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Effects of climate on timing of breeding, reproductive output and population growth of song sparrows… Wilson, Scott 2003

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Effects of Climate on Timing of Breeding, Reproductive Output and Population Growth of Song Sparrows (Melospiza melodia) in the Southern Gulf Islands, British Columbia by Scott Wilson B.Sc, The University of Calgary, 1999 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE in THE FACULTY OF GRADUATE STUDIES CENTER FOR APPLIED CONSERVATION RESEARCH DEPARTMENT OF FOREST SCIENCES We accept this theses as conforming to the required standard THE UNli^RSITY OF BRITISH COLUMBIA December 2003 © Scott Wilson L i b r a r y A u t h o r i z a t i o n In presenting this thesis in partial fulfillment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission. ScuH bji'lson Pec- %, 9001 Name of Author (please print) Date Title df Thesis: £ffi;cr3 faf rlfrvik ^ foifoo oP hrot^/ r ymdur iw D e 9 r e e : Maskr ticifAM Y e a r : '8003 A B S T R A C T In recent years, there has been a growing interest in the effects of global climate changes on avian population dynamics. Several studies have now shown that climatic oscillations such as the El Nino Southern Oscillation and the North Atlantic Oscillation affect a number of avian species through their impacts on local weather and ocean temperatures. In this thesis, I first reviewed the effects of these climatic oscillations on birds with a focus on identifying the underlying mechanisms by which they influence population change. I then examined the effects of climate on reproduction and population dynamics of song sparrows (Melospiza melodia) in the Southern Gulf Islands of British Columbia, Canada. Here, I first considered the influence of the El Nino Southern Oscillation on timing of breeding, fledgling production and population growth on Mandarte Island. I found that over the past 28 years, annual timing of breeding has not advanced in response to global warming, but has varied considerably among years in relation to variation in the El Nino Southern Oscillation. Females bred earlier in warmer El Nino years and later in colder La Nina years. Early breeding increased reproductive output, primarily because it increased the overall length of the breeding season and allowed females to make more nesting attempts. Despite this, timing of breeding had little effect on population growth, primarily because density-dependent effects on juvenile recruitment had an over-riding influence on population change. I then examined the regional influence of climate on reproduction in six adjacent song sparrow populations. Here, I predicted that if climate was the dominant factor affecting reproduction, we should observe synchrony in annual reproductive rates across all populations. In contrast, if local factors that vary among populations have the greatest influence on reproduction, we should observe asynchrony in reproductive rates. I found that populations displayed synchrony in the onset of egg laying, suggesting that climate affects timing of breeding similarly across all populations. However, populations displayed considerable variation in reproductive output, which was largely driven by differences in the extent of nest predation and brood parasitism. Rates of nest predation and brood parasitism were high in populations close to Vancouver Island, and lower in the more isolated populations. Overall, the results of this thesis suggest that populations will vary in their response to climate change depending on the influence of climate on demographic parameters that contribute most to population change and the relative influence of other ecological factors on reproduction and survival. u TABLE OF CONTENTS Page Abstract ii Table of Contents iii List of Tables v List of Figures vi Acknowledgements vii CHAPTER 1 1 Effects of Climatic Oscillations on Bird Populations 1 ABSTRACT 1 1.1 INTRODUCTION 1 El Nino Southern Oscillation 3 North Atlantic Oscillation 4 1.2 EFFECTS OF CLIMATIC OSCILLATIONS ON REPRODUCTION 4 Timing of breeding 4 Clutch size and reproductive output 8 1.3 COMPLEX INTERACTIONS BETWEEN CLIMATE AND REPRODUCTION 10 Density dependence 10 Competition 11 Lagged effects of climate on reproduction 11 1.4 EFFECTS OF CLIMATIC OSCILLATIONS ON SURVIVAL 12 1.5 FUTURE CONSIDERATIONS 14 CHAPTER 2 22 El Nino Drives Timing of Breeding but not Population growth in the Song Sparrow 22 (Melospiza melodia) ABSTRACT 22 2.1 INTRODUCTION 22 Effects of climate change on ecological processes and timing of breeding 22 El Nino and reproduction in birds 23 2.2 MATERIALS AND METHODS 24 Study area, data collection and statistical analysis 24 Climate data 25 2.3 RESULTS 26 El Nino Southern Oscillation and timing of breeding 26 Effect of timing of breeding on reproductive output and population growth .... 27 2.4 DISCUSSION 28 iii Mechanisms behind earlier breeding in El Nino years 28 Why does timing of breeding not influence population growth? 29 Relationship between ENSO, global warming and timing of breeding 29 CHAPTER 3 34 Effects of nest predation, brood parasitism and climate on reproductive variation in a song sparrow metapopulation 34 ABSTRACT 34 3.1 INTRODUCTION 34 Population synchrony and metapopulation stability 34 Local versus regional effects on population demography 35 Study predictions 36 3.2 MATERIALS AND METHODS 37 Study area and data collection 37 Statistical analysis 39 3.3 RESULTS 40 Variation in population size 40 Spatial and temporal variation in timing of breeding and reproductive output.. 40 Factors affecting reproductive output 41 Spatial and temporal variation in clutch size, nest predation and brood parasitism 42 3.4 DISCUSSION 43 Synchrony in timing of breeding 43 Local versus regional effects on reproductive output 44 LITERATURE CITED 52 iv LIST OF TABLES Page 1-1. Examples of the effects of the El Nino Southern Oscillation (ENSO) on reproduction and survival in birds. The table includes the mechanism suggested by the authors although in many cases, the mechanism was not directly tested 18 1 -2 Examples of the effects of the North Atlantic Oscillation (NAO) on reproduction and survival in birds. * indicates studies where the NAO was not mentioned but its influence was likely based on the location of the study 20 3-1 Annual variation in number of breeding female song sparrows on the six islands in this study (see Fig. 3-1 for island locations) 50 3-2 General Linear Model results showing factors that influence annual reproductive output (N=33, r = 0.84, r2= 0.71) 50 3-3 Types of nest failure, mean number of cowbird eggs per nest and mean clutch size on each of six islands during the study. Values indicate mean (+ 1SE) in each case. See methods for further descriptions of each type of nest failure. The total sample size of nests monitored on each island is included in brackets beneath the island name 51 v LIST OF FIGURES Page 1 -1. Time series showing patterns of the Southern Oscillation (top figure) and the North Atlantic Oscillation (bottom figure) over the past 100 years. Annual values for both are the averaged monthly indices from November through April 16 1- 2 Influence of El Nino events on food webs in marine ecosystems of the Eastern Pacific (based on Barber and Chavez 1983, left diagram) and terrestrial ecosystems in the Galapagos Islands using the Cactus Finch as an example (based on Grant et al. 2000, right diagram) 17 2- 1 Mean annual January through April values of the El Nino Southern Oscillation index during the study period. Low negative values indicate El Nino conditions, while high positive values indicate La Nina conditions 31 2-2 Influence of the El Nino Southern Oscillation on spring temperature in southwest British Columbia (r2=0.46, pO.OOl, N=31, regression: DDW = 2498.85 + 12.73 SOI). Higher values of degree-days of warming indicate colder spring temperatures 32 2-3 Influence of the El Nino Southern Oscillation on annual median date of first egg laid in song sparrows (r2=0.40, p<0.001, N=31, lay date = 107.39 + 0.57 SOI) 32 2-4 Effect of median date of first egg (Julian days) in year Xt on annual mean number of fledglings produced per female in year Xt (r2=0.42, p<0.001, N=31, mean fledglings = 13.75-0.09 lay date) 33 2- 5 Effect of median date of first egg (Julian days) in year Xt on change in population size (lambda) between years Xt and Xt+1 (r2=0.02, p=0.61, N=29, lambda=1.95 - 0.007 lay date) 33 3- 1 The study area in the Southern Gulf Islands of British Columbia, Canada showing the study region near Vancouver Island (left map), and the islands included in this study (in black, right map) 48 3-2 Spatial and temporal variation in date of first egg for song sparrows on six islands. Error bars represent + 1 SE 49 3-3 Spatial and temporal variation in mean number of young fledged per female song sparrows on six islands 49 v i ACKNOWLEDGEMENTS I would first like to thank my supervisor Peter Arcese for providing support, guidance and many helpful suggestions throughout the course of my thesis. This work also benefited greatly through helpful comments from my committee, Jamie Smith and Kathy Martin, and other students in the Arcese lab including Amy Marr, Katie O'Connor, Jane Reid and Simone Runyan. I also thank the many field assistants who have collected data over the course of the study. Most recently these include Danielle Dagenais, Andrew Johnston, Caroline Saunders, Rob Landucci and Andy Davis. Funding for this project was provided by the Natural Sciences and Engineering Research Council, the Faculty of Forestry, the Association of Field Ornithologists and the American Ornithologists Union. Last, but certainly not least, I would like to thank my wife Amy, for her helpful comments throughout the thesis, for patiently listening to many practice talks and for her never-ending support. vii CHAPTER 1 The Effects of Climatic Oscillations on Bird Populations Abstract. Through their effects on local weather patterns, climatic oscillations such as the El Nino Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) can influence ecological processes in both marine and terrestrial systems. This review highlights the effects of these oscillations on bird populations and attempts to identify the mechanisms underlying population change. Available evidence suggests that the ENSO and NAO influence several aspects of avian demography including timing of breeding, clutch size, nestling condition, juvenile recruitment and adult survival. These effects operate through a number of mechanisms including severe weather, physiological changes, effects on the food chain, competition, delayed effects and via interactions with other population processes. Because of spatial variation in the effects of the ENSO and NAO, a particular event may have very different effects on similar species or populations in different geographic locations. Furthermore, because of biological differences, two species in the same area may respond differently to the same climatic event. 1.1 Introduction It has long been recognized that local variation in temperature and precipitation can have marked effects on bird populations. For example, early studies found that populations of European songbirds were often lower after cold winters (Jourdain and Witherby 1918) and Australian songbirds often declined considerably during droughts (Finlayson 1932). At the time of these early studies, little was known about the broader sources of change in local weather patterns. However, we now know that variation in local weather is often driven by 1 changes in large-scale climatic forces that influence temperature and precipitation throughout much of the world. Two climatic forces that are particularly influential are the El Nino Southern Oscillation (ENSO, Allan et al. 1996) and the North Atlantic Oscillation (NAO, Hurrell 1995). Many of the ecological effects of the ENSO and the NAO were first observed in marine communities where altered ocean temperatures and currents resulted in changes in plankton density and community composition (Barber and Chavez 1983, Planque and Taylor 1998, Stephens et al. 1998, Chavez et al. 1999). These changes in turn influenced the abundance and spatial distribution of fish populations (Nakken and Raknes 1987, Cushing 1995, Lehodey et al. 1997, Helle and Pennington 1999) and caused fluctuations in seabird numbers (Schreiber and Schreiber 1984, Valle 1985, Boersma 1998). Studies have also recently provided evidence for effects of climatic oscillations on terrestrial ecosystems (reviewed in Holmgren et al. 2001, Ottersen et al. 2001, Stenseth et al. 2002). Some of these effects include advances in the phenology of vegetation (Myneni et al. 1997, Polis et al. 1997, Post and Stenseth 1999), earlier breeding in birds and amphibians (Forchhammer et al. 1998), altered predator-prey dynamics (Stenseth et al. 1999) and rodent outbreaks (Jaksic et al. 1997). Although we are beginning to understand the effects of the ENSO and the NAO on terrestrial processes, many questions remain about the degree to which climatic oscillations influence demography and population dynamics. This is despite the fact that a number of recent studies have greatly increased our knowledge on how these oscillations influence reproduction and survival in birds. Here, I review the effects of climatic oscillations on bird populations by examining our current understanding of the mechanisms by which the ENSO and the NAO are known to influence reproduction and survival, and thus population change among avian species. 2 El Nino Southern Oscillation The Southern Oscillation is a fluctuation in sea-level atmospheric pressure between the eastern and western areas of the south Pacific Ocean. It is typically measured by the Southern Oscillation Index, which is calculated as the pressure difference between Darwin, North Australia and Tahiti (Australian Commonwealth Bureau of Meteorology 2002). Although the underlying causes of the oscillation are unclear, one of its primary effects is a change in the strength of easterly trade winds that normally flow across the south Pacific Ocean. When the pressure is low in Tahiti and high in Darwin (low, negative values of the index), the trade winds weaken resulting in an accumulation of warm water in the eastern Pacific, while colder waters prevail in the western Pacific (El Nino events). When the pressure is high in Tahiti and low in Darwin (high, positive values of the index), the trade winds strengthen resulting in colder than normal sea temperatures in the eastern Pacific and warmer than normal temperatures in the western Pacific (La Nina events). Because of the close relationship between the Southern Oscillation and the El Nino/La Nina cycle, the two processes are often described by a single term, the El Nino Southern Oscillation (ENSO). El Nino and La Nina events have wide-ranging effects on temperature and precipitation. Some of the effects of an El Nino event on temperature include warmer than normal temperatures in western North America, southern Asia and northern South America, and colder temperatures in New Zealand and central Australia. The effects of an El Nino event on precipitation include increased rainfall in arid regions of western North and South America, the Galapagos Islands, and northwest Africa, and decreased rainfall in Australia, Micronesia, the Caribbean and northeast South America (Allan et al. 1996). The effects of La Nina events on temperature and precipitation are roughly opposite to El Nino effects. The strength and 3 frequency of El Nino events are difficult to predict although they tend to occur approximately every three to six years (Allan et al. 1996, Fig. 1-1). N o r t h A t l a n t i c O s c i l l a t i o n The North Atlantic Oscillation (NAO) is an alternation in the atmospheric mass between the sub-tropical high pressure center over the Azores and the sub-polar low pressure center over Iceland. The NAO index is typically calculated as the anomalies from the long-term average difference in sea level pressure between Lisbon, Portugal and Stykkisholmur, Iceland (Hurrell 1995). A high, positive NAO index is the result of a strong Icelandic low and a strong Azores high. Under these conditions, the westerly winds flowing across the North Atlantic move northward and increase in strength. This results in an increase in wintertime temperature and precipitation over Northern Europe and the eastern United States, and drier conditions over the northwestern Atlantic and the Mediterranean. A low, negative NAO index is associated with a lower pressure difference between Iceland and the Azores. This results in weaker westerly winds across the North Atlantic and roughly opposite temperature and precipitation effects to those observed during a positive NAO (Hurrell 1995, Visbeck et al. 2001). Although there is interannual variability in the NAO, there is a tendency for decadal trends where the oscillation is more consistently in one phase over a few decades (Hurrell, 1995). Since approximately 1975, the NAO index has generally been positive over the winter months (Fig. 1-1). 1.2 E f f e c t s o f c l i m a t i c o s c i l l a t i o n s o n r e p r o d u c t i o n Timing of breeding Many recent studies have analyzed for effects of climate change on the phenology of various taxa. For instance, recent changes in the growth of vegetation (Myneni et al. 1997, Post and Stenseth 1999), emergence date of insects (Visser et al. 1998) and timing of breeding in amphibians (Beebee 1995, Forchhammer et al. 1998) have all been linked to climate change. 4 Several studies also show that some species of European birds such as the chiffchaff (Phylloscopus collybita), the corn bunting {Miliaria calandra) and the great tit (Parus major) have bred earlier over the past 30 years (Crick et al. 1997, Forchhammer et al. 1998, McCleery and Perrins 1998, Sanz 2002). This advance in breeding date has been related to warmer wintertime temperatures as a result of the positive phase of the NAO since approximately 1970 (Forchhammer et al. 1998, Sanz 2002). In western North America, the ENSO cycle also influences winter and spring temperatures. Wilson and Arcese (2003) have recently shown that the ENSO cycle affects timing of breeding of song sparrows {Melospiza melodia) on Mandarte Island, British Columbia. Females bred earlier in warmer El Nino years and later in colder La Nina years. With earlier breeding, females made more breeding attempts and produced more fledglings per year. There are two likely mechanisms underlying the advance in breeding date with increased temperatures. First, because warmer temperatures influence plant growth (Myneni et al. 1997) and insect emergence dates (Ellis et al. 1997, Visser et al. 1998), earlier breeding may be triggered by increased food availability for females. In support of this mechanism is the fact that the provision of supplemental food advances breeding dates in a number of species including song sparrows (Arcese and Smith 1988), European starlings {Sturnis vulgaris ; Kallander and Karlsson 1993) and blue tits (Parus caeruleus; Nilsson and Svensson 1993). Second, as temperatures increase, the amount of energy individuals spend on thermoregulation declines (Weathers and Sullivan 1993, Cooper 2000, Stevenson and Bryant 2000). Thus, another potential mechanism for the observed trend to earlier breeding is that with warmer spring temperatures, females need to allocate less energy to thermoregulation and can invest more in reproduction, resulting in earlier laying dates. In support of this hypothesis is the fact 5 that great tit females in artificially warmed nest boxes bred significantly earlier than females in control nest boxes (O'Connor 1978, Dhondt and Eyckerman 1979). For any particular species, the degree to which breeding dates are influenced by the ENSO or NAO may depend on its life history strategy. For many resident species, timing of breeding is determined by spring temperatures on the breeding grounds (Forchhammer et al. 1998, McCleery and Perrins 1998, Wilson and Arcese 2003). Therefore, where the ENSO and NAO influence spring temperature we may observe a direct relationship between a particular phase of the ENSO or NAO and timing of breeding. For migrant species, the relationship between climate and timing of breeding is more complex because breeding dates are determined not only by conditions on the breeding grounds, but also conditions on the wintering grounds and along the migratory route. For instance, the pied flycatcher (Ficedula hypoleucd), a long-distance migrant, has not advanced its spring migration in response to warmer spring temperatures over the past 30 years (Both and Visser 2001). This is likely due to the fact that timing of migration for pied flycatchers is under endogenous control and thus not influenced by conditions on the breeding grounds. Although this species has shortened the period between arrival and initiation of egg laying, this advance has been insufficient to match selection for earlier breeding based on a shift in the emergence dates of their primary prey (Both and Visser 2001). In contrast to the above finding, two recent studies have also shown that for some species, the timing of migration and thus arrival on the breeding grounds, is related to the NAO, with earlier arrival dates in high NAO years (Forchhammer et al. 2002, Huppop and Hiippop 2003). For short-distance migrants, the earlier arrival in high NAO years may be related to both improved forage and weather conditions along the migratory route through Europe (Forchhammer et al. 2002). For long-distance migrants, earlier arrival in high NAO years may be a combination of increased food availability on the wintering grounds (Forchhammer et al. 2002) as well as beneficial weather conditions along the migratory route (Hiippop and Hiippop 2003). In these cases, the effect of a high NAO year on migration is consistent with its effect on spring temperature and selection for earlier breeding. However, these findings also suggest that if a particular climatic phase had opposing effects on timing of migration and timing of breeding, migrants may be unable to adapt to climate change on the breeding grounds and could suffer fitness consequences as a result (e.g. Visser et al. 1998). Most studies that have looked at relationships between timing of breeding and climatic oscillations have focused on a single population of a species. However, for the few species for which multiple populations have been studied, it appears that some populations display shifts in timing of breeding in response to climate change, while other populations do not. For instance, Visser et al. (2003) found evidence for shifts in timing of breeding in 5 of 13 great tit populations and 3 of 11 blue tit populations throughout Europe. This discrepancy was partially attributed to geographic variation in the effects of the NAO on temperature. In Finland, temperatures have not increased over that period and there has been no evidence for an advance in breeding dates for Finnish great tit populations. In contrast, both temperature and timing of breeding of great and blue tits in the United Kingdom have advanced over the past 30 years. However, some populations within the same geographic location show different patterns of change in laying date and thus variation in the response of populations cannot be fully explained by geographic variation in the effects of the NAO on temperature. One proposed explanation for these differences is that variation in habitat quality between sites affects the peak emergence dates of their primary prey. An alternative explanation is that there is a complex relationship between the value and frequency of double-brooding and the development time of the food resource in response to climate change (Visser et al. 2003). Overall, it is likely that populations of many species will show the type of variation to climate 7 observed among great and blue tit populations. Further study is thus required to understand the extent of variation in the response of populations to climate change and to better understand the mechanisms by which those differences might arise. Clutch size and reproductive output Climatic oscillations may also influence reproduction through their effects on clutch size and survival of young. These effects may occur either directly via severe weather, or indirectly by affecting food abundance. In marine systems, both direct and indirect effects of the ENSO cycle have had substantial impacts on reproductive success and population growth of seabirds. For example, heavy rains associated with the 1982-83 El Nino event caused the flooding of nests and near complete reproductive failure for a number of seabird species on Christmas Island (Schreiber & Schreiber, 1984). The effects of flooding were particularly severe for burrow nesting species such as shearwaters and petrels. A similar event occurred in central Chile during the 1997-98 El Nino, where Humboldt penguins (Spheniscus humboldti) abandoned their nests after heavy flooding (Simeone et al. 2002). In other marine examples, the effect of the ENSO cycle on reproduction has operated through the food chain (Fig. 1-2). In normal years, the waters in the eastern Pacific are cold and rich in nutrients (Allan et al. 1996). However, during El Nino events, warm waters prevail and this reduces the extent of upwellings that normally bring nutrients to the surface. This loss of nutrients sets off a cascade up the food chain where initial declines in phytoplankton are followed in turn by declines in zooplankton, fish and squid, and seabirds and marine mammals (Barber & Chavez 1983, Chavez et al. 1999). Boersma (1998) compared adult mass and reproductive output in Galapagos penguins (Spheniscus mendiculus) during El Nino and La Nina events. During the El Nino year, adult mass declined and nearly all breeding attempts failed, whereas during the previous La Nina year, adults were heavier and most nests were 8 successful. This variation in breeding success between the two years was attributed to declines in prey abundance with warmer sea temperatures during the El Nino event. The ENSO and NAO may also influence food chains in terrestrial systems and this has been shown to affect reproductive output (Fig. 1-2). In barn swallows (Hirundo rusticd) in Denmark, the size of the first clutch was larger during high NAO years (Moller 2002). Moller proposed that food abundance is greater during warm, wet years, and this improves the body condition of breeding females, allowing them to invest more in reproduction. Larger first clutches were an important determinant of both annual reproductive success and population growth the following year. Similar effects have been observed with variation in the ENSO cycle. In the normally arid Galapagos Islands, rainfall increases during El Nino years and this enhances the production of seeds, fruits, nectar, pollen and various arthropods. Grant et al. (2000) showed that two species of Galapagos finches produced more broods, larger clutches and fledged more young during these years. Because of the widespread variation in the effects of climatic oscillations on weather patterns, their effects on reproduction may differ considerably between areas. For example, in Hawaii, El Nino events are associated with decreased rainfall. Lindsey et al. (1997) found that the body condition and number of breeding attempts made by adult Hawaii amakihi (Hemignathus virens) and palila (Loxioides bailleui) declined during El Nino years. This decline was attributed to a reduction in flowering and seed production in mamane plants (Sophora chrysophylld), their primary food source. In New Hampshire, the condition of nestling black-throated blue warblers (Dendroica caerulescens) was lower in El Nino years and higher in La Nina years (Sillett et al. 2000). This difference was attributed to a reduced abundance of the warbler's primary prey, lepidopteran larvae, during El Nino events. Thus, while El Nino years bring increased rainfall and enhanced breeding success to finches in the 9 Galapagos Islands, they bring decreased rainfall and lowered breeding success to passerines in Hawaii and New Hampshire. For some populations, the effects of the ENSO or NAO on reproduction may operate through more than one mechanism. For example, annual reproductive output of rufous-crowned sparrows (Aimophila ruficeps) in California is higher than normal in wet El Nino years and lower than normal in dry La Nina years (Morrison and Bolger 2002). Elevated reproductive output during the El Nino year is thought to be predator-mediated. The cool, rainy weather of an El Nino event imposes thermoregulatory restraints on snakes, the primary nest predator. As a result, nest predation rates decrease during El Nino and this enhances reproductive output. Lowered reproductive output during the La Nina event is thought to be food-mediated. The dry weather associated with La Nina events lowers plant growth and insect abundance resulting in smaller broods and fewer fledged nests. 1.3 Complex interactions between climate and reproduction Density Dependence The effects of climate on reproduction may be enhanced or reduced depending on intrinsic population factors such as density dependent effects on reproduction or recruitment. For example, the cactus finch (Geospiza scandens) in the Galapagos Islands experiences increased reproductive output during El Nino years (Grant et al. 2000). However, clutch size, fledging success and survival of young from late broods all decline as the population size increases. Therefore, production of young in an El Nino year should be higher when the population density is lower. A similar situation is observed with song sparrows on Mandarte Island (Wilson and Arcese 2003). Although early breeding in warmer El Nino years enhanced reproductive output, there was little carryover to population growth because of density dependent declines in juvenile recruitment. These studies show that the response of some 10 populations to an ENSO or NAO event will not necessarily be linear, but will instead depend on interactions between climate and other population processes. Competition In some cases, the effects of the ENSO or NAO may vary among competing species in a community and this may in turn influence the competitive balance between the two. For example, in the Czech Republic, the collared (Ficedula albicollis) and pied flycatcher generally occupy different habitats (Saetre et al. 1999). Where the two species co-occur, the collared flycatcher is socially dominant and excludes the pied flycatcher from territories. Due to this interaction, nestling survival and breeding density of pied flycatchers declines. However, the collared flycatcher is sensitive to environmental conditions and numbers decline following cold spring weather associated with a low NAO. This decline allows numbers of pied flycatchers to increase and prevents competitive exclusion in areas where the species co-occur (Saetre et al. 1999). Lagged effects of climate on reproduction The effects of climatic oscillations on reproduction may also show a lagged effect for species that do not breed in the year following birth. In the north Atlantic Ocean, density and fledging success of breeding northern fulmars (Fulmaris glacialis) is higher during low NAO years (Thompson and Ollason 2001). However, fulmars delay breeding for at least five years following birth and so the effect of a low NAO winter on numbers of breeders and reproductive output is observed five years later when that cohort returns to breed (Thompson and Ollason 2001). A delayed response to climatic oscillations may also be observed in predatory terrestrial birds whose prey show irruptive responses to an ENSO or NAO event. In Chile, El Nino years bring increased rainfall, which enhances plant growth, seed production and rodent abundance (Jaksic et el. 1997). Raptors show a one-year lagged response to the rodent irruption with 11 enhanced reproductive output, immigration and population size in the following year (Jaksic et al. 1997, Jaksic 2001). 1.4 Effects of climatic oscillations on survival Many previous studies have examined the effects of weather on the survival of individuals during the non-breeding season (reviewed in Newton 1998). In many of these study locations, incidences of severe weather are in part driven by the NAO or the ENSO and thus, these oscillations may have pronounced effects on survival and population change in these regions. For example, nuthatches (Sitta europaea), golden plovers (Pluvialis pricaria), Dartford warblers (Sylvia undulata), blackbirds (Turdus merula) and song thrushes (Turdus philomelos) all show reductions in survival and/or population size following cold winters in Northern Europe (Karlsson and Kallander 1977, Nilsson 1987, Baillie 1990, Gibbons et al. 1993, Yalden and Pearce-Higgins 1997). In this region, cold winters are associated with a low NAO (Hurrell 1995). Other studies in Britain have found that increased winter snowfall, which is associated with a high NAO, may also influence overwinter survival. Populations of wrens (Troglodytes troglodytes), song thrushes and common sandpipers (Actitis hypoleucos) all declined following winters with increased snow depth, probably because snow reduced access to food resources (Greenwood & Baillie 1991, Holland & Yalden 1991). In Norway, Saether et al. (2000) found a positive correlation between the rate of population change in dippers (Cinclus cinclus) and warmer temperatures associated with a high NAO. Warmer temperatures increase the availability of stream habitat during winter, which raises the carrying capacity of the population and allows for higher rates of juvenile recruitment. The above examples indicate how a particular phase of the NAO may benefit some species while harming others. These differences are likely based on the biology of each species and how the NAO might influence features such as prey abundance, foraging methods and habitat availability. 12 Some of the most dramatic effects of the ENSO cycle on survival have been observed in seabirds. During the 1982-83 El Nino, declines in survival and/or population size was observed in a number of eastern Pacific seabirds including Galapagos penguins, flightless cormorants (Nannopterum harrisi), blue-footed boobies (Sula nebouxii), magnificent (Fregata magnifwens) and great frigatebirds (F. minor), and brown penguins (Pelecanus occidentalis) (Valle 1985, Boersma 1998). In Galapagos penguins, the population dropped by 77% immediately after the 1982-83 El Nino event and has since recovered slowly (Merlen 1984, Boersma 1998). The underlying mechanism behind these declines was most likely a reduction in food resources in response to increased ocean temperatures. These results indicate that populations of some seabirds decline quickly following El Nino events. So far, strong El Nino events have rarely come less than 5 to 10 years apart and this allows populations to recover to some extent between events. However, some models predict that global warming may increase the frequency and severity of El Nino events (Timmermann et al. 1999). If this prediction is correct, this shift may have severe effects on seabird populations in the central and eastern Pacific. The ENSO also influences overwinter survival in terrestrial birds through effects on precipitation or temperature. In Jamaica, Sillett et al (2000) showed that winter survival of black-throated blue warblers (Dendroica caerulescens) was high in wet, La Nina years and low in dry, El Nino years. This difference in survival was attributed to greater food availability that results from increased precipitation during a La Nina year. In New Zealand, El Nino winters are associated with increased rainfall and colder temperatures. Miskelly (1990) found that mortality of adult Snares Island snipe (Coenocorypha aucklandica heugeli) increased during El Nino years and this was likely due to a decline in numbers of invertebrate prey during cold weather. 13 1.5 Future Considerations This review indicates that climatic oscillations influence reproduction and survival of avian species over a range of geographic locations. These effects occur through a variety of mechanisms including severe weather, effects on the food chain, physiological changes, competition and via interactions with other population processes. Although the effects of the ENSO and the NAO are widespread, species vary in their response to a particular event. This is partly due to the spatial variation in the effects of the ENSO or the NAO. As a result, species in two different areas may experience very different temperature or precipitation effects. Also, two species in a single location may respond differently to a particular climatic event. This difference likely depends on how the change in temperature or precipitation affects the metabolic response, prey abundance, access to food resources and habitat availability for each species. Although we have improved our understanding of how climatic oscillations affect populations, at least three areas require further study. First, the effects of climate on populations will rarely be linear. Instead, they will depend on the interaction between climate and other factors that affect populations such as density-dependence, predation, disease and competition. Parallel, long-term demographic studies may reveal how these interactions vary from one population to another. Second, few studies have analyzed how individuals differ in their response to population change. However, one study on red-cockaded woodpeckers (Picoides borealis) found that females that were younger and more inbred were less able to respond to climate change (Schiegg et al. 2002). Thus, the response of populations to climate change may depend on population structure and in some cases, an individual based approach may be beneficial for predictive models. Third, there are a number of other climatic forces similar to the ENSO and NAO whose influence on bird populations is largely unknown. These 14 include the Madden-Julian Oscillation, the Pacific Decadal Oscillation, the Antarctic Oscillation, the Polar/Eurasian Pattern and the Scandinavian Pattern (for others see Climate Prediction Center 2003). These climatic forces are likely to affect population and ecosystem processes both on their own and through their interactions with one another. 15 30.00 -, o o CO -30.00 -I , ,—•• , , 1 1900 1920 1940 1960 1980 2000 Year 400 •D -400 A , , , , 1 1900 1920 1940 1960 1980 2000 Year Figure 1-1. Time series showing patterns of the Southern Oscillation (top figure) and the North Atlantic Oscillation (bottom figure) over the past 100 years. Annual values for both are the averaged monthly indices from November through April. 16 Seabirds/Marine Mammals T-Fish/Squid T-Zooplankton T-Phytoplankton I -Nutrients T Water Temperature T * El Nino Finch Density 1-Finch Reproductive Output V Finch Clutch Size/Fledging Success b Caterpillar Abundance r + Plant Growth + Rainfall T + El Nino Figure 1-2. 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Increasing evidence suggests that climate change affects the timing of breeding in birds, but there is less evidence to show how such changes affect the population dynamics of birds overall. Over the past 43 years, song sparrows (Melospiza melodia) on Mandarte Island, British Columbia, have not shown an advance in breeding date in response to global warming. However, this population did show considerable annual variation in timing of breeding correlated with the El Nino Southern Oscillation. Birds bred earlier in warmer El Nino years and later in colder La Nina years. Early breeding strongly increased reproductive output. However, annual variation in timing of breeding had little effect on population growth, perhaps because the population is strongly regulated by the rate of recruitment by juveniles. The juvenile recruitment rate declined with increasing population density but showed little response to climate. These findings suggest that populations will vary in response to climate change depending on how climate affects the demographic parameters that contribute most to population growth. 2.1 Introduction Variation in climatic conditions can have wide-ranging effects on local weather patterns (Hurrell 1995, Allan et al. 1996) and ecological processes (Stenseth et al. 2002). For example, a positive shift in the North Atlantic Oscillation during the last two decades has coincided with warmer, wetter winters over Northern Europe (Visbeck et al. 2001), advances in plant phenology *A version of this chapter is published. Wilson, S. and P. Arcese. 2003. El Nino drives timing of breeding but not population growth in the song sparrow (Melospiza melodia). Proceedings of the National Academy of Sciences 100: 11139-11142. 22 (Myneni et al. 1997, Post and Stenseth 1999), and an earlier spring emergence of some species of insects (Visser et al. 1998). Studies also show that the North Atlantic Oscillation can influence the timing of breeding of European birds (Crick et al. 1997, Forchhammer et al. 1998). Evidence for an effect of climate change on timing of breeding in North American birds is also growing. In response to warmer spring temperatures over the past few decades, Mexican jays (Aphelocoma ultramarina) and tree swallows (Tachycineta bicolor) have advanced their breeding dates by approximately 9 to 10 days (Brown et al. 1999, Dunn and Winkler 1999, see also Root et al. 2003, Parmesan and Yohe 2003). The tendency to breed earlier in temperate zone birds is expected under conditions of increased food supply and more favorable spring climate (Arcese and Smith 1988, Newton 1998, Kallander and Karlsson 1993). Thus, further advances in the timing of breeding are possible given that temperatures are expected to continue rising as carbon dioxide levels increase (Houghton et al. 1995). To the degree that breeding date affects reproductive output, we might also expect that climatic warming and early breeding will influence the overall dynamics of populations. Thus far, research suggests that climate change may influence population dynamics through effects on clutch size (M0ller 2002) and the rate of recruitment by young birds to populations (Sasther et al. 2000, Sillett et al. 2000). However, it is less clear that advances in the timing of breeding in response to climate also affect population growth. On the northern Pacific Coast of North America, the El Nino Southern Oscillation (ENSO) influences temperatures in winter and spring (Allan et al. 1996) and might therefore be expected to influence the timing of breeding in birds that rely on insects as food (e.g., Arcese and Smith 1988, Arcese et al. 1992). ENSO is known to affect reproduction in birds primarily through its effect on local precipitation (Curry and Grant 1989, Lindsey et al. 1997, Grant et al. 2000, Sillett et al. 2000), but it has yet to be shown that ENSO affects the timing of breeding in 23 North American birds via its effect on spring temperature. Here, I first show that over the past 43 years, the timing of breeding by song sparrows {Melospiza melodia) on Mandarte Island, British Columbia, has not advanced in response to global warming. However, timing of breeding was strongly correlated with variation in the El Nino Southern Oscillation and its effect on spring temperature. Second, I show that variation in the timing of breeding influenced annual reproductive output in the population but had little effect on population growth. This was apparently due to density dependent variation in juvenile recruitment, which acted to regulate population size (Arcese et al. 1992). 2.2 Materials and Methods Study Area I used 32 years of data (1960-63 and 1975-2002) from a long-term study of a population of song sparrows on Mandarte Island to assess the effects of ENSO on the median breeding date of female birds. Data from the first 4 years of the study were collected by F. Tompa (Tompa 1964). Mandarte Island is about 6 ha in size, with about 2 ha of shrub habitat used regularly for nesting by individually marked song sparrows. The size of the study population was estimated as the number of breeding females alive in the last two weeks of April each year. Because the island is small, the habitat is well dissected by trails, and all birds are individually identifiable, I assumed that the spring counts were without error. Throughout the March to July breeding period, territories were visited about every 5 days to record the behaviour of individual females, locate their nests and count the number of eggs laid and young fledged. I estimated the date the first egg was laid by each female annually by observing nests during the laying stage or back dating from the day of hatching. I then used the median date of first egg for all first nests as the estimate of the annual timing of breeding for the population. Annual reproductive success was estimated as the total number of young fledged per female breeder. I excluded from analyses all females and 24 nests with access to supplemental food in 1979 and 1985. I estimated the rate of juvenile recruitment as the proportion of juvenile females fledged in year t that survived to the last two weeks of April in year t + 1, assuming a 50:50 sex ratio in fledglings. I focused on females in the analysis because they are the limiting sex in this population (Arcese et al. 1992). Recruitment rates are absent from the early years of the study and therefore n (number of years with observations) is slightly lower for recruitment analyses. Population growth (k) was determined as female population size in year t+1/ female population size in year t. Further details of the methods used to band individuals, find and follow nesting attempts, and estimate demographic rates are given in Arcese et al. (1992). Statistical relationships among climate, reproductive parameters and population size were estimated by using general linear models. All variables were approximately normally distributed except for the annual rate of juvenile recruitment, which was right-skewed. I therefore normalized juvenile recruitment by square-root transformation, but used raw values in all other tests. The residuals of all models were approximately normally distributed. I tested for potential biases in the results as a consequence of including data from 1960-63, because the main data set was collected after 1974. The results were not markedly affected by including early data. I therefore report results based on the entire study period. 1 report simple regression equations for many analyses to facilitate their subsequent use by others. Climate Data Weather data were obtained from the daily summaries collected by the Olga weather station (National Oceanic and Atmospheric Administration, United States Department of Commerce) located on Orcas Island, 32 km east of Mandarte Island. For analysis of reproduction, I examined cumulative precipitation (cm) and the cumulative degree-days of warming (°F) (DDW) recorded from January through April each year. DDW was calculated as 65°F - ((daily maximum 25 temperature°F + daily minimum temperature°F) / 2), summed over all days from January through April (26). Thus, higher values of DDW indicate colder spring temperatures. Although the period January-April is not representative of "spring" in much of North America, the extreme south coast of British Columbia experiences a mild climate with a growing season that begins in late January, marked by the emergence of annual and perennial plants near sea level. Because song sparrows on Mandarte Island commence breeding as early as late February (Arcese et al. 1992), I considered it appropriate to summarize spring temperature and precipitation for January-April. I summed annual DDW for the period November-March to estimate the effect of winter-spring temperatures on juvenile recruitment because this is the period over which substantial juvenile dispersal and mortality occurs (Arcese 1989). The Southern Oscillation Index (SOI) is typically calculated based on monthly shifts in the air pressure difference between Tahiti and Darwin (Australian Commonwealth Bureau of Meteorology 2002, the source used here). Large negative values of SOI indicate El Nino conditions whereas large positive values indicate La Nina conditions. I used the mean monthly SOI value for the period January through April each year for analyses with timing of breeding and the following November through March for analyses with juvenile recruitment. 2.3 Results The El Nino Southern Oscillation has shown an erratic pattern over the past 42 years with major El Nino events in 1983, 1992 and 1998, and major La Nina events in 1971, 1974, 1989 and 1999 (Fig. 2-1). In southwest British Columbia, spring temperature was closely correlated with the SOI (rz=0.46, pO.OOl, N=31; Fig. 2-2). Temperatures were warmer in El Nino years and colder in La Nina years. In contrast, spring rainfall was unrelated to the SOI (C=0.04, p=0.39, N=31). The annual median lay date varied by 43 days over the study period, occurring as early as March 23 (Julian day 83) in 1992 and as late as May 6 (Julian day 126) in 1999. Median lay date was 26 related closely to spring temperature (r2=0.36, p<0.001, N=31, regression: lay date = 35.23 + 0.03 DDW) and to spring SOI (r2=0.40, pO.OOl, N=31, Fig 2-3), with the earliest breeding occurring in warm El Nino years and the latest breeding in cool La Nina years. The rate of juvenile recruitment showed little relation to temperature or the SOI either in the spring preceding birth (for temperature, r2=0.04, p=0.34; for SOI, r2=0.09, p=0.12, N=27) or the following winter (for temperature, r =0.02, p=0.51; for SOI, r =0.10, p=0.11; N=27). However, juvenile recruitment rate was negatively related to adult female density (r2=0.40, pO.OOl, N=27; recruitment = 0.70 -0.005 females). Breeding earlier had a strong positive effect on the annual number of young fledged by female song sparrows (r2=0.42, pO.OOl, N=31; Fig. 2-4), primarily because females had raised more successful nests when breeding began earlier (r =0.42, p<0.001, N=31; successful attempts = 4.70 - 0.03 lay date). The regression equation predicts that the average female breeding in the year with the earliest median lay date will raise about 1.29 more broods per year than females breeding in the latest year. The minimum time required for song sparrows to build a nest, lay eggs, incubate and raise young to independence is about 42 days (Arcese et al. 2002). This interval corresponds closely to the 43-day difference between the earliest and latest median lay dates recorded during the study. Despite the effect of timing of breeding on reproductive output, I did not find a relationship between median date of first egg in year t and population growth between year t and year t+1 (r =0.02, p=0.48, N=29; Fig 2-5). In contrast, variation in population growth was explained largely by the rate of juvenile recruitment (r =0.90, pO.OOl, N=27, lambda = -0.73 + 3.93 recruitment), with fewer juveniles being recruited on a per capita basis at higher population sizes. 27 I detected a weak linear increase in spring temperature over the entire 43-yr period 1960-2002 (r2=0.19, p=0.003, N=43, DDW = 15962.44 - 6.78 Year). In contrast, I found no evidence of a directional change in lay date over the course of the study (r2<0.001, N=31, p=0.61; lay date = 170.83 - 0.03 year), despite finding close correlations between spring temperature and median lay date. However, because I lacked data for the 11-year period 1964-1974, the analyses above are not directly comparable. Therefore, I cannot rule out the possibility that an advance would have been detected if data for the entire 43-yr period had been available. Discussion The El Nino Southern Oscillation has long been known to influence ecological processes in marine systems (Barber and Chavez 1983, Schreiber and Schreiber 1984), but its effects on terrestrial systems have been described only recently. One such effect concerns the influence of ENSO on reproductive success in land birds via its effect on rainfall (Curry and Grant 1989, Lindsey et al. 1997, Grant et al. 2000, Sillett et al. 2000). On Mandarte Island, ENSO had little effect on rainfall but a substantial effect on temperature. The period January through April was warmer in El Nino years and cooler in La Nina years, leading to a strong overall relationship between the Southern Oscillation Index and timing of breeding by song sparrows. I suggest that song sparrows responded to variation in the Southern Oscillation Index via one or both of two mechanisms. First, prior work on the Mandarte population has shown that the timing of breeding can be advanced with the provision of supplemental food (Arcese and Smith 1988). Thus, earlier breeding in El Nino years may be related to advances in plant and insect phenology, and consequent increases in food for breeding females. Second, warmer temperatures may also influence the energy budgets of females directly, allowing them to allocate less energy to thermoregulation and more to reproduction. The experimental insulation of nest boxes also advanced breeding in great tits (Parus major), probably by facilitating the allocation of energy to 28 reproduction (O'Connor 1978, Dhondt and Eyckerman 1979). Breeding earlier allowed female song sparrows to complete more successful nesting attempts and achieve higher reproductive output overall. Despite observing an increase in reproductive output in association with early breeding, I was unable to detect effects of timing of breeding on population growth. The lack of a correlation between breeding date and population growth may have resulted from a number of factors but especially because density-dependent variation in the rate of juvenile recruitment acts strongly to regulate population growth (Arcese et al. 1992). Recruitment declined with increasing density but showed little relation to climate. Thus, earlier breeding and enhanced reproductive output in El Nino years may act to accelerate population growth at low densities, but as population size increases, density-dependent declines in recruitment may over-ride the effects of breeding early. Recruitment may also be influenced by predation. If predation varies among years and operates independently of climate, then this could further reduce any association between timing of breeding and population growth. Finally, it is also possible that high reproductive output in association with earlier breeding simply translated into higher rates of emigration by juveniles from Mandarte Island. Annual reproductive output on Mandarte generally exceeds that on other small islands nearby. If emigration is higher in years with early breeding at high population size, it becomes plausible that earlier breeding on Mandarte Island acts indirectly to stabilize the regional metapopulation (Smith et al. 1996). I did not detect a long-term trend toward earlier breeding by song sparrows over the period 1960-2002, despite evidence that January through April temperatures have increased over the same period. This result contrasts with those from two other studies of North American birds that report 9 to 10 day advances in breeding dates over a similar period (Brown et al. 1999, Dunn and Winkler 1999). The inability to detect an advance in the timing of breeding by song 29 sparrows may be due to the erratic behavior of ENSO. Although spring temperature has increased, substantial annual variation in temperature and timing of breeding exists, and longer-term data may therefore be required to demonstrate a statistically reliable trend towards earlier breeding in this population. In conclusion, I found that annual shifts in climate influenced timing of breeding, which in turn enhanced reproductive output of individual female song sparrows in our study population. However, because climate had little effect on juvenile recruitment, the key factor regulating population size on Mandarte Island, annual variation in climate had no detectable effect on population growth. In contrast, Saether et al. (2000) found that climate influenced the amount of winter habitat available to dippers (Cinculus cinculus), their rate of recruitment to populations, and their rate of population growth. Similar effects of climate on influential demographic rates have been observed in populations of swallows (Moller 2002) and warblers (Sillett et al. 2002). Overall, these results suggest that populations will vary in their response to climate change depending in part on the strength and mechanism of population regulation. 30 30 1960 1965 1970 1975 1980 1985 1990 1995 2000 Year Figure 2-1. Mean annual January through April values of the El Nino Southern Oscillation index during the study period. Low negative values indicate El Nino conditions, while high positive values indicate La Nina conditions. 31 Figure 2-2. Influence of the El Nino Southern Oscillation on spring temperature in southwest British Columbia (r2=0.46, pO.OOl, N=31, regression: DDW = 2498.85 + 12.73 SOI). Higher values of degree-days of warming indicate colder spring temperatures. Figure 2-3. Influence of the El Nino Southern Oscillation on annual median date of first egg laid in song sparrows (r2=0.40, p<0.001, N=31, lay date = 107.39 + 0.57 SOI). 32 8 © 7 CO E £ 6 CD Q . CO D ) _C "5> X I 0) LL c CO CD -1 • 1 1 1 --• - -\ • •• • X • • \ • • • • * \ • • \ ^ - • • • 1 i i T 80 90 100 110 120 Median Date of First Egg 130 Figure 2-4. Effect of median date of first egg (Julian days) in year Xt on annual mean number of fledglings produced per female in year Xt (r2=0.42, pO.OOl, N=31, mean fledglings = 13.75 -0.09 lay date). 3 2 ro XI E 0 1 • 1 1 1 • - • -• • • • • • • • • • - • • • • * • * • 1 1 1 • 1 80 90 100 110 120 130 Median Date of First Egg (Xt) Figure 2-5. Effect of median date of first egg (Julian days) in year Xt on change in population size (lambda) between years Xt and Xt+1 (r2=0.02, p=0.61, N=29, lambda=1.95 - 0.007 lay date). 33 CHAPTER 3 Effects of Nest Predation, Brood Parasitism and Climate on Reproductive Variation in a Song Sparrow Metapopulation. Abstract. Several studies have examined how climate promotes synchrony in the fluctuations of adjacent populations, but few have examined whether climate synchronizes demographic rates. In this study, I examined spatial and temporal variation in timing of breeding and reproductive output in a song sparrow (Melospiza melodia) metapopulation, to determine the relative roles of local versus regional factors on variation in reproductive output among populations. The onset of egg laying in spring varied considerably among years but was more or less synchronous among islands within years. Despite synchrony in the onset of breeding, populations showed considerable asynchrony in reproductive output. Differences in reproductive output among populations was largely the result of spatial variation in nest predation and brood parasitism. In general populations on small, isolated islands had low levels of nest predation and brood parasitism and high reproductive rates, while populations on islands closer to Vancouver Island had higher levels of nest predation and brood parasitism, and lower reproductive rates. Thus, in this system, the regional influence of climate on reproduction was largely over-ridden by other ecological factors that varied among populations. 3.1 Introduction The degree of spatial synchrony among populations has important consequences for the persistence of metapopulations (Harrison and Quinn 1989, Foley 1997). If neighboring populations fluctuate asynchronously, the movement of individuals between populations should stabilize the overall metapopulation (Levins 1969). In contrast, if fluctuations are synchronous, there is an increased likelihood that all populations will decline simultaneously, thereby lowering 34 the persistence time of the metapopulation (Harrison and Quinn 1989, Heino et al. 1997). Examples of each situation have recently been described in nature. For example, populations of butterflies in the United Kingdom (Thomas and Hanski 1997), amphibians in Sweden (Sjogren Gulve 1994) and white-tailed ptarmigan (Lagopus leucurus) in Colorado (Martin et al. 2000) each displayed asynchronous fluctuations over time and a spatial dynamic similar to the classical Levins metapopulation (1969). In contrast, many studies on a wide range of taxa including moths and butterflies (Hanski and Woiwod 1993, Sutcliffe et al. 1996), spruce budworm {Choristoneura fumiferana, Williams and Liebhold 2000), Pacific salmon (Beamish et al. 1999), grouse (Ranta et al. 1995, Lindstrom et al. 1996), snowshoe hare (Lepus americanus, Sinclair et al. 1993) and reindeer (Rangifer tarandus platyrhynchus, Aanes et al. 2003) have found evidence of spatial synchrony in population trends. A key factor thought to promote synchrony in metapopulations is climate (Hanski 1991, Ranta et al. 1997), which can entrain the dynamics of adjacent populations that share a similar density dependent structure (Moran 1953). Although several studies have examined the effect of climate on synchrony in population trends, (e.g. Ranta et al. 1995, Post and Forchhammer 2002, Aanes et al. 2003) fewer have estimated the influence of climate on spatial synchrony in demographic rates, such as reproduction and survival. Weatherhead et al. (2002) found that climate promoted synchrony in annual survival and recruitment in two populations of black rat snakes (Elaphe o.obsoletd) in Eastern Canada. In southeast Asia, green sea turtles (Chelonia mydas) at four rookeries displayed synchrony in egg production that was related to annual variation in the El Nino Southern Oscillation (Chaloupka 2001). These studies provide support for the notion that spatial correlation in climate tends to synchronize the dynamics of adjacent populations in nature through effects on reproduction and survival. 35 In contrast to the above results however, Ringsby et al. (2002) found that climate promoted asynchrony in reproductive output in a population of house sparrows (Passer domesticus) in Norway. Ringsby et al. concluded that this asynchrony occurred because timing of breeding varied across populations and that led to nestlings of the same age experiencing different weather conditions. Even where populations do experience a common climatic influence on reproduction or survival, asynchrony in these traits may still arise if local factors that vary among populations over-ride the regional effect of climate. Such factors may include variation in food abundance, number of competitors, predator abundance, population density and age structure (Martin 1995, Newton 1998, Wilson and Arcese 2003). Therefore, the degree of demographic synchrony among populations should depend on the relative influence of local versus regional factors on reproduction or survival. Here, I examine spatial and temporal variation in reproductive rates of six song sparrow (Melospiza melodia) populations in the Southern Gulf Islands of British Columbia, Canada. In chapter 2,1 showed that a large-scale climatic phenomenon, the El Nino Southern Oscillation exerted marked influence on annual variation in the timing of breeding and reproductive output on one of these populations on Mandarte Island (Wilson and Arcese 2003). Because all of the populations examined in this study are located within 8 km of each other, I assumed that they experienced this temporal variation in climate similarly. Thus, based on this assumption and the earlier work on Mandarte Island, I expected to observe synchrony in reproductive output among populations if climate was the dominant factor affecting reproduction across all populations. In contrast to my prediction above, however, previous studies have also shown that nest predation and brood parasitism can have marked deleterious effects on reproductive rates of song sparrows on Mandarte Island (Arcese et al. 1992, 1996, Smith and Arcese 1994, Arcese and Smith 1999) and elsewhere in the region (Rogers et al. 1997, Smith et al. 2002, Zanette et al. 36 2003). If the frequency and intensity of nest predation and brood parasitism varies among populations, asynchrony in reproductive rate may exist. To test these predictions further, I first examined the extent of spatial and temporal variation in timing of breeding and reproductive rate across our study populations. I then examined how climate and local factors each affect the degree of reproductive synchrony in these same populations. 3.2 Materials and Methods The study was conducted from 1998 to 2003 on seven small islands (Mandarte, Reay, Rubly, Dock 1, Dock 2, Ker, Imrie) located in the Southern Gulf Islands of British Columbia, Canada (48°N, 123°W, Fig. 3-1). The Southern Gulf Islands are comprised of >300 islands of varying size located between Vancouver Island and the Strait of Georgia. The islands in this study are separated from one another by one to eight km. The two Dock Islands are separated by approximately 75-m of inter-tidal zone that is exposed at very low tides. Thus, I treated Dock 1 and 2 as a single island here. The study islands vary in size from 0.05 km2 (Imrie) to 0.6 km2 (Mandarte). Habitat on four of the islands (Mandarte, Docks, Reay and Imrie) consists of patches of shrubs interspersed with open grassy areas. On the other two islands (Ker, Rubly), the habitat consists of a Douglas fir (Pseudotsuga menziesii) dominated canopy, with shrubs and open meadows in the understory and along the edges. On all islands, common shrub species were snowberry (Symphoricarpos albus), nootka rose {Rosa nutkana), saskatoon (Amelanchier alnifolia), ocean spray (Holodiscus discolor) and blackberry (Rubus discolor, R. ursinus). Song sparrows on all islands were individually marked with color bands and a numbered aluminum band, and breeding population sizes were estimated as the number of territorial adults alive in the last two weeks of April each year (Table 1). Fieldwork was conducted throughout the breeding season from about the end of March to mid-August each year. All territories were mapped and then monitored every 3-5 days to record the behavior of individual females and 37 locate nests. Each nest was monitored to estimate the date of first egg laid, the clutch size and the number of young fledged. The date of first egg laid by each female annually was estimated by observing nests during the laying stage or back dating from the day of hatching. I then used the mean date of first egg for all first nests as our estimate of the annual timing of breeding for the population. Clutch sizes were estimated as the maximum number of eggs laid in nests that were observed during the incubation or nestling stages and did not contain eggs or nestlings of brown-headed cowbirds (Molothrus ater). Cowbirds routinely remove host eggs prior to laying their own (Arcese and Smith 1999, Smith et al. 2000). Thus, clutch size estimates for nests found with cowbird eggs are likely underestimated and were not included in analyses here. The number of cowbird eggs was also recorded for all parasitized song sparrow nests. When young were 5-8 days old, we visited the nest and banded nestlings. The nest was re-visited at day 10-15 to determine if the nest was successful (> 1 sparrow fledged) and estimate the number of young fledged. Nests were considered successful if they were empty and intact post-fledging, and the parents were alarmed and observed feeding young. During the first few days after fledging, the young are secretive and difficult to observe. Therefore, the number of young fledged was estimated based on the number heard begging and the number of locations to which parents delivered food. On all subsequent visits to the territory we recorded observations of banded young in the event that some were missed during the fledge check. However, it remains possible that some young were not detected on the initial fledge check and died before subsequent territory visits. As a result, the number of nests producing fledglings and nestlings leaving the nest successfully may have been underestimated in some cases. I assigned failed nests into four categories: (1) Nest predation was assumed to have occurred when all eggs or young were removed from the nest, no young were detected post-fledging and parents were not observed to carry or deliver food. Potential nest predators in our 38 study area include the northwestern crow (Corvus caurinus), Cooper's hawk (Accipiter cooperi), brown-headed cowbird, mink (Mustela visori), deer mice (Peromyscus maniculatus), and northwestern garter snake (Thamnophis ordinoides), (2) Brood parasitism was indicated as a cause of failure when the presence of cowbird young was associated with the disappearance of all sparrow young or the failure of all sparrow eggs to hatch, and where active nests were abandoned by the female sparrow after cowbirds removed sparrow eggs and replaced them with their own. A nest was judged to be deserted when the eggs were cold and the female was absent from the nest on two checks > 5 days apart, (3) Other causes of failure included: the effects of severe weather and starvation (young dead in nest without sign of injury), nest abandonment in the absence of egg loss and cases of infertility (eggs failed to hatch despite incubation >15 days, 4) Unknown causes of failure were recorded when I was unable to assign a source of failure to one of the three categories described above. I excluded nests from this analysis if they were found while being built but not finished or if they failed due to human disturbance (n=4). General linear models were used to test for spatial and temporal variation in laying date and clutch size among islands. I then used Tukey's test for post-hoc comparisons to compare means among classes. Date of first egg and clutch size were approximately normally distributed. Other variables including the number of failed nesting attempts per female, number of depredated nesting attempts per female, number of cowbird eggs laid per female song sparrow and the number of young fledged per female followed a Poisson distribution. To examine spatial and temporal variation in these variables, I used a generalized linear model specifying a Poisson distribution and controlling for the total number of nesting attempts. I also used general linear models to estimate the influence of various factors on the annual reproductive rate observed on individual islands (mean number of young fledged per female). Variables in this analysis included nest predation rate, mean number of cowbird eggs per song 39 sparrow nest, mean clutch size and mean laying date. Mean clutch size and laying date were approximately normally distributed, however nest predation rate and mean number of cowbird eggs per nest were right skewed. Thus, I applied a square-root transformation to these two variables before analysis. Because the sample size of nests differed markedly among islands and years, I weighted each estimate for islands in a year by the square root of the number of nests observed (following Arcese et al. 1992). It is also possible that average age and level of inbreeding in a particular population may contribute to variation in reproductive output (Nol and Smith 1987, Martin 1995, Keller 1998). However, other than Mandarte Island, there are no measures of inbreeding for the populations and the age of birds present on the islands at the start of the study in 1998 are unknown. Therefore, I was unable to test for any differences in these variables among islands, or ask how they may contribute to reproductive output in this study. 3.3 Results Variation in population size Table 3-1 shows the number of breeding females on all islands over the six-year study period. Population sizes on Reay and Rubly Islands were relatively constant over the period and typically averaged between 2 and 5 breeding females. On Mandarte, Ker and the Dock Islands, populations dropped during the first two years. By the end of the study, those numbers had remained low on the Dock Islands, but had recovered on Mandarte and Ker Island. Imrie Island was extinct during the first three years of the study but was colonized in 2001 and has grown by one pair per year since. Spatial and temporal variation in timing of breeding and reproductive output The mean date of first egg for all females combined was earliest in 1998 (April 9) and latest in 1999 (May 1), and differed significantly among years ( F 5 2 1 5 = 25.76, p<0.001). Mean date of first egg also varied among islands 215 = 3.87, p=0.002), mainly because breeding on Imrie 40 Island averaged about 13 days earlier than on the other five islands (Fig. 3-2). I found no significant differences in timing of breeding among the other five islands. In general, all islands showed concordant variation in breeding date between years (Fig. 3-2). The number of fledglings produced per female differed significantly among islands (X2=57.94, df=5, pO.OOl, n=238, Fig. 3-3), but only marginally among years (x2=10.89, df=5, p=0.053, n=238). When averaged across all years, Imrie and Reay Islands had the highest annual reproductive output (8.17 + 0.60 (1SE) and 5.73 + 0.83 fledglings per female, respectively), Mandarte and Rubly Islands had intermediate levels of annual reproductive output (4.01 + 0.20 and 4.00 + 0.58 fledglings per female, respectively) and Ker and the Dock Islands had the lowest annual reproductive output (2.51 + 0.46 and 2.82 + 0.34 fledglings per female, respectively). Factors affecting reproductive output Because annual reproductive output varied among islands, despite a similar onset of breeding, 1 next asked what other factors influenced reproductive output. To do so, I entered four variables (mean clutch size, mean date of first egg, proportion of nesting attempts depredated and the mean number of cowbird eggs laid per song sparrow nest) into a GLM to estimate the influence of each on the annual mean number of fledglings produced per female. Together, these four variables accounted for 71% of the annual variance in reproductive output across islands and years (Table 3-2). Nest predation and brood parasitism had the greatest effects followed by clutch size and date of first egg. Thus, reproductive output on islands was higher when fewer nests were depredated, fewer cowbird eggs were laid, clutch sizes were larger and breeding began earlier. Including 'Island' as a blocking variable only increased the total variance explained by 1.1% (r2=0.721). The variable'Island' was also not a significant predictor of reproductive output in the overall model (F5] 23 = 1.07, p=0.40). Thus, after accounting for nest predation, brood parasitism, 41 timing of breeding and clutch size, other differences between islands contributed little to variation in reproductive output by female song sparrows in our study area. Spatial and temporal variation in clutch size, nest predation and brood parasitism Mean clutch size differed significantly among islands (Fs, 431 = 5.77, p<0.001, Table 3-3) and years (F5,431 = 4.50, p=0.001). Post-hoc comparisons revealed that this variation in clutch size among islands resulted mainly because clutches on Mandarte Island were smaller than those recorded on Reay and Rubly Islands (p<0.05 in each case, other pairwise comparisons not significant). There were no significant differences in clutch size among the five islands excluding Mandarte. In all cases where nests failed, I tried to assign the source of failure. Across all islands and years, 214 of 569 nesting attempts failed (37.6%, Table 3-3). Overall, nest predation was the most common source of failure (53.7%), followed by brood parasitism (21.0%) and other known failure (17.7%). I was unable to assign a source of failure for 16 (7.5%) nests. The number of failed nesting attempts per female varied among islands (x =16.07, df=5, p=0.007, n=238, Table 3-3), but not among years (x2=4.39, df=5, p=0.49, n=238, Table 3-3). The number of nests depredated also varied among islands (x2=12.18, df=5, p<0.05, n=238, Table 3-3), but not among years (x2=4.94, df=5, p=0.42, n=238). Imrie, Rubly and Reay Islands experienced rates of nest predation that averaged one half to one fifth lower than those experienced by females breeding on Mandarte, Ker and the Dock Islands (Table 3-3). The number of cowbird eggs laid per female song sparrow also differed strongly among islands (x2=66.60, df=5, pO.OOl, n=238, Table 3-3) and years (x2=29.06, df=5, pO.OOl, n=238, Table 3-3). Imrie and Mandarte Island experienced the lowest levels of brood parasitism, while Ker and the Dock Islands experienced the highest (Table 3-3). Among years, the highest number of cowbird eggs per female was observed in 2000 and 2001, and the lowest in 1998. 42 3.4 Discussion Synchrony in timing of breeding I found that timing of breeding varied considerably among years, but was more or less synchronous among islands within years. In chapter two it was shown that annual variation in timing of breeding on Mandarte Island was linked to the El Nino Southern Oscillation and its effect on winter and spring temperatures (Wilson & Arcese 2003). Many other studies have also shown that temperature is a key factor affecting timing of breeding in temperate zone passerines (Lack 1966, Slagsvold 1976, Forchhammer et al. 1998, Brown et al. 1999). Thus, because timing of breeding on Mandarte Island is determined primarily by spring temperature and timing of breeding on the other islands fluctuates synchronously with Mandarte, it is likely that annual variation in temperature also drives annual variation in timing of breeding across all islands. Temperature might affect laying dates via two mechanisms. First, warmer temperatures may result in greater food availability for females, resulting in earlier laying. In support of this hypothesis, two previous studies on Mandarte Island found that breeding dates could be artificially advanced with the provision of supplemental food prior to the breeding season (Smith et al. 1980, Arcese and Smith 1988). Other experimental studies of European starlings (Sturnis vulgaris ; Kallander and Karlsson 1993), magpies (Pica pica; Hochachka & Boag 1987) and blue tits (Parus caeruleus; Nilsson and Svensson 1993) reported similar results. Second, warmer temperatures may influence females directly by lowering the energetic costs of thermoregulation (Weathers & Sullivan 1993). Females that expend less energy to maintain a constant body temperature may be able to invest more in reproduction and breed earlier. Synchrony in the timing of breeding of female birds in a local area has been observed in a variety of species (e.g. geese, Findlay & Cooke 1982; blackbirds, Westneat 1992; grouse, Keppie 2000). This synchrony is suggested to be the result of females responding to the same 43 environmental cue, although other ecological and sociobiological factors may also play a role (see Ims 1990). However, synchrony in the timing of breeding of females in discrete areas is less commonly reported. In such studies, females breeding in separate populations (Ringsby et al. 2002) or habitats (Brawn 1991, Jarvinen 1993) can vary in timing of breeding, perhaps because either habitat quality or the interaction between climate and habitat varies among sites. In our study, islands were separated by only 1 to 8 km and had similar breeding habitat. Because the ENSO operates at regional scales (Allan et al. 1996), the effects of annual variation in climate were probably experienced equally by females breeding on adjacent islands. Local versus regional effects on reproductive output A number of studies have found that large-scale effects of climate can entrain the dynamics of spatially separate populations. Post and Forchhammer (2002) found that the population dynamics of caribou (Rangifer tarandus) and musk oxen (Ovibos moschatus) in separate areas of Greenland showed spatial synchrony related to variation in the North Atlantic Oscillation (NAO) index. The authors proposed that synchrony may have been caused by effects of the NAO on juvenile survival or offspring production among populations of both species. Early age classes of brown trout (Salmo truttd) in streams throughout France also show population synchrony due to common climatic effects on stream conditions during emergence (Cattaneo et al. 2003). In birds, climate is known to influence a number of reproductive traits including timing of breeding (Forchhammer et al. 1998, Wilson and Arcese 2003), clutch size (Sanz 1995), nestling survival (Ringsby et al. 2002) and fledging success (Solonen 2001). Therefore, when adjacent populations experience a common climate that affects reproduction, we might expect that they will display synchrony in rates of reproductive output. In our study, the annual onset of egg-laying was very similar across populations. Despite this, the populations differed markedly in annual reproductive rates within years. This variation 44 in breeding success appears to have been driven in large part by differences in the frequency and intensity of nest predation and brood parasitism among islands. In general, the smaller, more isolated populations (e.g. Reay and Imrie Islands; Fig. 3-1) experienced low rates of nest predation and brood parasitism, and high reproductive rates. In contrast, islands closer to Vancouver Island (e.g., Ker and the Dock Islands) experienced higher rates of nest predation and brood parasitism, and lower reproductive rates. A number of previous studies in this area have also shown that nest predation and brood parasitism can have marked effects on breeding success of song sparrows (Arcese et al. 1992, 1996, Smith and Arcese 1994, Arcese and Smith 1999, Zanette et al. 2003, Arcese & Marr 2003). Furthermore, the type of spatial variation in nest predation and brood parasitism observed in this study appears to be common throughout the region. For instance, in an earlier study on some of these same populations, Smith and Myers-Smith (1998) also found that rates of brood parasitism among populations declined with increasing distance from Vancouver Island (Smith and Myers-Smith 1998, see also Arcese and Smith 1999). The higher activity of cowbirds on the near-shore islands may be because these islands are more accessible for individuals that must commute from foraging areas located on Vancouver Island (Smith and Myers-Smith 1998). Variation in breeding success also occurs between our study islands and populations in nearby mainland areas, which experience higher rates of nest predation and brood parasitism, and consequently, lower reproductive output per female (Rogers et al. 1997, Smith et al. 1996, Smith et al. 2002, Smith et al. 2003). Studies in other geographic areas have also found that the extent of nest predation and brood parasitism can vary over small spatial scales in relation to habitat type (Seitz and Zegers 1995, Chalfoun et al. 2002) and the size and location of habitat patches in fragmented landscapes (Moller 1991, Burke and Nol 2000, Robinson et al. 2000). 45 Previous studies on Mandarte Island have found that both age and inbreeding influence reproduction in song sparrows (Nol and Smith 1987, Keller 1998). Therefore, we might expect some variation in reproductive output per female if age structure or degree of inbreeding varies among populations. I was unable to measure variation in these traits or their effects in this study. However, after accounting for nest predation, brood parasitism, mean clutch size and mean date of first egg, the variable "Island" only explained a small fraction of variance in reproductive output. This may suggest that age or level of inbreeding do not vary among populations or that they have relatively little influence on reproductive output compared with the effects of nest predation and brood parasitism. However, it also remains possible that the variation in clutch size among populations may have been a consequence of differences in age structure or degree of inbreeding. Variation in demography among populations may also arise due to differences in habitat quality. For example, Broome (2001) found that, despite sharing a common regional climate, five local populations of pygmy-possums (Burramys parvus) displayed asynchronous demography as a consequence of differences in habitat structure and quality, which likely influenced food abundance. Similar results have been reported in American pikas (Ochotona princes; Kreuzer and Huntly 2003) and black-throated blue warblers (Dendroica caerulescens; Holmes etal. 1996). Overall, our results suggest that the populations in our study follow a source-sink structure with small, isolated populations acting as sources and larger populations closer to Vancouver Island acting as sinks. However, the degree to which this may be the case also depends on two aspects. First, it depends on whether dispersal from productive populations actually helps augment less productive populations. The movement of juveniles among islands in this system is fairly common (Wilson and Arcese, unpublished data). However, further study is 46 required to determine what aspects of individuals and populations influence patterns of dispersal and recruitment. Second, the degree to which our observations of reproductive asynchrony translate into population dynamics also depends on the relative influence of juvenile recruitment and adult survival on population growth. In chapter 2,1 found that population growth was determined primarily by density-dependent effects on juvenile recruitment, rather than by factors that influence reproductive output. If density-dependent effects on recruitment and population growth are experienced similarly across all populations, then differences in reproductive rates among islands may have very little influence on the stability of the individual populations. Further study that compared differences in juvenile recruitment and adult survival across all islands would be valuable to determine if this is the case. 47 Figure 3-1. The study area in the Southern Gulf Islands of British Columbia, Canada showing the study region near Vancouver Island (left map), and the islands included in this study (in black, right map). 4 8 145 135 o) 125 cn L U I 115 I 105 Q c CO CD 95 85 75 1998 1999 2000 2001 YEAR 2002 2003 D O C K S KER MANDARTE REAY IMRIE RUBLY Figure 3-2. Spatial and temporal variation in date of first egg for song sparrows on six islands. Error bars represent + 1 SE. ro E CD 0) CL GO CO g T> <D 1998 1999 2000 2001 YEAR 2002 2003 DOCKS KER MANDARTE REAY IMRIE RUBLY Figure 3-3. Spatial and temporal variation in mean number of young fledged per female song sparrows on six islands. 4 9 Table 3-1. Annual variation in number of breeding female song sparrows on the six islands in this study (see Fig. 3-1 for island locations). Island 1998 1999 2000 2001 2002 2003 Mandarte 30 11 14 10 25 26 Dock 13 12 7 5 4 5 Reay 3 2 2 3 3 2 Rubly 4 4 3 5 2 4 Ker 6 4 3 7 8 7 Imrie 0 0 0 1 2 3 Table 3-2. General Linear Model results showing factors that influence annual reproductive output (N=33, r = 0.84, r2 = 0.71). Factor b + SE statistic P Nest Predation Rate -4.83 ±1.27 -3.82 0.001 Cowbird eggs per song sparrow nest -2.34 ± 0.64 -3.67 0.001 Mean clutch size 2.13 ±0.88 2.42 0.022 Mean date of first egg -0.05 ± 0.03 -1.94 0.062 50 T3 3 CD . f l -k^ 00 a ' § -d CO -a .2 'co O X 5 o Cd CD fl O CD N co X 3 CD H—» J3 "o u CD a -A CO CD C k n CD OH CO 00 00 CD I O CD CkH O kH CD - O a A A U CD a CD S3 CO CD fl fe O CO CD OH p cn cn CD fe O CD _ N 'co CD ! 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O N o o +1 T f C N OO o ©' +1 00 o o +1 T f VO © ' 00 o o +1 r-in ©' co CD a k H CD OH CO 00 00 CD X> O O oo o VO cn © + 1 T f VO T f o ©' +1 C N cn O N o ©' +1 O N cn cn oo p © ' +1 cn T f cn CD _ N 'co -a CD "o S CD L I T E R A T U R E C I T E D Aanes, R., B-E Saether, E. J. Solberg, S. Aanes, 0. Strand, and N.A. 0ritsland. 2003. Synchrony in Svalbard reindeer population dynamics. Canadian Journal of Zoology 81: 103-110. Allan, R., J. Lindesay, and D. Parker. 1996. El Nino Southern Oscillation and climate variability. CSIRO, Collingwood, Australia. Arcese, P., and J.N.M. Smith. 1988. Effects of population density and supplemental food on reproduction in song sparrows. Journal of Animal Ecology 57: 119-136. Arcese, P. 1989. Intrasexual competition, mating system and natal dispersal in the song sparrow. Animal Behaviour 38: 958-979. Arcese, P., J.N.M. Smith, W.M. Hochachka, C M . Rogers, and D. Ludwig. 1992. 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