second growth.I laid out a trail system following each of the ridge lines and creeks on the study site.25Every 25 m along each trail I placed a permanent, numbered, reference marker. I then surveyedthe study site and made a topographical map on which these trails and permanent markers wereplotted.TelemetryBirds were captured using mist nets, weighed, measured and each individual wasgiven a unique combination of coloured leg bands. Females were identified from themorphometric data (see Westcott and Smith 1994). Radios (O.8g when mounted) from Biotrackand Holohil Systems Ltd. were attached to females using harnesses made from gauze. All birdsflew strongly upon release and appeared not to be bothered nor hindered by the radios. In allcases where birds were recaptured or resighted 6 weeks after being tagged, n6, the radio hadand harness been dropped.Since my aim was to identify correlations between lek location and femaledistributions and movements, it was necessary to control for any influence of males or leks onfemales’ movements. An obvious way of doing this was to track females only during the non-breeding season. The short duration of the non-breeding season, the heavy rainfall (as much as ametre per month), and the fact that males are present, though not necessarily actively displaying,on their display territories all year round, all conspired to make this a less than satisfactoryapproach. Consequently, I did what telemetry was possible during the non-breeding season, aquarter of my total sample, and did the remainder during the breeding season.To avoid any confounding effects resulting from the presence of males on displayterritories, only non-breeding females were selected for telemetry during the breeding season.Females were selected if they met the following criteria: i) were known to have recently bredsuccessfully within the previous month, so presumably were unlikely to renest, or, ii) showed nosign of tail feather wear indicative of ongoing incubation, iii) did not differ from the nonbreeding female mean mass, and iv) did not have an enlarged brood patch. This procedure wassuccessful in selecting non-breeding females. Only 3 of 25 tagged females showed any sign of26breeding activity during this work. One female made repeated visits to leks over a period ofseveral days, one visited a lek and subsequently laid a clutch, and another was already incubatingwhen tagged. These females were excluded from the analysis. I sampled four females during thenon-breeding season.With the exception of two birds early in the study, I followed only one female at atime. As far as possible, I followed the females continuously from prior to their leaving theirroost site in the morning until they settled down again in the evening. Location fixes were madeby triangulation, using bearings taken from the permanent markers along the trails. A new fixwas determined whenever a change in the bird’s location could be detected. Triangulations wereanalyzed using “Locate II” (Nams 1990). Mean 95%-error ellipses (Maximum LikelihoodEstimator) on triangulation fixes were 282 ± 78 S.E. m2, n=4060.Radio-telemetry data was analysed in the following manner. A grid of 25 m2 cellswas laid out over the map of the study site. Within each of these cells, all occurrences of thefemale distribution parameter under consideration were tallied. These distribution parametersincluded: i) total number of fixes from all females recorded in that cell, ii) total number offemales recorded in the cell, and iii) number of instances of travel through the cell (outlined inmore detail in the results section). This resulted in matrices of scores that were overlaid on themap of the study site. Randomization tests were then used to analyze the distribution of leksrelative to these female distribution parameters.Randomization tests were performed as follows. There were 13 leks on the study siteduring the period of the study. For each of the three distribution parameters, the score for the cellin which each lek’s centre occurred was determined and the mean for all 13 leks calculated, theobserved mean lek score. Thirteen computer “leks” were then randomly settled onto the patternin question and their mean score was determined and recorded. This process was repeated 5000times. The location of the observed mean lek score in the distribution of simulated mean lekscores was then determined. An observed lek score that lay in either the upper or lower 2.5%tails of the distribution indicated that leks were either more or less associated with a pattern of27female distribution than would be expected by chance alone.RESULTSIn total, 16 females, or approximately 25% of the estimated female population(unpubi. data), are included once in this analysis. Four of these were followed during the non-breeding season. A further 6 tagged females were not included in the analysis because they werefollowed for less than 4 days. This occurred due to presumed radio failure (dropped radio orbattery failure, n=3), the female leaving the study site (n= 1), being taken by a raptor (Acipitersuperdiiosus, n=l), or by a snake at night (n=1). Three additional females were excludedbecause they showed signs of breeding activity during the time they were being tracked (seeabove). Females were tracked for a mean of 8 days, the range being 4-16 days.The mean female home range size was 28.4 ha, ± 3.7 S.E., and included 4.7, ± 0.5S .E., leks. Home range size increased with each additional day of tracking (r=0. 6, n16,P=0.0 16), though the strength of the correlation was due largely to a single female. Though stilla trend, the correlation was no longer significant when this female was removed (r=0.46, n= 15,P=0.096). There was no correlation between the date on which tracking of a female commencedand the size of her home range (r=-0.19, n=16, P=0.49), nor did home range size vary withseason (t-0.02, d.f.=14, P=O.984). Neither the number of leks within a female’s home range(t=0.066, d.f.=14, P=0.948), nor the mean proportion of time that females spent at leks (MannWhitney U=38, n=16, P=0.488) differed between the breeding and non-breeding seasons.Indeed, even during the breeding season females’ visits to leks appeared not to be concerned withmales; they simply passed through the territory, and males were often not even present.The pattern of use of the study site by the females is shown in Figure 3.1. Use of thearea was extremely heterogeneous. Females spent a higher proportion of their time in areaswhere fruiting plants were plentiful. Areas of extremely high usage were localized and28associated with fruiting events of long duration, e.g. fruiting by the tree Clusia tovomitosis. Inthe randomization test 48% of the 5000 iterations had mean lek scores higher than that observedin the field. Thus leks were not associated with areas where females spent their time than wouldbe expected by chance (P = 0.96), a result that is inconsistent with the time/resource model of thehotspot hypothesis.The distribution of female densities, i.e. the number of different females recorded atleast once in each cell, is shown in Figure 3.2. The greatest female densities tended to be foundin the gullies. In the randomization test only 3.4 % of the simulated mean lek scores were higherthan that observed for leks in the field. Thus, leks tended to be associated with areas of highfemale densities (P = 0.068).In general, females tended to spend relatively long periods of time, sometimes almostenitre days, in a single area, e.g. in a patch of fruiting bushes or at a large fruiting tree, beforemoving on to a new area. Movement between such “patches” took two forms. In the first,females moved in a stop-start fashion, often changing direction. This pattern of movement wassuggestive of exploration or foraging and such behaviour was confirmed by direct observation.This first movement type was similar to the circumscribed within-patch movements, whichwould re-cover the same ground. The second form was characterised by movement with arelatively constant speed and heading, and is hereafter called directional movement. In thefollowing analysis I have extracted all recorded instances of directional movement. Eachoccurrence of directional movement was plotted as a vector on the map of the study site. In orderto analyze their distribution, the directional movements traversing each cell were tallied.Movement between patches followed distinct routes and such routes weredocumented in all areas of the study site (Figures 3.3, 3.4). Each route was used by severalfemales and also used repeatedly by individual females. Routes tended to follow lines of leasttopographical resistance, e.g. to run along drainages and to cross ridges at cols (Figure 3.4). Inthe randomization test, only 43, or 0.0086 % of the 5000 mean lek scores were greater than thoseobserved in the field. Thus, leks are very significantly associated with areas of high female29traffic (P = 0.0172).DISCUSSIONThe Influence of Males on Female MovementWhen attempting to document the effect of female movement on male distributions, itis important that the males are not themselves influencing female movements. This possibilityrepresents the major potential shortcoming of this study as most females were tracked during thebreeding season. Even though some females were tracked during the non-breeding season thiswas not a complete solution to the problem. First, males maintain their territories and singthrough out the year (pers. obs.), though active display is rare during the non-breeding season.Second, adverse weather conditions severely reduced both the time available and the quality ofthe work that could be done during the non-breeding season. To minimise the effect of thepresence of males during the breeding season, I selected and used data from non-breedingfemales only. This approach was permitted by the fact that for a significant proportion of thelong breeding season females are in non-breeding condition. The marked influence oftopography on movements (Figure 3.4) and the lack of differences between the breeding and non-breeding season in female movement patterns, e.g. home range sizes, number of leks per femalehome range and the proportion of time that females spend at leks, suggest that the influence ofmales was successfully controlled.Female Distributions and the Hotspot HypothesisMy telemetry results support several predictions of the hotspot hypothesis. Bothverbal and simulation models (Bradbury and Gibson 1983; Bradbury et al. 1986) suggest that ifhotspot mechanisms are involved in lek evolution, inter-lek distances should be less than theaverage female home range diameter, and females should visit more than one lek prior to mating.30Though the irregular shape of home ranges here makes the concept of “average home rangediameters” difficult to deal with, the presence of 4.7 display sites within the average female homerange is consistent with the prediction. Since females regularly passed more than one lek in aday’s foraging, it seems likely that they would visit more than one lek prior to mating.Observations of the same individual females being displayed to by males at different leks on thesame and subsequent days (unpubl. data), and of the one radio-tagged female visiting multipleleks (n=3) confirm this.My most important result is the support for the hotspot hypothesis’ more generalprediction; that male settlement patterns are a function of female distributions. Leks were moststrongly associated with the routes that females use to travel through their environment (Figure3.3). Leks tended to occur where the greatest numbers of different females were recorded(Figure 3.2). Leks were not associated with areas used heavily by females, such as fruiting plants(Figure 3.1).To understand why males might distribute themselves based on traffic routes, ratherthan some other aspect of female distribution, requires consideration of the circumstances underwhich they are attempting to meet receptive females. Three points are important. First, receptivefemales are spread unpredictably and sparsely through time. Not only is M. oleagineus’ breedingseason long, but females are also highly asynchronous, with nesting occurring throughout thisperiod. Second, female mate choice appears to be based on long-term sampling of males ratherthan on song performance on the day of a visit (Westcott 1992, Chapter 4). Third, the locationsof fruiting events varies both within and between breeding seasons (Worthington 1982; D.Graham, pers. comm.). Males must, therefore, approximate the optimal solution to severalconflicting demands. Ideally, they should maintain the maximum exposure to females, and bereadily locatable by females over the entire length of the breeding season, both for assessment formate choice and for subsequent copulations. However, the locations where females can befound, e.g. at fruiting plants, will be unpredictable both within and between breeding seasons.Males will benefit from displaying at a fixed location when females need to reliably31relocate them. This will occur when mate choice is based on information gathered over a longperiod of time. It will also be the case if females return to a male to copy the mate choice ofothers (Wade and Pruett-Jones 1990), or to remate either within (Kruijt and Vos 1988; McDonald1989a; Trail and Adams 1989; Baimford 1990) or between breeding seasons (McDonald 1989a;Pruett-Jones and Pruett-Jones 1990; Liii 1 974a). When females do need to relocate mates, it isunlikely that for males settlement at sites where females spend time, i.e. feeding sites, wouldprovide an adequate solution to the problem of display site selection. The ephemeral nature offruiting events (Worthington 1982; D. Graham, pers. comm.) means that it will be difficult tochoose a single site which remains close to good feeding habitat throughout the breeding season.A second problem with this strategy is that settlement at resources exposes males only to thefemales that use that particular patch; potentially only a small fraction of the females in an area(compare Figures 3.1 and 3.2). Males at these sites will maximize their exposure to females interms of total time, but not always in terms of the number of receptive females encountered.Under the second hotspot formulation, males attempt to settle at those sites wherethey can encounter the greatest number of females. While leks showed some association withthis pattern of distribution (Figure 3.2), this result was not quite significant, nor was it thedistribution pattern to show the strongest association with ieks. Given the expectation that lekbreeding males are attempting to maximize the number of potential mates they encounter, thisresult is surprising. This result may, however, be a function of the nature of the “site” rather thanof the logic that lead to the expectation of males settling in areas of high female densities. Whilemany of the sites with high numbers of females are located along travel routes (see below), othersare located at fruiting plants. Once again, the variability in the location of these fruiting sitesover time probably makes them a poor choice for display site location.Leks were most strongly associated with areas used by females traveling betweensites (Figure 3.3). Settlement along travel routes is an appropriate solution only when their useby females is predictable. In this study, females traveled along these routes and throughparticular points irrespective of precisely where they had come from in one area or where they32were going to in another (Figure 3.5). Such predictability is most likely to arise whenenvironmental conditions restrict female movements to certain sites (e.g. Apollonio et al. 1990).However, such strong constraint is unlikely to have been involved in the case ofj oleagineus.For oleagineus the predictability of female movements through particular sites appears tohave more to do with travel along lines of least resistance, e.g. moving around the end of a ridgerather than climbing over it (Figure 3.4). In other cases, females appear to cross ridges at certainpoints simply because the gullies and associated small ridge lines that originate at these pointsfocus and channel, rather than constrain, movement.Previous Tests of The Hotspot HypothesisBased on the mixed results of tests of the hotspot hypothesis, Gibson (1992) andBalmford et al. (1993) argued that if hotspot mechanisms are involved in lek evolution, theyoperate only at a very coarse level. Two observations from my study suggest that such a generalconclusion is premature. Following Bradbury and Gibson’s (1983) statement that the hotspothypothesis would be falsified if it was found that leks did not occur in areas where the greatestnumber of female home ranges overlap, several studies have concentrated on female home rangesize and distributions. However, in my study home ranges commonly encompassed areas that theindividual did not actually use and some areas of very high home range overlap were notrecorded being used by some or all of the females in whose home ranges they occurred. This isan artifact of standard home range estimation procdures. So, firstly, estimates of home rangeoverlap may be of limited value when an accurate assessment of female distributions is required.Secondly, few studies have considered more than one measure of female distribution; differentaspects of female distribution may be relevant under different conditions. For example,Bradbury et al. (1989) concluded that, while female densities could predict lek location on acoarse level in sage grouse, they failed to do so on a fine scale. These authors suggested that thiswas possibly due to the over-riding influence of specific habitat preferences of males at this level.In my study, the three female distribution parameters showed very different associations with33leks. If male sage grouse face a similar set of conditions to male M oleagineus, an alternativehypothesis might be that they are settling according to local female travel routes (pers. comm.).SummaryIn summary, hotspot mechanisms appear to affect lekking in M oleagineus. Inparticular, my study identifies a clear role for hotspot mechanisms in determining the preciselocation of leks of M oleagineus’. Leks were most strongly associated with travel routes. Iinterpret this pattern to be a result of the males’ solution to the “mate encounter problem” (Lill1976). Females nest asynchronously and assess males and mate throughout the long breedingseason. Individual males then, need to be easily relocatable and, therefore, should display from asingle location. This location, however, cannot be at feeding areas because these are bothephemeral and unpredictable within and between breeding seasons. Males may settle alongtravel routes because all females use these routes and route locations remain constant over time.My results suggest that it is too early to write the hotspot hypothesis off as responsible only forminor background variation in lek mating systems. Ephemeral and unpredictable resources, largedaily movements, a long breeding season, and low female synchrony are characteristic of manyother tropical, lek-breeding, frugivorous birds. I suggest that the hotspot hypothesis may stillprove to be useful for explaining lekking in these species.34Figure 3.1:Distribution of use of the study site in terms of time spent in each 25 m2 cell by all females. Leksare shown as hollow diamonds, creeks as black lines. Females spent less than 58 minutes inwhite areas. The time spent in each cell increases as the shading darkens. There is noassociation between leks and areas of high female use (P=O.96).35in cell0 500(mins)I I I I I Imeters36Figure 3.2:Female distribution in terms of the number of individual females recorded in each 25 m2 cell.Leks are shown as hollow diamonds, creeks as black lines. White shading represents cells wherefewer than 2 females were recorded. Leks showed slight association with areas with highnumbers of different individual females (P=O.068).370 500I I I I I Ifemales/cellmeters38Figure 3.3:Distribution of travel routes over the study site, see text. Leks are shown as hollow diamonds,creeks as black lines. Leks showed a significant association with areas with high female traffic(P=O.0172). White shaded areas were transected by <4 paths.39paths/cell0500meters40Figure 3.4:Travel routes used by females in relation to topography. a) tracing of travel routes, dark line,over the travel routes of females weighted by the number of females recorded in a cell (# of pathstransecting cell/# of females recorded in cell). b) the same tracing of travel routes superimposedon the topographical map. Darker shading represents increased altitude, maximum altitude is150m.41paths/cellIi0 500I I I Imeters0 500I I Imeters42Figure 3.5:Examples of the travel routes of three females that used the same part of the study site. (a)Female 18, non-breeding season, (b) female 9, breeding season, (c) female 6, breeding season,(d) all three combined. Note that the leks are associated with junctions in the travel routes andthat females tend to travel around rather than over the ridge to the east of the two leks in thecentre of the map. Shading indicates topography, pale lines are creeks and the hollow diamondsare leks. Darker shading represents increased altitude.430 500meters0meters0meters0meters44CHAPTER FOUR.CONSEQUENCES OF VARIATION IN LEK SIZE IN THE OCHRE-BELLIEDFLYCATCHER, MIONECTES OLEAGINEUS.ABSTRACTSeveral hypotheses suggest that the costs and benefits of display in aggregations of different sizesplay a major role in both the evolution of leks and the patterns of distribution of males across leksof different sizes. I examined the consequences of variation in lek size for both males andfemales in a study of the ochre-bellied flycatcher, Mionectes oleagineus. In total 41 solitarydisplay sites and leks, ranging in size from 1 - 5 males, were observed over three breedingseasons. Although mean visitation rate by females was positively correlated with lek size, femalevisitation rate per male remained constant across lek sizes. The rate at which females visited themale with the highest female visitation rate at each lek was positively correlated with lek size.Neither mean nor per capita intrusion rates were correlated with lek size. For the top rankedmale, however, there was a significant correlation between intrusion rates and the size of the lekat which he displayed. Intrusion at leks may be costly, since 28% of female visits wereinterrupted by intruders. Solitary males suffered no such interference. Females show nopreferences for larger leks, visiting and mating at solitary sites as well as at leks. However,females preferentially visit males with high singing rates and this male trait may determinevisitation patterns. My data argue that preferences for larger leks are not important in theevolution of lekking in this species, nor do they affect lek size. Instead, males settle according tothe distribution of female movement, while females choose mates without consideration of leksize.45INTRODUCTIONIn lek mating systems, the number of males displaying at a site can vary from just asingle individual, as in some birds of paradise (Pruett-Jones and Pruett-Jones 1990) and manakins(Lill 1974, 1977; Théry 1990, 1992), to over 400 males in the most extreme example, sagegrouse, Centrocercus urophasianus (Scott 1942). Marked variation in lek size occurs within aswell as between species. Within a single population it is common to find that some males displaysolitarily while others display at leks (Kruijt et al. 1972; Lill 1974; Sexton 1979; de Vos 1983;Rolstad and Wegge 1987; Hoglund et al. 1993; Westcott and Smith 1994). In recent years therehas been increased interest in the costs and benefits associated with variation in lek size. Thisshift in emphasis away from studies of within-lek dynamics has been prompted by the realizationthat the variation in payoffs between leks may be a function of lek size and might provideinsights into the processes underlying lek evolution. A number of hypotheses attribute lekevolution to an increase in benefits with increased lek size for one or both sexes. Thesehypotheses, refered to as “preference” hypotheses, suggest a number of ways in which a positiverelationship between reproductive success and lek size may evolve.Preference hypotheses suggest that females have a specific preference for mating atlarger leks. This could occur if increased lek size was positively associated with factors such asi) decreased predation rates (Koivisto 1965; Hjorth 1970; Wittenberger 1978; Gosling 1986), ii)an increase in the ease or probability of detecting the lek (Otte 1974; Alexander 1975), iii) moreefficient comparisons of mates (Emlen and Oring 1977; Parker 1978), or, iv) decreasedharassment by males (Clutton-Brock et al. 1993; Stillman et al. 1993). Preferences for larger leksmay also occur if lek size functions as a male secondary sexual characteristic (Queller 1987),perhaps in a manner similar to the bowers of bowerbirds (Borgia 1985; Borgia et al. 1985) andwidowbirds (Andersson 1991). The reasons for using leks as secondary sexual traits would bethe same as those suggested for the evolution of other epigamic traits, because males at leks aremore attractive (Lande 1981; Kirkpatrick 1982) or viable (Zahavi 1975; Hamilton and Zuk 1982;46Andersson 1986).Other preference models rely on indirect mechanisms to produce preferences forlarger leks. A preference of females for mating with specific individual males may result inunsuccessful males clustering around those preferred individuals in an attempt to parasitize theirsuccess (Beehier and Foster 1988; Hoglund and Robertson 1990a). Because females matepreferentially with these “hotshot” males, irrespective of the number of other males present at thelek, and because the number of males around a successful individual should reflect the quality ofthat individual, females will appear to show a preference for larger leks. Although the variouspreference hypotheses differ in mechanism, they all share two predictions: 1) one or both sexesbenefit from mating at larger aggregations, and, 2) one or both sexes show a preference for largerleks.Variation in the costs and benefits associated with attendance at leks of different sizesis an important assumption not only of preference models of lekking. Such variation may affectthe settlement decisions of individual males even when these decisions are made according to thephysical distribution of receptive females, i.e. the hotspot hypothesis (Bradbury 1981; Bradburyand Gibson 1983; Bradbury et al. 1986). In the simplest versions of the hotspot hypothesis malessettle over the distribution of receptive females according to ideal free distribution rules (sensuFretwell 1972; Parker 1978). Under these conditions, male settlement should result in the meanpayoff to males being equal across all lek sizes. It has also been suggested that a model assumingideal free distribution of unequal competitors provides a more appropriate description of malesettlement patterns (Sutherland and Parker 1985; Parker and Sutherland 1986; Bradbury et a!.1986; Alatalo et al. 1992; Hoglund et al. 1993), given the high variance in male mating success inmost lek mating systems (Bradbury and Gibson 1983). This model assumes that males distributethemselves not only according to resources, as in the ideal free case, but also according thedistribution of competitors and their relative competitive abilities. Thus males should settle atleks of different sizes as a function of their competitive abilities. High quality males should befound at large leks where, despite higher costs, payoffs will be greater because of higher female47densities. Low quality males, on the other hand should settle at sites where both costs andrewards are lower. The result of this process should be that as lek size increases, so too shouldthe mean payoffs associated with display at that lek.To date, slightly less than half of the cases examined have failed to demonstrateincreased payoffs associated with larger leks (Hammerstrom and Hammerstrom 1955; Koivisto1965; Liii 1976; Pruett-Jones 1985; Bradbury et al. 1989b; Deutsch 1994). Conversely, theremaining experimental (Kruijt et al. 1972; Lank and Smith 1992) and observational (Shelley1990; Alatalo et al. 1992; Hoglund et al 1993; Baimford et al. 1992) studies have found a percapita increase in female visitation or copulation rates at larger leks. The prediction of modelsthat consider settlement by unequal competitors, that males of different competitive abilities willhave preferences for different lek sizes, has also received empirical support in two studies(Alatalo et al. 1992; Widemo and Owens 1995).In this paper I examine the consequences of variation in the size of ieks of Moleagineus, a Tyrannid flycatcher. The study had two aims. The first was to examine the costsand benefits associated with display at leks in the light of the hypotheses discussed above. Thesecond was to explore the reasons why territorial males attempt to prevent new males fromsettling adjacent to their display territories. In particular I was interested in answering thefollowing two questions: 1) are females more likely to visit males at larger leks?, and 2) do malesbenefit by displaying at larger, rather than smaller, leks?METHODSStudy Species.M oleagineus is a small (12 g) understory resident of wet, lowland forests fromsouthern Mexico to southern Amazonia, including Trinidad and Tobago (Meyer de Schauensee1966). In Costa Rica it occurs at elevations of up to 1200 m on both the Pacific and Atlanticslopes (Stiles and Skutch 1989). In south-western Costa Rica the breeding season of M.48oleagineus lasts from February to August (unpublished data), though this varies with rainfall.This period spans the last half of the dry season and the beginning of the wet season.There are three “classes” of male in my study population: territorial, satellite andfloaters. Territorial males held display territories with a mean size of 763, ± 88 S.E., m2.Display territories were both solitary or clustered into small leks (size range = 2-6 males, mean =2.6 ± 0.83 males). With the exception of 2 two-male exploded leks, where territories wereseparated by an undefended interstitial buffer of 10 and 13 m respectively, display territories atleks share contiguous boundaries. The average solitary male was 114.7, ± 28.9 S.E., m from hisnearest neighbour (Westcott and Smith 1994). In this paper I use the term “display territory” torefer to the area defended by a territorial male against other males and to which he attempts toattract females by singing. I also use the term display site to refer to any location where one ormore males defend display territories.Non-territorial males include “satellites” and “floaters” (Westcott and Smith 1994).Satellites do not “own” a territory; instead they surreptitiously use the territories of another male.When the owner is present the satellite behaves in an inconspicuous manner; when he is absent,the satellite behaves like the owner. Floaters have no territorial association; they wander widelyand visit display sites at many leks. They are not tolerated by territorial males and are generallyseen only briefly on display sites before being chased off by the owners.Data Collection.Birds were caught using mist-nets and banded with unique combinations of colourbands. All territory owners were banded. The sexes are almost entirely monomorphic and thesex of individuals was ascertained either through their behaviour or from morphometric data (seeWestcott and Smith 1994).Males were observed on their display territories, one at a time, during observationperiods of either 1 (1990 and 1992) or 2 (1989) hours duration. Observations were conductedbetween 0530 and 1130 hours, the time of peak display activity (Westcott and Smith 1994). All49males were observed approximately once a week and both the sequence in which males wereobserved, and the time of day that each male was observed on was randomized. Behaviour ofmales and of their visitors was documented using continuous sampling (Altmann 1974) andrecorded using an event recorder program running on a Tandy TRS-80 computer. The datapresented in this paper were collected between May and August in 1989, and March and Augustin 1990 and 1992. Because much of the analysis of this data is based on rates, the underlyingdistributions of which are uncertain, I used non-parametric statistical methods except whereparametric tests were appropriate. All rates are events per hour.When the same individual males displayed at the same lek in two years, the data forthose years were pooled. This only occured in the case of solitary males. If the lek’s sizeremained constant but one or more males were replaced between years, or if lek size changed, thelek was considered to have changed and was included more than once in the analysis. Theoutcome of the statistical tests are qualitatively identical if data from different years is combinedwhen leks at a site remained the same size but were composed of different males.Data Analysis.Because a suite of a posteriori correlations was performed against lek size, sequentialBonferoni adjustments were used in assessing their significance level (Rice 1989). SequentialBonferoni adjustments at alpha=0.05 had no effect on the statistical significance of the results,with results significant prior to the adjustment being significant afterwards and non-significantresults remaining so. Consequently, for ease of interpretation, I present only the non-adjusted Pvalues.RESULTSA total of 74 males was studied at 41 display sites over the 3 years of this study. The50number of display sites, both lekking and solitary, and the number of males studied in each yearare given in Table 4.1. One to 5 males held territories at a given display site during the study.The mean rate of visitation by females to leks was positively correlated with the sizeof the lek at which the territory was located(r5=0.42, n=40, P<0.01; Figure 4.1 .a). This increasein female visitation rate occurred in proportion to the number of males present at the lek and,consequently no significant relationship was detected between per capita rate of female visitationand lek size (r5 = 0.24, n=40, P>0. 10; Figure 4.1 .b). However, if the female visitation rates of thetop ranked male (the male at each display site with the highest female visitation rates) at eachdisplay site and lek are considered alone, there is a significant correlation between femalevisitation rate and lek size(r5—_O.53, n=40, P<0.0O1; Figure 4.1.c). Female visitation rate to thedisplay territories of second-ranked males was not correlated with lek size (r5=0. 18, n=40,P>0.20, Figure 4.1 .d). There was also no significant increase in the variance in female visitationrates among the territories of a lek along with increasing lek size (r5=0. 139, n=40, P>0.20).Copulations were rarely seen, but there was no detectable preference on the part offemales for mating at leks over solitary display territories. In the 585 hours of observationsincluded in this analysis, 2 copulations were observed; one involving a solitary male and theother a male at a lek of 2.For a male, one potential result of displaying at a lek is that exposure to the stimuli ofother males displaying may elevate his own display performance. Though I found a positiverelationship between the rates of female visitation to a male’s territory and his song rates(r=O.36,n=74, P<0.002; Figure 4.2.a), I detected no tendency for mean song rates to increasewith lek size(r5=0.159, n=40, P>0.20 Figure 4.2.b). Nor was tenure on a territory greater formales at leks than for solitary males; at both solitary territories and at leks the median tenure of aterritorial male was 1 breeding season with a range of 0.5 -4 breeding seasons (Mann-WhitneyU-Test, U=278, P=0.285).The males most visited by females at a lek sang more in larger leks (r5=0.50, n=40,P<0.002; Figure 4.2.c). This is not to say, however, that the best solitary males have low song51rates. Song rates of more than 1500 songs/hour were recorded from just 3 males: one a solitarymale, one a male at a lek of two, and one a male at a lek of five. Song rates of greater than 1000songs/hour were recorded from all lek sizes. The mean song rate across all males was 687, ±40S.E., songs/hour. A male’s song rate rank on the study site in a given year was positivelycorrelated with his female visitation rank that year (r=0.49 1, n=74, P<0.0001; Figure 4.2.d).Rates of intrusion on to a territory by other males are only weakly correlated with thesize of the lek at which the territory was located(r5=O.233, n=40, P>0. 10 (Figure 4.3.a). Nor wasthere significant correlation between per-capita rates of intrusion and lek size (r5=-0.20, n=40, P>0.20; Figure 4.3.b). As in the case of female visitation rates, however, there wasa significant positive correlation between lek size and the intrusion rates on the territories of themales with the top female visitation rank(r5=O.46, n=40, P<0.005; Figure 4.3.c). No correlationwas detected between intrusion rates on the territories of the second ranked males and lek size(r=O.2l,n=23, P=0. 10; Figure 4.3.d). Males at leks were more likely to suffer intrusion duringdisplay to females than were solitary males. Intrusion occurred during 13 (28%) of 46 bouts ofdisplay to females by 9 males at leks while no intrusion occurred during 17 bouts of display bysolitary males (Fisher’s Exact Test, P=0.013).DISCUSSIONSmall Sample Sizes and Non-Significant Correlations.As with other studies of variation in lek size, many of my reported correlationcoefficients are low and non-significant. Despite my relatively large sample sizes, n=23 and 40(c.f. Alatalo et a!. 1992; Hoglund et al. 1993; Deutsch 1994), the statistical power associated withthese correlations is low, always less than 0.33. Consequently, these correlations do not showthat relationships do not exist. However, within the same system and at the same level of power,I did detect significant correlations with lek size. While these non-significant correlations may52describe real relationships, I suggest they are not dominant influences on lekking.Do Females Prefer Leks?Preference hypotheses suggest that there is an adaptive advantage to females frommating at leks. This advantage might accrue through improved mate choice (Otte 1974;Aiexander 1975; Bradbury and Vehrencamp 1977; Bradbury 1981; Queller 1987), decreasedpredation (Koivisto 1965; Hjorth 1970; Wittenberger 1978) or decreased harrassment (CluttonBrock et al. 1993; Stiliman et al. 1993). For M oleagineus, there was no per capita increase infemale visitation rates with lek size, and females visit and copulate (Westcott 1992) with solitarymales. These observations suggest that females have no preferences for males at leks oversolitary males, or for large over small leks. This argues against a role for female preferencebased models in determining lek size in M± oleagineu& mating system.The lack of a preference for larger leks is probably because males with high songrates, a correlate of female visitation rates (Figures 4.2.a and 4.2.d), can be found at leks of allsizes (Figure 4.2.c). The strong correlation between a male’s song rate rank and his femalevisitation rate rank indicates that females? preferences are for the male with the highest song rate,or for some closely correlated trait, at a lek. Given the lack of any per capita increase in meanfemale visitation rates with lek size, I suggest that females prefer to visit the males in their homerange with the highest song rates, independently of the number of other males at the display site.Having selected a set of potential mates based on average song rate, females may then choosetheir mate from among these individuals on the basis of the males’ complete display performance(see Westcott and Smith 1994).The Costs and Benefits of Lekking for Males.I found no detectable relationships between either mean intrusion rates onto displayterritories or per capita intrusion rates and lek size. This is surprising since the presence of apermanent pool of potential intruders in the form of neighbours at leks would seem to be the53perfect recipe for increased intruder pressure. However, the bulk of intrusion is performed bynon-territorial males, and the behaviour of these individuals suggests that they visit the displayterritories within their home range during their daily foraging movements, apparently prospectingfor potential territories (Westcott and Smith 1994). Under the maxim that any territory is betterthan no territory, floaters might be expected to investigate solitary and lek territories equally, thusno difference in intrusion rates across lek sizes would be expected.Territorial intrusion, however, clearly does constitute a cost in j oleagineus’ matingsystem. Top-ranked males suffered higher intrusion rates when they displayed at larger leks(Figure 4.3 .c). Based on this relationship, the top-ranked male at a large lek could expect doublethe intrusion rate of a solitary male. The costs associated with such increased intrusion arepotentially large. First, increased intrusion requires a greater investment in territory defense.Costs associated with territory defense include the extra time and energy that is diverted fromdisplay and self-maintenance, and the risk of injury during encounters with intruders. This riskof injury may be significant. Twice I recorded raptor attacks on males preoccupied withintruders, though both attempts were unsuccessful. On one occasion the predator, a collaredforest-falcon, Micrastur semitorquatus, was unsuccessful only because I inadvertently flushed itas it was walking up to the male and his intruder. These two were on the ground together,helplessly entangled in spider web, Nephila spp., and still completely engrossed in their mutualattack.Second, and more importantly, intrusion results in lost copulations. This may occurin two ways. First, when a male is absent from his territory, intruders, both satellites and floaters,often sing on the territory and will even display to visiting females (Westcott and Smith 1994). Itis possible that females will occasionally mate with these impostors. Second, the display ofsolitary males was never interrupted by intruders; at leks, however, 28% of female visits wereinterrupted by intruders. When a visit was interrupted, the female usually left the territory andwas not seen visiting that territory again by us. This intrusion most likely decreases the netbenefits to top ranked males of displaying at larger leks. For lower ranked males, interference54during display may be even more severe due to their lower female visitation rates.Do males prefer displaying at larger leks? The lack of a detectable per capitaincrease in female visitation rates with lek size means that for the average male there should beno advantage in displaying at larger leks. Indeed, only the top male at a lek experiencedincreases in female visitation rates with lek size. Nor did lekking provide other advantages suchas increased display performance, or increased tenure on a territory. Thus, I conclude that theaverage male gains no advantage from larger leks, and thus should have no preference fordisplaying at them. With no detectable advantage to larger leks for males or for females,preference hypotheses for lek evolution can be discounted as explaining lek size foroleagineus.Hotshots and Lek SizesAn alternative to the preference hypotheses is the hotshot hypothesis (Beehier andFoster 1988). This hypothesis suggests that less attractive males settle around the malespreferred by females. If this is the case then lek size should increase as a function of the successin attracting females of a lek’s hotshot. The correlation between lek size and female visitationrates on the territory of the male with highest female visitation rates is consistent with thisprediction (Figure 4.1 .c). However, two pieces of evidence run contrary to the hypothesis’predictions. First, the distribution of males did not match hotshot predictions in three regards.Not only were the highest mean female visitation rates recorded at two leks of 2 males and one of3 males rather than at the largest leks on the study site, but males with low female visitation ratesremained at some leks of two and three males despite the extremely paucity of female visitors tothose leks. Also, consistently high female visitation rates at several 2-male leks did not lead to anincrease in the size of these leks over the four years of the study, as predicted by the hypothesis(Beehler and Foster 1988). Second, the long breeding season and low rates of female visitationto males suggest that subordinate males should use the same cues for choosing their hotshot as dofemales for choosing mates, i.e. song rate, rather than direct observation of female visits. Over55the period of this study, this hotshot selection process should have resulted in the largest threeleks forming around males that actually held territories at display sites of one, two and fivemales.Though these results are not consistent with Beehier and Foster’s (1988) version ofthe hotshot hypothesis, they might be expected if hotshots, unable to prevent interruption anddisturbance by neighbours, attempt to circumvent the problem by preventing new males fromsettling nearby (Hoglund and Robertson 1990a). Because males do attempt to prevent settlementadjacent to their territories (Chapter 5), the distribution of males across M oleagineus leks may,at least partially, be explained by hotshot processes.Female Visitation. Lek Size and Hotspots.The hotspot hypothesis suggests that males settle according to some aspect of thedistribution of females (Bradbury 1981; Bradbury and Gibson 1983). In the simplest hotspotmodel, male settlement occurs such that the mean access to females is constant across leks, i.e.according to ideal free distribution rules (Bradbury et al. 1986). There is good evidence from myearlier studies that hotspot mechanisms explain some aspects of male display dispersion in Moleagineus. As predicted by the hotspot model, leks of yL oleagineus are not only clusteredalong with the leks of other understory lekking species, but they also show a tendency to changein size in unison with neighbouring leks of other frugivorous species (Westcott 1994). Also, leksare located along travel routes used by females and are most closely associated with locationswith high female traffic (Chapter 3). The results of this study also provide support for thehotspot model. Female visitation rates increased significantly with lek size, but did so inproportion to the number of males at the lek, as would be expected if lek sizes are a function ofthe distribution of females.However, this picture of males settling according to simple ideal free rules is marredby the fact that female visitation rates are not equal across the males at a lek; female visitation56rates increased with lek size for the male with the top female visitation rank at each lek but didnot do so on a per male basis (Figures 4.1 .a and 4.1 .c). If some males are more attractive thanothers, an ideal free model with unequal competitors might provide a more realistic description ofmale settlement processes. However, the unequal competitor model predicts that, despiteincreased competition, high quality males should settle at larger leks because these are the siteswith the highest rewards (Sutherland and Parker 1985; Parker and Sutherland 1986; Alatalo et al1992; Hoglund et al. 1993). Lower quality males should settle at smaller leks where both thecosts and the rewards are lower. The absence of any detectable per-capita increase in femalevisitation rate with lek size suggests that current unequal competitor models cannot adequatelyexplain male distribution in this species.Another explanation for the unequal distribution of female visitation within leks isthat the territories of the top ranking males may be located closer to, or directly on, the points ofhighest female traffic. If environmental features force females to pass through very small areas,e.g. a break in a thick scrub (Apollonio et al. 1990), individual males may be able to monopolizethese areas. Other males at the site may have much lower visitation rates simply because theirterritories are located on the edge of, rather than directly over, this site. While female M.oleagineus do use habitual travel routes when moving through their environment (Chapter 3),there are no locations where either their movements or the environmental features around lekssuggest such severe constraints (Chapter 3).Alternatively, while males may settle in an ideal free manner over femaledistribution, all males may not be equal in the sight of those females. If females exhibit strongpreferences for particular individuals, then the benefits associated with ideal free settlementpatterns may not match those predicted by theory. This is probably what is happening in M..oleagineus. The observation that the female visitation rate a male receives is strongly correlatedwith his song rate (Figure 4.2.a) suggests that females choose males on the basis of song rate, orsome closely correlated trait. If females do exhibit strong preferences within leks for this traitthen a male’s rank at a lek in terms of song rate should be strongly correlated with his female57visitation rate rank at that lek. This is in fact the case (Figure 4.2.d).Summary.The results of my study rule out the preference hypotheses for lekking: there is noincrease in female visitation, or in any other possible benefits that I measured with increased leksize for the average male. Females also show no preference for males at larger aggregations.The study is not, however, so clear in distinguishing between the hotspot and the hotshothypotheses. Given the evidence from my previous work in favour of a role for hotspotmechanisms in determining both the location and size ofj oleagineus leks, the hotshothypothesis might be discounted on the basis of parsimony alone. However, the two hypothesesare not mutually exclusive and additional tests of their specific predictions are needed. If hotspotmechanisms are the primary determinants of lek size, then the distribution of males acrossdisplay sites will be a function of the number of females passing through an area and the abilityof territorial males to prevent settlement at that site by others. The existence of a large nonterritorial male population suggests that despotism is an effective means of preventing settlement.While intruder pressure will eventually force many males to accept additional settlers at a lek,despotism might limit lek sizes to levels significantly below those that would be expected undersimple ideal free rules.58Table 4.1:A summary of the number of males and display sites that were present on the study site in eachyear of the study. ‘In 1990 the study site was extended and included 4 additional solitary malesthat were not included in 1989 or 1992.Year 1989 1990’ 1992# of solitary 6 9 3display sites#ofleks 6 7 9# of males 18 16 22at leksmean males/ 3 2.3 2.4lekTotal # of 24 25 25males59Figure 4.1:Female visitation rates (number/hour): a) visitation to leks of various sizes, b) visitation permale, c) visitation to the territory of the male at each lek with the highest female visitation rates,d) visitation to the territory of the male at each lek with the second highest female visitation rates.To reveal overlapping points a small amount of uniform random error has been added to eachpoint, resulting in some non-integer lek sizes and negative rates.CD Cl) N CD CD 0) N CDID Co N CD ID (I) N CDmeanfemalevisitationrate00COP0000P omeanfemalevisitationrateppo0000P00oI’3tIIII0-..•:•I.C.).cn 0)S•••.8••P.)••..C.)•C 0)000) 00 F’.)C.)meanfemalevisitationrate0000popppp--.-.-oIClCDCII010)100-.C)IIIIIIIIIIIIS..IIIIIIIIIII0 P.)C.)cli 0)meanfemalevisitationratepp00000P0p.-.-.—-0-O.t.b-.ICD100-,IIIIIIIIIIIII,pg.•1IIIIIIIIIII061Figure 4.2:a) Mean female visitation rate/hour for each male plotted against his mean song rate/hour: datafrom 1989 and 1990 from Westcott (1992). b) Mean song rate at each lek plotted against leksize. c) Mean song rate of the male at each lek with the highest female visitation rate plottedagainst his lek size. d) The rank, in terms of female visitation rate, of each male that held aterritory at a lek in any given year plotted against his rank in terms of song rate at that lek. Toreveal overlapping points a small amount of uniform random error has been added to each point,resulting in some non-integer lek sizes and negative rates.620 400 800 1200 1600mean song ratec)I I I.I.•4I I0 1 2 3 4 5 6lek sized)I I I I I I I I I IS—I I I I I I I I I I I Ib)a). ..• S• S.... •I . • •••I• •. • SI I I—2000180016001400a)12001000800E4002000• • S:•.1.41.2a)1.00.80.60.4a)E0.20.0200018001600- 14001200a)0) 1000C0800Ca)6004002000CCOa)C0a)0>a)a)Ea)30282624222018181412108S4200 1 2 3 4 5 6 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30song rate ranklek size63Figure 4.3:Mean intrusion rate (number/hour): a) intrusion to territories at leks of various sizes, b) intrusionrates per male at leks, c) intrusion onto the territory of the male at each lek with the highestfemale visitation rates, d) intrusion on to the territory of the male at each lek with the secondhighest female visitation rates. To reveal overlapping points a small amount of uniform randomerror has been added to each point, resulting in some non-integer lek sizes and negative rates.64b)a)I I I.eI I2.01.5C0CotoC0)0)a0.5I I I I.•. .••.•e. : I I2.01.5 -a)CS0Co2 1.0 -CCCOa)E0.50.02.01.50)CSC01.0CCCSa)20.50.00 1 2 3 4 5lek sizec)I I I I I. •.I •I • •I I I I0 1 2 3 4 5lek size0.00 1 2 3 4 5lek sized)I I I2.0 -1.5 -0)•C0Co21.0 •CCS •Coa •0.5-0.0- ••I I• I0 1 2 3 4 5lek size65CHAPTER FIVE.NEIGHBOURS STRANGERS AN]) MALE-MALE AGGRESSION AS ADETERMINANT OF LEK SIZE.ABSTRACTInteractions between males on leks may play a role in lek formation and the regulation of leksize. In this paper I present the results of a playback experiment that simulated ‘de novo’settlement at sites adjacent to currently existing display territories of the ochre-bellied flycatcher,Mionectes oleagineus. In the study population, males displayed both solitarily and at small leks.A large proportion of males held no display territory at all. A stranger’s song was played to bothsolitary and lekking males from 10 m outside their territorial boundaries. In separate playbacks,lekking males were also played neighbour’s song. Both lekking and solitary territorial malesreacted to the playback by decreasing their song rate, approaching the playback speaker and, onoccasion, attacking the model. Solitarily displaying males responded more aggressively toplayback of stranger’s song than did lek males. Lek males were able to distinguish between theirneighbour’s and a stranger’s song and did so irrespective of whether it was played from theneighbour’s territory or from outside the lek. The nature of the response to such playbacks,however, depended on where the playback originated. These results indicate that male-maleinteractions can be influential in structuring leks. In M oleagineus, interactions between malesare aggressive and act to limit rather than augment lek size.66INTRODUCTIONThe factors that structure leks and the role of interactions between males in thisprocess have been of interest since the earliest research on leks (Selous 1906,1927). Male-maleinteractions may affect lek size and structure in several ways. First, dominance hierarchieswithin leks, for example, are probably fundamental to lek stability in many species (Foster 1981;Avery 1984; Beehier and Foster 1988). Display is often directed towards other males (LeCroy etal. 1980; Avery and Sherwood 1982), and is necessary for territory maintenance in many lekkingspecies; failure to display can result in increased intrusion and territory loss (Snow 1974;Westcott 1992). In some species, dominance interactions between males have been ritualisedinto complex, coordinated displays (Prum 1985, 1986; Théry 1990). In extreme cases, thesedisplays have become essential components of display to females (Foster 1977, 1981). Incooperatively breeding manakins stable, long-term dominance hierarchies not only allow malesto co-exist at a lek (Foster 1977, 1981; Schwartz and Snow 1978; Robbins 1985; McDonald1 989b) but are imperative for successful attraction of mates (McDonald 1 989a). The breakdownof the dominance hierarchy at a lek can result in increased display disruption and can even causethe disintegration of the lek (Trail 1985; Robel and Ballard 1974). Similarly, the lack of adominance hierarchy and the continual destruction of competitors’ bowers is suspected to beresponsible for the absence of lekking in bowerbirds, Ptilonorhynicidae (Pruett-Jones and PruettJones 1982, 1994; Borgia 1985).Second, males might monitor the mating success of others and modify their ownbehaviour at the lek correspondingly. Unsuccessful males might cluster around males that aresuccessful in attracting females and attempt to parasitize their success (Beehler and Foster 1988;Hoglund and Robertson 1990a). Finally, differences in the competitive abilities of males mightdetermine the lek at which a male eventually settles (Alatalo et al. 1992; Hoglund et al. 1993). Ifmales of a particular competitive ability encounter higher net rewards at leks over a particularrange of sizes they should move preferentially to leks of those sizes (Alatalo et al. 1992; Hoglund67et al. 1993). In ruff, Philomachus pugnax, larger leks favour low-ranked males while medium-sized leks favour high-ranked males. At small leks all males attempt to attract passing males tojoin the lek (Hogan-Warburg 1966). Once a ruff lek reaches a medium size, about 4 males, thetop ranked males cease enticement displays towards other males, low ranked males, however,continue to display (Widemo and Owens 1995).In this paper, I consider the influence of male-male interactions on lekking in theochre-bellied flycatcher, Mionectes oleagineus, a small, lek-breeding, Neotropical Tyrannid(Skutch 1960). Males defend display territories either solitarily or at small leks in the forestunderstory (Snow and Snow 1979; Westcott and Smith 1994). Song is involved in both territorialdefence and mate attraction (Westcott 1992). This study was initially prompted by the puzzlingobservation that a large proportion of the banded male population, approximately 50%, do nothold display territories (Westcott and Smith 1994). These non-territorial males behave either asfurtive satellites on the territories of other males, or as wandering floaters with no apparentterritorial attachment. That such a large proportion of males are non-territorial is especiallysurprising given that only territorial males have been seen to display to, and copulate with,females. Non-territorial males thus appear to be foregoing current reproductive opportunities.Why these males do not establish display territories is not immediately obvious. InM± oleagineus, and in lek mating systems in general, the majority of the habitat remainsunoccupied and apparently available for settlement (Westcott 1993). There are at least 3 possibleexplanations for this failure of non-territorial males to establish territories. First, non-territorialmales, while able to establish a territory, may not be able to bear the energetic costs of territoryownership, perhaps because they are young and inexperienced. Second, inexperienced males,though able to bear the costs of territoriality, may not garner sufficient copulations to make theeffort worthwhile. In both these instances young individuals may postpone territoriality untiltheir prospects have improved, e.g. when they are older (Wiley 1991). Third, settlement atexisting display sites by non-territorial males may be resisted by the territorial males alreadypresent at the site. This is likely to be particularly important when there are preferred sites for68settlement (e.g., at traditional mating sites: Warner 1988), or locations where females are likelyto pass (e.g. Westcott 1994 and Chapter 3) and can be defended by an established male as aresource.Here I describe a playback experiment conducted to determine whether territorialmales attempt to prevent the establishment of new territories, adjacent to their own. If territorialmales benefit from increases in lek size, as suggested by several theories for lek evolution and anumber of field studies (Kruijt et al 1972; Lank and Smith 1992; Shelley 1990; Alatalo et al.1993; Hoglund et al. 1993), then territorial males should not respond aggressively to new settlersand might actively encourage settlement. On the other hand, if increased lek confers a net cost,they should respond aggressively and try to limit lek size. I tested this hypothesis with aplayback experiment. My results support the latter hypothesis. They also show that males candistinguish the songs of neighbours from those of a stranger, and that they modify their responseto these different signals depending on location of the playback.METHODSStudy Site.The study was conducted between April and June 1992 at Estacion Sirena (lat. 80 29’N, long. 83° 36’ W) in Corcovado National Park, Costa Rica. The study site is divided betweenlow rugged hills (to 140 m in height) and flat areas. There is a distinct wet season between Mayand November, with peak rainfall occurring between September and November. Mean annualrainfall is 5,305 ± 301 mm S.E., n 10 years (Servicio de Parques Nacionales de Costa Rica).The vegetation is pre-montane, wet forest (Tosi 1969) of which 80% is primary forest, theremainder being 15 year-old second growth.69Playbacks.Recordings for the playbacks were made during March 1992 using a Sennheiser ME-80 directional microphone and a Sony Professional Walkman WM-D6C. Each territorial malewas recorded and a 2 minute section was selected for each according to the following criteria: i)the focal male was singing at a standard rate of between 18 and 20 songs/minute, ii) there was alow level of ambient noise, iii) the male had not had a visitor of either sex on his territory for atleast 1 hour, and, iv) none of his neighbours were singing. The “strange” male recording wasmade subject to the same criteria and obtained near the junction of the Cedral and Sirena rivers,some 11 km north of the study site. It is unlikely that any of the males subjected to this playbackhad been previously exposed to this individual’s song. All males sing the three songs recordedfrom M oleagineus (Snow and Snow 1979; Westcott and Smith 1994). In all playbackrecordings singing occured at a relatively constant rate throughout the playback, i.e. with nonoticeable pauses.I conducted playbacks between April 20th and June 4th 1992. I kept the duration ofthe experiment brief to minimize effects of stage of the breeding season on males responses.This period represented approximately 30% of the breeding season and began after mostterritories had been established for 2 months.Solitary males were subjected to a single session of playback of the stranger’s songfrom outside their territory. Males at leks were subjected to four playback sessions; i) thestranger’s song played from outside the lek, ii) the stranger’s song played from within aneighbour’s territory, iii) neighbour’s song played from outside the lek, and iv) neighbour’s songplayed from within that neighbour’s territory. An individual lek male was played the sameneighbour’s song in both neighbour playbacks. Each male at a lek was played the song of adifferent neighbour. In addition, a random sample of solitary and lekking males was subjected toa playback of background noise. No male experienced more than one playback per week. Toavoid association of my presence with the playback by the males, all focal males experienced atleast two 1 hour observation periods between consecutive playback sessions. The sequence of70presentation of the different types of playbacks was random and differed between males.Because my aim was to simulate the settlement of a new male adjacent to the focalmale, playbacks were performed from a point 10 m outside the territory boundary of the focalmale. This distance corresponds approximately to the radius of a circle the size of an averagemale display territory (760 m2,Westcott and Smith 1994). Playbacks were performed only if thefollowing conditions were met: i) there had been no visitors of either sex to the territory of thefocal male within the previous half hour, ii) no other birds were currently on the territory, iii) thefocal male was in the centre of his territory, iv) his neighbours were not singing and, if it was aneighbour playback that was to be performed, his neighbour was not present on his territory, andfinally, v) ambient noise levels were low. I considered these restrictive criteria necessary butthey resulted in unequal sample sizes in the different playback categories.In the actual playback session the playback speaker was set up 1.5 m above theground and a stuffed male M oleagineus specimen in an upright position was placed 30cm aboveit. A preliminary half hour observation period was then conducted. At the end of this half hour atwo minute “before” observation period was begun followed by the two minute playback. Duringboth the before period and the playback period male song rate, distance from the speaker, andbehaviour was recorded onto tape. Distance was estimated to the metre each time the birdchanged perches.Statistical Analysis.I compared males’ behaviour during the 2 minutes of playback with their behaviourduring the 2 minutes before playback. For song, I determined the proportion of the total amountof song sung during the experiment (i.e. song of before phase plus song of during phase) that wassung during the playback. If males showed no reaction and continued to sing as in the beforephase, this proportion would be 50% on average. Proportions of less than and greater than 50%would occur if males decreased or increased song rates respectively. Proportions were arcsinetransformed for analysis. I calculated the mean distance of focal males from the speaker in the71during playback phase and subtracted it from their mean distance in the before phase. In thisinstance a relative distance of 0 m represents no reaction on the part of males, and positive andnegative scores represent movement towards or away from the speaker respectively.Both parametric and non-parametric tests were used as appropriate. The sample sizefor solitary males, n=5, was set by the number of solitary males present on the study site duringthe experiment. Because of the low sample sizes in this category, I also used a randomizationtest with 10,000 iterations (Potvin and Roff 1993) when comparing stranger from outsideplaybacks for lekking and solitary males. Since in each case the results of the randomization testwere qualitatively similar, and sometimes quantitatively identical to those obtained usingparametric tests only the latter are presented.RESULTSDuring playbacks, males typically ceased singing and approached the speaker,apparently searching for the culprit. Sometimes males would approach the speaker only briefly,and then return to their territories. However, sometimes, and especially in response to astranger’s song played from outside the lek, they would vigourously search the area of thespeaker and even attack the mounted specimen. Males did not visibly react to the controlplayback nor was any statistically significant effect detected during controls (control fromoutside, song: Mann-Whitney U-Test, W=121, P=0.69; control from out side, distance: MWU,W=100, P>0.l).Lek Type and Male Responses.Solitary males showed a greater decrease in song rate than did lek males in responseto playback of stranger’s song from outside the territory (t-test t=2.8 1, df=17, P=0.049, Figure5.1). Lek and solitary males did not differ in their closest approach to the speaker (Mann-72Whitney U-Test, T=45, P=O.66), nor in their relative mean distance from the speaker during theplayback (t=O.835, df= 17 P=O.41, Figure 5.2). However, solitary males were more likely toattack the model than were lek males (lek males mean # of attacks/playback=O.07 1, solitarymales=1.8, MWU T=69, P=O.022).In this population, M oleagineus shows three categories of male display dispersion:solitary males, exploded leks and classical leks. At exploded leks territories did not sharecommon boundaries and only 2 males were ever present. There was no difference between thesong rates of the top-ranked males at exploded leks and those of solitary males (t=- 1.55, df= 1,P=O.261, Figure 5.3). However, when top-ranked exploded lek and solitary males are combinedand compared with lekking males, the combined exploded lek and solitary males show a greaterdecrease in song rate in response to the playback (t=-4.4, df 7, P=O.003, Figure 5.3).Playbacks at Leks: Effects on Song.Males decreased their song rates during all playbacks with the exception ofneighbour’s song played from that neighbour’s territory (Figure 5.1). A two-way ANOVA ofplayback type x playback location was performed with proportion of song sung during theplayback as the response variable. There was a significant effect of the type of playback, i.e.neighbour or stranger song(F1,45=6.012, P=O.018; Figure 5.1). Neither playback location, i.e.outside the lek or from a neighbour’s territory, nor the playback type*location interaction hadsignificant effects (location:F145=l.266, P=O.266; type*location:F145=O.538, P=O.467). Posthoc tests revealed that the difference between playback types was due to males singing lessduring stranger playback from a neighbour’s territory than during neighbour playback from thatneighbour’s territory (F1,45=4. 9, P=O.038; Figure 5.1).Playbacks at Leks: the Effects on Distance from Speaker.The effect of playbacks on relative mean distance from the speaker, i.e. meandistance prior to the playback minus mean distance during playback, is shown in Figure 5.2.73Two-way ANOVA with relative mean distance from the speaker as the response variablerevealed significant effects of playback type(F1=26.695 P<O.OOl), playback location(F1,46=7.948, P=0.007) and type*location interaction(F1,46=13.598, P=O.OO1). Because of thesignificant interaction effect some caution in interpreting the main effects is warranted. Post hoctests show that males spent more time close to the speaker during playback of a stranger’s songfrom outside the lek than during playback of the same song from a neighbour’s territory=23.015, P<0.001). Males responded more strongly to stranger’s song from outside the lekthan they did to a neighbour’s song from the same location(F146=2.774, P>0.00l). However,males did not differ in their response to stranger’s and neighbour’s song when these were playedfrom the neighbour’s territory(F146=l.052, P=0.31 1).DISCUSSIONDistinguishing between Neighbours and Strangers.Playback of a stranger’s song consistently elicited a stronger response thanneighbour’s song, irrespective of whether it was played to a solitary or lek male, or whether it wasplayed from outside a lek or from a neighbours territory. Of the stranger’s song playbacks,solitary males exhibited the strongest response (Figures 5.1 and 5.2). While solitary males didnot differ from lek males in their mean distance from the speaker, or in their closest approach tothe speaker during the playbacks, they did show a significantly greater decrease in the amountthat they sang and appeared to search more intensely. The aggressive nature of the solitarymales’ response is underscored by the fact, that with one exception, only solitary males actuallyattacked and struck the model, or, in one case, a female blue-crowned manakin (Pipra coronata)unfortunate enough to be about the right size and colour and in the wrong place.For lekking males, the reaction to a stranger’s song varied according to where thesong was played from. When the stranger’s song was played back from a neighbour’s territory,males did not approach the speaker as closely as when it was played from outside the lek. The74effect on song rate, however, was identical and resulted in a sharp decrease in song output duringthe playback (Figure 5.1). These results suggest that males always recognize a stranger as such,but also that they regard them as potentially threatening. It seems, however, that they attackstrangers only when the stranger is outside the lek. Stranger’s song emanating from a neighbour’sterritory is considered either as the neighbour’s problem, or as a situation that, at least initially,need only be monitored. Though males are not loath to cross territory boundaries when femalesare present (Chapter 4), defence of a neighbour’s territory probably has few benefits.A criticism of the experimental design used in this study is that I have subjected apopulation of males to the playback of a single stranger male thereby limiting the generality ofmy conclusions (Kroodsma 1989). However, I believe my results can be generalized to allstrangers for two reasons. First, all male M oleagineus sing a simple song made up of the samethree components (Snow and Snow 1979; Westcott and Smith 1994). This means that there is novaritation in song repetoires and very little in song structure. Second, in a pilot study conductedin 1991, 10 lek and 8 solitary males were subjected to playbacks of the songs of two additionalstrangers. The mean response of these males to playback from outside the lek did not differ fromthose reported for the same experiment in 1992 (for lek males, song: t-test, t=-0.436, df=23,P=0.667; distance:t=-0.703, df=23, P=0.49; for solitary males, song: t= 0.97, df=12, P=0.35;distance: MWU T=1.623 P=0.239).The only response males made to the playback of a neighbour’s song was to decreasesinging when the playback came from outside the lek (Figures 5.1 and 5.2). In this instancemales paused and then resumed singing. Since it is common for territorial males returning totheir territories to commence singing a short distance from the territory, neighbour’s song fromoutside the lek should not be a novel stimulus for most males. The slight decrease in song ratesprobably represents no more than a pause to confirm the identity of the singer.These striking differences between the reactions of lek males to the playback ofneighbour’s and stranger’s song are clear evidence that males can distinguish neighbours andstrangers on the basis of song. Given that territory ownership is relatively stable within and even75across breeding seasons (Westcott and Smith 1994, Chapter 4), males have ample opportunity tolearn the songs of their neighbours. In addition, since territorial defense is a significantcomponent of a male’s activity in both the breeding (Westcott 1992; Westcott and Smith 1994;Chapter 4), and non-breeding seasons (pers. obs.), it should pay a male to discriminate betweensingers that do and do not require a response. Interestingly, not only do males appear todetermine the identity of the singer but they vary their response depending on where the playbackcomes from.Individual recognition capabilities, based on song and other characteristics, have beendemonstrated in a number of bird species (Weeden and Falls 1959; Falls 1969; Baker et al. 1981;Falls 1982). However, such abilities have not previously been demonstrated in lek-breedingbirds. In several lek-breeding species there are indications of individual recognition. Forexample, females have been observed returning to, and sometimes re-mating with, particularindividual males both within and between breeding seasons (e.g. Snow 1962, Lill 1974, Trail andAdams 1989, Gibson et al. 1992). While these observations suggest that females can identifyindividual males, it is not possible to rule out alternative explanations such as the territory historyor location (Gibson 1992), or even whether a consistently high rank of the male in the pool ofavailable males makes him a preferred mate. To my knowledge, this is the first experimentaldemonstration of song-based, individual recognition in a lek breeding species.Male-Male Interactions and Lek Evolution.In previous work on this species (Westcott 1992; Westcott and Smith 1994),territories were defined as the areas encompassed by a male’s song posts; from which he wouldevict intruders and beyond which he did not pursue intruders. By performing playbacks fromlOm outside these boundaries, I placed the speaker in locations from which males did notnormally sing or evict intruders. Excursions by focal males outside their display territoriesboundaries to seek out and attack a simulated territorial male in these previously undefendedareas can reasonably be interpreted as attempts to deter settlement adjacent to their display76territories. This conclusion is supported by observations of the defense of extra-territorial areasin the field. In two cases, one a solitary territory, the other at a lek (unpubi. data), this defensecontinued for a period of months. In both instances it was successful in preventing the repeatedsettlement attempts of several individuals in areas that were not otherwise used or defended bythe resident male. In the case of the solitary male, this successful defense lasted for one fullbreeding season at least. Part way through the second breeding season a male finally managed toestablish a new territory adjacent to this solitary male. Thus, males seem to be actively seekingto prevent other males from settling at a site and are capable of doing so for long periods.The results of this study suggest that most current models for lek evolution areunlikely to provide much insight into lekking in M oleagineus. The clear disinclination ofmales, particularly solitary males, to allow settlement adjacent to their territories indicates apreference for solitary display. This effectively rules out hypotheses that propose a benefit tomales from display at large leks (predation reduction, Koivisto 1965; lek size as a sexuallyselected trait, Queller 1987; mate retention, Clutton-Brock et al. 1993; signaltransmission/detectability enhancement, Otte 1974). Models that propose a benefit to females(Lack 1968; Bradbury 1981; Bradbury and Gibson 1986), however, cannot be ruled out on thebasis of these data since it is possible that female preferences for leks conflict with the malepreference for solitary display. Observations of female visitation and copulation patterns,however, have failed to identify a female preference for larger leks (Chapter 4).An alternative model is the hotspot hypothesis (Bradbury 1981). In the simplesthotspot models, male settlement follows ideal free distribution rules, with males settlingpreferentially in the areas of highest female traffic in a manner such that each male encountersthe average number of females. Under more complex versions of the model, however, malesdiffer in their competitive abilities and individuals choose their lek on the basis of their qualityrelative to that of the other males present, i.e. settle according to ideal free rules with unequalcompetitors (Sutherland and Parker 1985; Parker and Sutherland 1986; Bradbury et al. 1986;Aiatalo et al. 1992; Hoglund et al. 1993). Unfortunately, both simple and complex models77assume that males can settle where they choose, an assumption that is precluded if establishedmales can prevent settlement. Settlement models for this species, therefore, should be despotic(Bradbury et al. 1986), with highly competitive males not only obtaining a disproportionate shareof the matings at a site, but also limiting the settlement options of other males at that same site.Such a scenario is particularly appropriate if males are in fact defending not just thearea of their territories but rather the whole hotspot itself. If this were the case, a solitaryterritorial male would be defending the “rights” to a display site where many females pass on aregular and predictable basis. All other things being equal, the cost to the original male of thesettlement of a second male would be 50% of the copulations at the site. This represents a sharpdecrease in the fortunes of any males’ reproductive prospects (pers op.). As additional malessettle at the site, the original male must share more and more of his access to females. However,as the cost per additional male decreases, so too might the aggressiveness of his response.The data presented here fit this scenario nicely; males do defend extra-territorial areasand the response to playback does decrease from solitary to lek males. Neighbours also representa cost in terms of increased intrusion rates. While intrusion rates remained constant across therange of lek sizes for the average male, they increased for the top ranked male at each lek(Chapter 4). The top-ranked male at the largest lek could expect an intrusion rate approximatelydouble that of a solitary male. Perhaps more importantly, intrusion and interference at leksoccurred during display to females, and sometimes resulted in the female leaving the territory.This did not occur at solitary display sites. Thus, losses in terms of both lost copulations to a newneighbour and of interference in display, are largest when the number of males goes from one totwo. As this cost diminishes I predict a decrease in the level of aggression displayed by theoriginal male towards additional settlers.In summary, male-male interactions in M. oleagineus act to limit, rather thanincrease, lek size through direct interference by territorial males. The existence of a large pool ofnon-territorial males in this population is probably due to the high cost of territorial establishmentin the face of interference by territorial males.78Figure 5.1:Proportion of total amount of song that was sung during the playback, ± S.E.. A score of 50%represents no change in song rate during the playback, less than 50% indicates a decrease. Thenumbers above the x axis represent sample sizes in each category. N>N - playback ofneighbour’s song from a neighbour’s territory, N>O - playback of neighbour’s song from outsidethe lek, S>N - playback of stranger’s song from a neighbour’s territory, S>O - playback ofstranger’s song from outside the lek. “Lek” and “solo” refer to playbacks performed to males atleks and solitary display territories respectively.790.60,50)0.4C0C’)0.10.0 13 12 13 14 5oplayback category80Figure 5.2:Change in mean distance, ± S.E., from the playback speaker in the two phases of the experiment.A positive score indicates that males were y m closer to the speaker during the playback thanprior to it. The numbers above the x axis represent sample sizes in each category. N>N -playback of neighbour’s song from a neighbour’s territory, N>O - playback of neighbour’s songfrom outside the lek, S>N - playback of stranger’s song from a neighbour’s territory, S>O -playback of stranger’s song from outside the lek. “Lek” and “solo” refer to playbacks performedto males at leks and solitary display territories respectively.relativedistanceinm-0-r\3C.)4C)10)-JCD(00-MCi)3IIII1--i•iO(0j5(\-R1•I(f)C)___-•Hi5_ CO(\i00___(I.I00_______0)IIIII0282Figure 5.3:Comparison of the proportion of song sung by males at the three categories of display site. Thenumbers above the x axis represent sample sizes in each category.830.40.3-C)C0Cl)0.1 -0.0 13 5exploded lek solodisplay site type84CHAPTER SIX.CONCLUSION.Community Level Patterns of Lekking.Patterns in lekking within a community of lek-breeding species have only rarely beenconsidered. When such comparison have been done, the purpose has always been to look for acorrelation between the sizes of female home ranges and the degree of clustering in males acrossspecies (Bradbury et al. 1986; Théry 1992). At Sirena, not only are the leks of all lek-breeding,understory birds spatially aggregated, but they tend to occur close to environmental features thatwould channel the movements of birds through the habitat, e.g. saddles in ridge lines, the ends ofridgelines and the confluences of drainages. In addition, the sizes of the leks of frugivores tendedto change in unison. These observations suggest that leks form as a response to the temporal andnumerical distribution of females in space. The results are consistent with the predictions of thehotspot hypothesis (Bradbury et al. 1986) and suggest a role for hotspot mechanisms indetermining both the location of leks and their sizes in lek-breeding tropical forest birds.Female Movements and Distributions.The results of my first chapter suggest that topographical features influence femalemovements and that leks are associated with some aspect of female distributions. The telemetrydata presented in Chapter 2 confirm these suggestions. Not only do females tend to follow linesof least resistance when they travel between areas, but leks are strongly associated with areas ofhigh female traffic. In contrast, leks are less strongly associated with sites with high femaledensities, and not associated with areas where females spend the most time. It appears that theproblem males face is to maximize their exposure to as many females as possible when the exactlocations of these females, and the times when they are sexually receptive, are uncertain. Malesachieve this by settling at sites where females reliably pass when travelling through the habitat.85The observation that females use habitual travel routes suggests two avenues for future research.First, manipulation of these routes would allow an experimental test of the female traffic versionof the hotspot hypothesis. Second, the effect of such structured movement patterns of seeddispersers on the distribution and recruitment patterns of the plants whose fruit they eat warrantsconsideration.Is Lekking a Better Strategy?Contrary to the expectations of several current hypotheses for lek evolution, i.e. thepreference hypotheses and hotspot models with unequal competitors, display at larger leks is notassociated with any apparent advantages in IYL oleagineus. While female visitation ratesincreased with lek size, they did not do so on a per-capita basis as predicted by these hypotheses.Indeed, for males, rather than being advantageous, display at leks incurs the additional cost ofinterference during display. The patterns of the distribution of female visitation to leks and leksize failed to match the predictions of the hotshot hypothesis (Beehier and Foster 1988).However, the lack of a per capita increase in female visitation rates with lek size is consistentwith the predictions of both the hotspot hypothesis (Bradbury et a!. 1986) and the attractivenessversion of the hotshot hypotheses (Hoglund and Robertson 1990a). To determine whether one orboth of the hypotheses are relevant to this system will require additional work; I have no ideawhat at the moment.Despots and Lek Size.The observation that a large proportion of males do not hold display territories(Westcott and Smith 1994) and that increased lek size is associated with increased costs, bothsuggest that established territorial males might be acting despotically, i.e. preventing thesettlement of additional males at their lek. This was confirmed by the playback experiment. Thisexperiment also demonstrated that males can distinguish neighbours from strangers on the basisof their songs.86The Evolution of Lekking in Mionectes oleaineus.The picture that emerges from this study is that the maj or impetus for aggregation inL oleagineus is provided by hotspot mechanisms. Males attempt to maximize their reproductivesuccess by settling at sites where they will encounter the greatest number of receptive females.Because of the unpredictability in the locations of females and of the resources that they requireover the 7-month breeding season, males cannot predict exactly where females will spend time orwhere the greatest female densities will be encountered. Males appear to overcome theseuncertainties by settling at sites where female traffic is predictable, irrespective of the season andthe exact locations between which females are travelling.While hotspot mechanisms alone are sufficient to explain lek location and play a rolein determining lek size (Chapter 2, 4, Westcott 1994), interactions between males, perhaps notsurprisingly, are also involved in determining the number of males that settle at a site (Chapters 3and 4). Clustering of other males around successful individuals, i.e. the attractiveness version ofthe hotshot hypothesis (Hoglund and Robertson 1990a), is possibly part of this process. The netresult is that, up to a threshold level of intruder pressure, territorial males limit the settlement byother males, thus keeping lek size below the level predicted by ideal free settlement. Beyond thislimit, territorial males are forced to allow other males to settle adjacent to them. The manner inwhich territorial males attempt to prevent settlement in extra-territorial areas suggests they aredefending the entire hotspot rather than just the area within their “usual’ territory boundaries.Several observations suggest that hotspots are involved in lekking in other taxa,particularly other tropical frugivores. First, my comparison of lek locations used members ofthree distinct families, the Tyrannidae, Pipridae and Trochilidae. All exhibited the same patternof aggregation. Second, ‘leks of leks’ occur in other parts of the Neotropics and can involvemembers of at least one additional family, the Cotingidae (M. Théry, pers. comm.), as well asabout 10 additional species not included in my analysis (M. Théry, pers. Comm.; Westcott pers.ohs.). Third, evidence for hotspot mechanisms has been found in at least two other studies oflekking tropical birds, the Pipridae (Thery 1992) and the Paradisaeidae (Pruett-Jones and Pruett87Jones 1990).I suggest that this convergence in lekking in tropical species is due to three factors: i)the extreme length of the breeding season, ii) the ephemeral nature of their food resources and theresulting lack of predictability in female distributions and, iii) low female reproductivesynchrony. I predict that as the effect of these factors is diminished, so too will be theimportance of hotspot mechanisms in lek evolution. Under such circumstances, e.g. grouse andshorebirds, the influence of other mechanisms, such as hotshots and female preferences foraggregated males, should predominate.Lekking in an Evolutionary Time Frame.The current mating systems paradigm postulates that mating systems are adaptiveresponses to current social and ecological conditions (Emlen and Oring 1977). Several recentstudies have suggested that this paradigm is flawed and that phylogeny may override ecologicalinfluences in determining a species’ mating system (Bjorklund 1991; Edwards and Naeem 1993;Rendall and DiFiori 1993; Prum 1994). This raises the question: to what extent can my results beapplied to the evolution of lekking. I have considered lekking in an ecological time frame but thefactors involved in an evolutionary time frame may be very different (Prum 1994). I believe,however, my results can be extended to an evolutionary time frame for two reasons. 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