"Science, Faculty of"@en . "Zoology, Department of"@en . "DSpace"@en . "UBCV"@en . "Gilchrist, H. Grant"@en . "2009-04-17T22:24:46Z"@en . "1995"@en . "Doctor of Philosophy - PhD"@en . "University of British Columbia"@en . "The glaucous gull (Larus hyperboreus) is a generalist predator with a circumpolar distribution. It\r\ncommonly depredates the eggs and chicks of birds nesting in the Arctic, and often breeds in\r\nassociation with colonial nesting waterfowl and seabirds. The aim of this thesis was to examine\r\nhow environmental factors constrained the ability of glaucous gulls to depredate cliff-nesting\r\nthick-billed murres (Uria lomvia), and to determine whether these constraints vary through time,\r\nin space, or between individuals. I also wanted to determine how changes in gull foraging\r\nconstraints could affect the impact of gulls on murre reproductive success and population\r\ndynamics. This study was conducted from 1990-1992 at a murre colony on Coats Island,\r\nCanada, and in 1993 at colonies in the Upernavik region of Greenland.\r\nBased upon results from egg placement experiments and field observations of predation,\r\ngull foraging success was constrained by high murre nesting densities, collective murre defence,\r\nand the accessibility of narrow cliff ledges. However, windy conditions enhanced the ability of\r\ngulls to overcome these constraints. Wind improved the aerial maneuverability of gulls, and\r\nenabled gulls to reach weakly-defended narrow ledges and avoid contact with murres during\r\nattack. Murre defence on narrow cliff ledges was less effective because murres had difficulty\r\nturning to face attacking gulls without dislodging their own eggs and chicks. Gull search\r\nactivity, attack activity, and predation rates were strongly correlated with windy conditions.\r\nConsequently, the impact of gull predation on murre reproductive failure, which ranged from 0%\r\nto 21% depending on nest type, was determined largely by wind.\r\nUnder calm wind conditions, adult gulls were often inactive and they rarely fed their\r\nchicks. This inactivity may reflect the reluctance of gulls to forage on foot, because although this\r\nwas the most successful attack mode, it also incurred the greatest contact with defending murres.\r\nAlternatively, gulls could have been responding to the varying energetic demands determined by\r\nchanging weather conditions, so that the danger of injury while foraging did not influence mode\r\nselection or periods of foraging activity. I explored these two alternatives using a dynamic\r\nsimulation model which integrated field data and energetic estimates of gull foraging behaviour.\r\nThe model suggested that the first explanation, which was based upon energy considerations\r\nalone, was not sufficient to explain gull foraging inactivity under calm wind conditions.\r\nHowever, the model supported the idea that gulls were sensitive to risk of injury, and that they\r\nshould select low-danger foraging modes that provide low energetic gains. The model also\r\nrevealed that gulls can afford to use foraging modes that yield low energetic gains relative to\r\nmore productive and dangerous ones (e.g. scavenging vs. foraging on foot under calm\r\nconditions), because even poor foraging modes are sufficient to meet their energetic demands\r\nunder most circumstances.\r\nI predicted that a decline in the density of nesting murres should enhance the ability of gulls\r\nto overcome the constraints of calm wind conditions, cliff ledge accessibility, and prey defence.\r\nThus, murre colony declines should increase the impact of gull predation on murre reproduction.\r\nTo examine the impact of gull predation at declining thick-billed murre colonies, I compared gull\r\nforaging mode selection, predation rates, and murre nest site selection at Coats Island, N.W.T.,\r\nwith that at declining murre colonies found near Upernavik Greenland. I found that gulls at\r\ndeclining colonies foraged on broad cliff ledges and were less constrained by calm wind\r\nconditions, apparently because population declines increased the availability of low nesting\r\ndensity ledges where gulls could maneuver on foot and attack murres with little risk of injury.\r\nPerhaps because of this, predation rates at declining murre colonies were consistently higher than\r\nat the stable Coats colony, particularly at low wind speeds."@en . "https://circle.library.ubc.ca/rest/handle/2429/7356?expand=metadata"@en . "2933157 bytes"@en . "application/pdf"@en . "THE FORAGING ECOLOGY OF GLAUCOUS GULLS PREYING ONTHE EGGS AND CHICKS OF THICK-BILLED MURRESbyH. GRANT GILCHRISTHonors B.Sc., Trent University, 1990A THESIS SUBMHthD IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFDOCTOR OF PHILOSOPHYinTHE FACULTY OF GRADUATE STUDIESDEPARTMENT OF ZOOLOGYWe accept this thesis as conformingto the required standardNIVERSITY OF BRITISH COLUMBIASeptember 1995\u00C2\u00A9 H. Grant Gilchrist, 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, CanadaDate \u00C2\u00B0DE-6 (2/88)ABSTRACTThe glaucous gull (Larus hyperboreus) is a generalist predator with a circumpolar distribution. Itcommonly depredates the eggs and chicks of birds nesting in the Arctic, and often breeds inassociation with colonial nesting waterfowl and seabirds. The aim of this thesis was to examinehow environmental factors constrained the ability of glaucous gulls to depredate cliff-nestingthick-billed murres (Uria lomvia), and to determine whether these constraints vary through time,in space, or between individuals. I also wanted to determine how changes in gull foragingconstraints could affect the impact of gulls on murre reproductive success and populationdynamics. This study was conducted from 1990-1992 at a murre colony on Coats Island,Canada, and in 1993 at colonies in the Upernavik region of Greenland.Based upon results from egg placement experiments and field observations of predation,gull foraging success was constrained by high murre nesting densities, collective murre defence,and the accessibility of narrow cliff ledges. However, windy conditions enhanced the ability ofgulls to overcome these constraints. Wind improved the aerial maneuverability of gulls, andenabled gulls to reach weakly-defended narrow ledges and avoid contact with murres duringattack. Murre defence on narrow cliff ledges was less effective because murres had difficultyturning to face attacking gulls without dislodging their own eggs and chicks. Gull searchactivity, attack activity, and predation rates were strongly correlated with windy conditions.Consequently, the impact of gull predation on murre reproductive failure, which ranged from 0%to 21% depending on nest type, was determined largely by wind.Under calm wind conditions, adult gulls were often inactive and they rarely fed theirchicks. This inactivity may reflect the reluctance of gulls to forage on foot, because although thiswas the most successful attack mode, it also incurred the greatest contact with defending murres.Alternatively, gulls could have been responding to the varying energetic demands determined bychanging weather conditions, so that the danger of injury while foraging did not influence modeselection or periods of foraging activity. I explored these two alternatives using a dynamicIIsimulation model which integrated field data and energetic estimates of gull foraging behaviour.The model suggested that the first explanation, which was based upon energy considerationsalone, was not sufficient to explain gull foraging inactivity under calm wind conditions.However, the model supported the idea that gulls were sensitive to risk of injury, and that theyshould select low-danger foraging modes that provide low energetic gains. The model alsorevealed that gulls can afford to use foraging modes that yield low energetic gains relative tomore productive and dangerous ones (e.g. scavenging vs. foraging on foot under calmconditions), because even poor foraging modes are sufficient to meet their energetic demandsunder most circumstances.I predicted that a decline in the density of nesting murres should enhance the ability of gullsto overcome the constraints of calm wind conditions, cliff ledge accessibility, and prey defence.Thus, murre colony declines should increase the impact of gull predation on murre reproduction.To examine the impact of gull predation at declining thick-billed murre colonies, I compared gullforaging mode selection, predation rates, and murre nest site selection at Coats Island, N.W.T.,with that at declining murre colonies found near Upernavik Greenland. I found that gulls atdeclining colonies foraged on broad cliff ledges and were less constrained by calm windconditions, apparently because population declines increased the availability of low nestingdensity ledges where gulls could maneuver on foot and attack murres with little risk of injury.Perhaps because of this, predation rates at declining murre colonies were consistently higher thanat the stable Coats colony, particularly at low wind speeds.111TABLE OF CONTENTSAbstract iiTable of Contents ivList of Tables viiList of Figures viiiAcknowledgments xChapter 1: Introduction 1Predator foraging behaviour 1Study sites 3Ecology of the glaucous gull 4Glaucous gull ecology at Coats Island 5Aims of this thesis 9Chapter 2: Effects of murre nest density, cliff ledge width,and wind on predation by gulls 10Introduction 10Methods 11Study area and species 12Egg placement experiments 12Behavioural observations 13Statistical analysis 14Results 16Nesting density 16Ledge width 16Wind speed 20Vulnerability of eggs relative to timing of laying 22Discussion 26Constraints on gull foraging efficiency 26Consequences of predation for thick-billed murres 28Chapter 3: Wind and prey nest sites as foraging constraintson an avian predator, the glaucous gull 30Introduction 30Methods 31Study area and species 31Behavioural observations 32Gull gliding-flight dynamics 33Effects of gull predation on murre reproductive success 33Statistical analysis 34ivResults 36Murre colony structure and environment 36Gull search activity 36Gull attack rates 41Gull attack selectivity relative to murre nest site characteristics 41Gull predation rates 41Gull attack success 46Wind and gull maneuverability in flight 51Murre responses to gull attack 51Path diagram 53Colony-level effects of gull predation on seasonal murre reproduction 57Discussion 59Wind: a foraging constraint for glaucous gulls depredating murres 59The currency of glaucous gull foraging decisions 61Population level consequences of gull foraging behaviour for murres 63Conclusions 64Chapter 4: Foraging mode selection of glaucous gulls provisioningyoung: consequences of a danger-reward trade-offmediated by wind? 65Introduction 65Methods 67Study site and species interactions 67Field studies of gull foraging behaviour 67Dynamic optimization model 68Brood energy dynamics 69Model assumptions concerning provisioning 74Weather conditions 76Metabolic rate estimates 76Limits of chick fat stores 80Flight energetics 82Energy consumption 83Foraging mode energetic s and danger of contact with murres 84Modeling risk of fatal injury 89Results and Discussion 90Foraging mode selection with no risk of injury 90Foraging mode selection with risk of injury 90Comparisons with field data of gull foraging behaviour 94Further predictions 101Conclusions 102Chapter 5: Effects of glaucous gull predation at decliningthick-billed murre colonies: An inter-colony comparison 104Introduction 104Methods 105Study sites 105Gull foraging behaviour and predation rates 107Numerical response of glaucous gulls to murre colony declines 107VMurre nest site selection and breeding density 108Results 109Gull foraging mode selection, attack activity, and predation rates 109Numerical response of glaucous gulls to murre colony declines 112Murre nest site selection 115Discussion 117Gull foraging behaviour and predation rates 117Numerical response of glaucous gulls to murre colony declines 118Murre nest site distribution at declining colonies: a consequenceof gull predation? 119Population-level effects of gull foraging behaviour: multiple-stable states? 121Conclusions 123Appendix I 124Chapter 6: Conclusions 125Thesis synthesis 125Comparisons with other studies of gull foraging ecology 126From Larus to leo: identifying future research directions 132Literature cited 134viLIST OF TABLESpageTable 2.1 Description of attack modes of gulls and murre responses to attack. 15Table 2.2 Murre responses to gull attacks in relation to cliff ledge width. 19Table 2.3 Gull attack techniques in relation to cliff ledge width. 21Table 2.4 Murre response to gull attacks in relation to timing of murre egg laying. 25Table 3.1 General linear models of factors affecting rates of aerial search by gulls. 40Table 3.2 General linear models of factors affecting gull attack activity. 42Table 3.3 Analysis of covariance of factors affecting rates of gull attack inrelation to murre nest site characteristics. 43Table 3.4 General linear model of factors affecting gull predation rates. 47Table 3.5 Multiple logistic regression of factors affecting gull attack success. 49Table 3.6 a) General linear model of factors affecting murre response to gullattack, and b) multiple logistic regression of factors affecting theprobability that a gull was struck by murres during attack. 54Table 4.1 Probability of wind conditions in the future in relation to the windconditions in the current time interval 72Table 4.2 Estimates of Resting Metabolic Rates for a glaucous gull adult and chickin relation to weather conditions 81Table 4.3 Proportion of gull foraging time devoted to flight in relation to wind 85Table 4.4 Parameter estimates of glaucous gull foraging mode energetics and riskof injury in relation to wind conditions 87Table 6.1 A review of studies examining the foraging ecology of gulls 127viiLIST OF FIGURESpageFigure 2.1 Survival of exposed murre eggs in relation to thick-billed murrenesting densities. 17Figure 2.2 Survival of exposed murre eggs in relation to cliff ledge width. 18Figure 2.3 Survival of exposed murre eggs in relation to wind speed. 23Figure 2.4 Survival of exposed murre eggs in relation to timing of murre egg laying. 24Figure 3.1 Wind conditions in relation to date at Coats Island. 37Figure 3.2 Examples of glaucous gull aerial search activity in relation to windand time of day. 38Figure 3.3 Attack activity of glaucous gulls in relation to wind and murre nest sitecharacteristics. 44Figure 3.4 Glaucous gull predation rates in relation to date and year. 45Figure 3.5 Predation rates of murre eggs and chicks in relation to wind, murrenest site characteristics, and year. 48Figure 3.6 Glaucous gull attack success in relation to wind speed and year. 50Figure 3.7 a) Glaucous gull foraging patrol duration over murre breeding areas inrelation to wind, and b) gull attack-hover duration in relation to wind. 52Figure 3.8 Path analysis of the interactions between factors affecting gull predationrates and risk of contact with murres during attack. 55Figure 3.9 Proportion of murre eggs and chicks lost to gull predation in relation tomurre nest type 58Figure 4.1 Survival function of gull brood in relation to energy stores. 70Figure 4.2 Dynamic model predictions of gull foraging mode selection under calmconditions in relation to time, brood energy stores, and risk of injury. 91Figure 4.3 Dynamic model predictions of gull foraging mode selection under windyconditions in relation to time, brood energy stores, and risk of injury. 92Figure 4.4 Individual variation in attack mode selection in relation to wind conditions. 96Figure 4.5 Time budgets of aerial foraging specialists, and pedal foraging specialistsin relation to wind conditions. 99viiiFigure 5.1 Wind conditions at Coats Island and Kingittoq murre colonies. 106Figure 5.2 Attack mode selection of glaucous gulls in relation to wind conditionsand colony identity. 110Figure 5.3 Glaucous gull attack activity in relation to cliff ledge width, wind speedand colony identity. 111Figure 5.4 Gull predation rates in relation to wind speed and colony identity. 113Figure 5.5 The relationship between gull numbers and murre colony size 114Figure 5.6 Murre nesting characteristics in relation to the magnitude of declines atthree Arctic murre colonies. 116ixACKNOWLEDGMENTSThis study benefited greatly from the support and input of many people, and Iwould like to take this opportunity to thank them. Most notably, I thank my supervisorsJamie Smith and Tony Gaston who generously provided support and encouragementthroughout this project, and who contributed many ideas during the field work andwrite-up stages. Both Jamie and Tony were great supervisors, and I feel very fortunateto have studied with them. They struck a balance of involvement with this researchwhile at the same time allowing me to make key decisions relating to the work (for betteror worse). They also tempered my overly optimistic approach to field work withquestions like, \u00E2\u0080\u009C...so what will you do if that doesn\u00E2\u0080\u0099t work?\u00E2\u0080\u009D at opportune times (i.e.prior to plane departure). I would also like to thank Erica Nol for introducing me toboth Jamie and Tony. I also greatly appreciate the input of my other research committeemembers Tony Sinclair, Ron Ydenberg, and David Jones, and also Carl Walters. Theyencouraged me to integrate a sound understanding of an organism\u00E2\u0080\u0099s Natural Historywith theoretical ideas. I particularly thank Ron Ydenberg for our many conversationsand his substantial contributions to chapter 4.At the University of British Columbia, I was also fortunate to be surrounded by adynamic and friendly group in the \u00E2\u0080\u009CSmith Lab\u00E2\u0080\u009D consisting of Lance Barret-Lenard,Fiona Schmiegelow, Linda Dupuis, Dave Westcott, Wesley Hochachka, David Ward,Arnon Lotem, Alice Cassidy, Victoria Campbell, and Christine Adkins. I benefitedespecially from my discussions with David Ward, Arnon, and Wesley. Thanks also toChristine Adkins for helping me translate gull behaviour into Quick Basic.While on Coats Island, I was fortunate to share my summers and bear encounterswith Leah deForest, Garry Donaldson, David Andrews, David Noble, Don Crol, MarkHiphner, Marco Passeri, Paul Prior, and Thomas Alogut. The experiences we sharedon Coats have been the basis for many long-lasting friendships. I especially thank \u00E2\u0080\u009Cthegull guys\u00E2\u0080\u009D, Thomas Alogut, Marco Passeri and Paul Prior for their assistance andtireless commitment to this work. While in the Canadian arctic, I also thank BobLongworth and Lyne Paplinski of the Iqaluit Research Center for their logistical supportand Carribean weather reports over the radio. For the research conducted in Greenland,I thank the Greenland Home Rule Government, Henning Thing, and the community andMayor of Upernavik for their assistance and permission to work in the Upernavikregion. I would also like to thank Kaj Kampp, Jamie Smith, Tony Gaston, DavidNettleship, Vernon Byrd, David Nysewander, William Sydeman, and Daniel Roby forhelping me reach Greenland. I also appreciate the assistance and companionship ofGabanguak Bidstrup, Thomas Alogut, and Tara Gilchrist while in the field.This project was funded by the Canada Life Assurance Co., World Wildlife FundCanada, the American National Fish and Wildlife Foundation, the John K. CooperMemorial Trust, the Erickson Memorial Scholarship, the Hull Group, Sierra DesignsCanada, Mountain Equipment Co-op, Sigma Xi Research Support Fund, the CanadianWildlife Service (Student Research Support Program to H.G.G. and operating grants toAnthony J. Gaston), Natural Sciences and Engineering Research Council of Canada(grant to James N. M. Smith and student support for H.G.G.), the University of BritishColumbia Graduate Fellowship, the Northern Studies Trust Program, and the ScienceInstitute of the Northwest Territories.Finally, I would like to acknowledge the support of my family, Tara Gilchrist,Nancy and Brian, Susan and Cohn, and my parents Hugh and Myirea Gilchrist whohelped me in so many ways. I am especially grateful to my parents for encouraging meto pursue my interest in nature and for not pushing the more practical \u00E2\u0080\u009Carchitecturecareer path\u00E2\u0080\u009D too hard. Finally, I thank my wife Tara for her tremendous support andpatience, particularly during our times apart.x1CHAPTER 1INTRODUCTIONWhen I was young, I used to lie in the grass on summer afternoons to watch the aerialacrobatics of a kingbird (Tyrannus tyrannus) catching insects. I was amazed at how easily itcould fly up from its perch to snatch insects out of the air. On one of these days, a dense fogrolled over the countryside and the kingbird rarely moved; its feathers fluffed out against thedampness. As I was about to leave, I noticed a Cooper\u00E2\u0080\u0099s hawk (Accipiter cooperii) suddenlyappear out of the fog in a grey blur as it dived towards the kingbird on the fence. The kingbirdclimbed vertically into the air to avoid the attack, and the hawk followed effortlessly. At thepeak of its climb the kingbird dived straight back to the ground and at the last second, pulled upto glide neatly through the wire fence with the hawk in close pursuit. The hawk rocketedthrough the fence in a puff of feathers, and tumbled lifelessly into the grass on the other side;one of its wings shorn off by the wire. The kingbird re-appeared out of the grass and returnedsilently to its perch in the fog.Early observations like this convinced me that foraging and anti-predator behaviour arecritical components of an animal\u00E2\u0080\u0099s life history, and also important influences on the interactionsbetween species in the wild. Even predators like the Cooper\u00E2\u0080\u0099s hawk face constraints and riskswhen foraging, and in the most extreme cases, foraging decisions can cause crippling injury oreven death (as above). As I continued to study biology, I often found it useful to examineforaging behaviour from an economic perspective (Stephens and Krebs 1986). Early foragingstudies used the premise that animals should select foraging strategies which maximize their netenergetic gain. However, the simplicity of these early models obscured critical factors thataffect foraging decisions in the wild (learning, changing physiological states, predation risk,risk of injury during attack; Real and Caraco 1986; Lima and Dill 1986; Mangel and Clark1986). More recent foraging studies have attempted to identify factors that constrain theabilities of animals to maximize net energetic gain. Rather than asking, \u00E2\u0080\u009Cdo animals behave2optimally?\u00E2\u0080\u0099, foraging theory now considers why animals often select foraging strategies that donot yield the highest energetic gain (Dill 1986; Stephens 1990; Ward 1990).For example, foragers often appear to minimize mortality risks while foraging, so that atrade-off exists between immediate energetic gain and risks of mortality (Lima and Dill 1986).For predators like the Cooper\u00E2\u0080\u0099s hawk, foraging can be dangerous if prey fight back(physically or behaviourally), or if larger predators prey on them while they hunt (Curio 1974).In the first case, the value of attacking a prey should decrease as the danger of injury duringattack increases, because an animal should rarely jeopardize its current and future reproductionfor a small immediate increment in fitness (Clark 1994). The relative costs and benefits ofalternative foraging decisions should also vary with: 1) the availability of prey, 2) the dangersassociated with subduing prey, 3) environmental conditions, and 4) the energetic state of thepredator. Consequently, the impacts of predators on prey should also be dynamic.Only a few field-studies of vertebrate predator-prey interactions have quantified howchanges in the foraging constraints of predators influence the behaviour and populationdynamics of predators and prey (Werner et al. 1981; Spear 1993; Young 1993; Goss-CustardCt al. 1995). Young (1993) identified several environmental factors which determined theimpact of south polar skuas (Catharacta niaccorinicki) on the reproductive success of Adeliepenguins (Pygoscelis adeliae). The ability of skuas to take penguin chicks increased in windyconditions because wind increased their ability to avoid the defensive attacks of adult penguins.Late in the breeding season, penguin chicks also grew too large for skuas to kill, and thisplaced skuas in an energetic bottleneck at a critical time in their own chicks development. Inaddition, heavy ice conditions at sea affected the availability of alternative food sources and thisaffected the value of foraging at the penguin colony for skuas, and the risks skuas were willingto take when attacking penguins. These examples illustrate how seemingly unimportantenvironmental factors can interact to influence predator foraging behaviour, and consequently,the impact of predators on prey populations.3Interactions between changing environmental conditions and predator foragingconstraints like those described above, may affect the ability of glaucous gulls (Larushyperboreus) to prey on the eggs and chicks of colonial nesting thick-billed murres (Urialomvia). These topics are examined further in the chapters that follow using egg placementexperiments, behavioural observations, and computer simulations.Study sitesI studied the predator-prey interaction between glaucous gulls and thick-billed murresbreeding on Coats Island, N.W.T. Canada (1990-1992), and in the Upernavik region of northwest Greenland (1993). The Coats Island murre colony has expanded since the 1970?s,whereas the Upernavik murre colonies have experienced severe population declines due to acommercial hunt conducted by the local community between 1970 and 1985 (Kampp et al.1994).The stability of the cliff at the Coats colony permitted safe access with climbingequipment and un-obstructed views of the birds. In addition, 3 factors permitted me to studyan avian predator-prey system in unusual detail: 1) the numbers of birds present at the colony(ca. 32000 breeding pairs of murres, 13-16 breeding pairs of glaucous gulls), 2) my closeproximity to the birds allowed me to study many predator-prey interactions at close range andin a degree of detail that is rarely possible in the wild, and 3) several of the physical andenvironmental factors that affected these interactions were easily quantified (e.g. cliff ledgestructure, wind speed).A further benefit of the field site was that other recent research on the reproductiveecology of the thick-billed murre at Coats Island (Noble 1990; deForest 1993), provided mewith insights into this predator-prey system that I could not have achieved alone.4Ecology of the glaucous gullDISTRIBUTION - The glaucous gull is the second largest of all gull species. It is ageneralist predator with a circumpolar distribution. In North America, it is widely distributedin arctic regions and breeds along coasts from northern Alaska to Baffin Island, and Labrador.In the Atlantic, it is replaced by the greater black-backed gull (Larus marinus) below Labrador.In the Pacific, it is replaced by the glaucous-winged gull (Larus glaucascens) in the AleutianIslands and in British Columbia. Glaucous gulls from the eastern Arctic winter in the Maritimeregions of Canada, southern Greenland, and rarely on the Great Lakes. Gulls that breed in thewestern Arctic and Alaska winter along the west coast of North America as far south asnorthern California.NESTING BEHAVIOUR - Glaucous gulls use a variety of nesting habitats. Like manyother Arctic bird species, glaucous gulls select nest sites that are inaccessible to Arctic foxes(Alopex lagopus). They may nest on coastal islands and cliffs (Manning et al. 1956; Gaston etal. 1985), on islands in freshwater lakes near coasts (Martin and Moitoret 1986), and on sandyislets at river mouths (Sage 1974). In the Canadian and Greenland Arctic, glaucous gulls alsonest on inaccessible cliffs in association with colonies of black-legged kittiwakes (Rissabrevirostris), thick-billed murres (Uria lomvia), and Iceland gulls (Larus glaucoides).The density of breeding glaucous gulls ranges from single nests spaced kilometers apartover flat tundra, to colonies of more than 100 breeding pairs on small islands or cliffs (Johnsonand Herter, 1989). The selection and distribution of glaucous gull nests likely reflects aninteraction between limited nest sites that are inaccessible to foxes, and of the availability offoraging opportunities needed for successful reproduction.For example, at Digges Island, Quebec, Canada, all glaucous gulls nest on a cliff that isinaccessible to foxes (Gaston et al. 1985). Some gull nests are scattered within a thick-billedmurre colony, and the owners of these nests forage primarily on murre eggs and chicks(Gaston et al., 1985). In contrast, the gulls nesting at a colony of approximately 40 pairs nearthe upper margin of the murre colony apparently feed at distant shorelines and on the open5ocean. These gulls do not maintain feeding areas around their nests, and consequently, theirnests are spaced only meters apart within the gull colony (Gaston et al., 1985). A similardichotomy (i.e. where seabird-feeding specialists maintain territories and generalists nestcolonially), apparently occurs among glaucous gulls at most other seabird colonies in theeastern Canadian and Greenland Arctic (Gaston and Nettleship, pers corn., and pers. obs.),and among some other gull species (Spear 1994; Watanuki 1993; Nettleship 1972).DIET DURING THE BREEDING SEASON - Glaucous gulls are omnivores, although animalmaterial predominates in the diet of most individuals. Major categories of food during thebreeding season include: 1) marine invertebrates from the intertidal zone, 2) bird eggs andchicks, especially of colonial-nesting waterfowl and seabird species, 3) invertebrates and fishfrom rivers, 4) carrion, especially items washed up on shore or present on the sea ice, 4) smallmammals, especially lemmings where they occur, 5) human refuse at community garbagedumps or fish plants, and 6) food items pirated from other foraging birds (Ingolfsson, 1976;Gaston and Nettleship 1981; Strang 1982; Johnson and Herter 1989; Barry and Barry 1990).One of these food categories may predominate in the diet of some individuals. For example,some glaucous gulls nest within seabird or waterfowl colonies and prey primarily on the eggsand chicks of these other species (Gaston and Nettleship 1981). However, it appears that mostglaucous gulls in the Arctic have a more varied diet, and that these specialized gulls form asmall portion of the entire breeding population (Barry and Barry 1990).Glaucous gull ecology at Coats IslandNESTING BEHAVIOUR AN]) REPRODUCTIVE TIIvIING - Glaucous gulls arrive at CoatsIsland during the first and second weeks of May, which is several weeks prior to the arrival ofthick-billed murres at the colony. At this time, land-fast ice typically extends for over 50 kmfrom shore and the tundra of the island is largely snow-covered. Snow squalls occurfrequently in May and well into early June. Gulls construct their nests out of plant materialwhich they collect from ridges that have been blown free of snow. At Coats Island, gull nests6are constructed on cliff ledges and are spaced widely apart (>35 meters between most nests).This nesting distribution reflects the fact that breeding pairs defend feeding territories within themurre colony, and prevent other gulls from establishing nests in these territories by directattacks. Banding of gulls has shown that breeding pairs at Coats Island are philopatric to boththeir mates and to specific nesting ledges between years. This nesting distribution is unlike thatobserved in most other gull species, which typically nest colonially and commute to distantfood sources (e.g. Siegel-Causey and Hunt 1981; Sibly and McCleery 1983; Pugesek andWood 1992; Spear 1993). The number of glaucous gulls that have bred at the Coats Islandmurre colony since 1986, when researchers began to visit the colony regularly, has rangedfrom 12 to 16 pairs. A further 3 to 5 non-breeding glaucous gulls are usually present.Thick-billed murres begin to arrive at the cliff sporadically and briefly in early June. Theearly attendance patterns of murres appears to be determined largely by weather and thedistances they must travel to open water. During bad weather, murres often abandon thecolony and return to sea until weather conditions improve. In rare years where open wateroccurs close to shore, murres will return to the colony one to two weeks earlier in the year.The peak of murre egg laying typically falls between the 24th and 26th of June, whichcoincides with the hatching of most glaucous gull chicks. One factor that influences glaucousgull reproductive success is the weather that coincides with hatching period of gull chicks.Hatching chicks may be susceptible to exposure during wet and windy weather, becausebrooding adults sometimes stand up off their eggs when hatching is taking place. Once chickshave hatched, however, they are brooded more consistently, and are less vulnerable to theeffects of wind and rain.During late June, July and August, glaucous gull chicks remain on cliff nesting ledges.If chicks accidentally fall to lower ledges during this time, they are either killed outright bybreeding murres, or are forced by murres to leap to the ocean where they eventually die.Gull chicks fledge during the third and fourth weeks of August, so that most gull chickshave left the cliff prior to the peak of murre chick departures. Gull chicks remain in the area7around the colony and are sometimes attended by their parents at this time. Thus, breedingglaucous gulls spend approximately four months of the year at the Coats Island colony.Although very little is known about their migration pathways, it is likely that the glaucous gullsof Coats Island spend the winter in southern Greenland or Newfoundland, Canada.FORAGING OPPORTUNII1ES DURING BREEDING - When gulls first arrive at Coats islandin early May, foraging opportunities may be limited. They have poor access to the open ocean,and murres are absent from the colony. Examination of samples of regurgitated pellets (n=32)during May and early June revealed that gulls scavenge seal species (F. phocidae) and walrus(Odobenus rosmarus) carcasses on the sea ice.Coats Island supports a resident caribou herd (Rangfer tarundus) which experiencesmass die-offs on a five to eight year cycle. The carcasses from these winter die-offs, however,are mostly consumed by Arctic foxes prior to the arrival of breeding glaucous gulls. Anexception to this occurred in 1991, when two caribou fell to their deaths at the colony and werecovered by drifting snow and glaucous gulls scavenged these carcasses for several weeks inMay and June.In June, Arctic foxes provide gulls with scavenging opportunities on the sea ice belowthe colony. Murres often fall to the ice during intra-specific fights and when departing from thecolony in calm winds. Murres are rarely killed by the fall, but remain on the ice and are oftenunable to take flight. During calm wind conditions, it was common to see 10 to 20 murresstranded on the ice below the colony. Arctic foxes often killed these murres in quicksuccession, so that several carcasses remained to be scavenged by glaucous gulls. Followingthe break-up of the sea ice in late June, murres are no longer accessible to foxes and this foodsource no longer exists for gulls.Once murres begin to lay in late June, glaucous gulls concentrate much of their foragingeffort on stealing murre eggs. Observations of predation events, of food items provided tochicks, and the analysis of pellets regurgitated on nesting ledges (GG, unpublished data),8indicated that murre eggs and chicks make up >85% of gull diet in July and August. This isthe period of the breeding season that is the focus of this thesis.Murre eggs and chicks are a highly profitable food for glaucous gulls for the followingreasons. First, murre colonies provide gulls with a dependable and predictable energy sourcefor 2 and one half months during the breeding season. Second, adult gulls can almost forageand guard their chicks simultaneously, because they can hunt within feeding territories neartheir nest. As a result, glaucous gull chicks are sometimes left alone at the nest at a very earlyage while both parents forage in the vicinity. This contrasts with the situation in most colonial-nesting gull species, where one member of a pair must be present at the nest to defend theirchicks against cannibalistic conspecifics (Pierotti and Annett 1991 a). A further benefit to gullsbreeding in association with murres is that glaucous gulls can kill and easily ingest murrechicks throughout the breeding season. In some other avian predator-prey systems, prey cangrow too large for the predator to kill them (e.g. skuas and adelie penguin chicks, Young 1994;Glaucous gulls preying on snow goose goslings, Keith Hobson, C.W.S. personalcommunication.).In summary, the diet of breeding glaucous gulls at Coats island is apparently less diversethan the diets of glaucous gulls nesting elsewhere in the Arctic, and also less diverse than thediets of most other large gull species nesting in temperature regions. Glaucous gulls in thewestern Arctic have a diverse diet which can include rodents, intertidal organisms, scavengedcarcasses, mollusks, fish, and waterfowl eggs (see references above). Gulls in more temperateregions have a similarly diverse diet and may supplement natural prey with human refuseduring the breeding season (Siegel-Causey and Hunt 1981; Sibly and McCleery 1985; Pierottiand Annett 1986; Spear 1993). Coats Island supports no rodent species or Inuit communities.The shores near the murre colony at Coats Island also support few intertidal organisms,apparently due to scouring by sea ice. As a result, glaucous gulls that breed at the Coats Islandmurre colony feed primarily on murre eggs and chicks, fish at sea, and rarely on scavengedcarcasses. Thus, the foraging opportunities for breeding gulls at Coats Island are largely9determined by weather and ice conditions around the colony, and the reproductive timing ofmurres.Aims of this thesisMy objectives were to examine the foraging behaviour of glaucous gulls preying on theeggs and chicks of thick-billed murres, and to determine their impacts on murre reproductivesuccess. Specifically, I studied how environmental factors constrain the ability of glaucousgulls to attack murres, and if these constraints varied through time, in space, or betweenindividuals. I also considered the impact of gulls on the reproductive success and populationdynamics of murres.In chapter 2, I describe an experiment in which I placed murre eggs in the colony andmonitored their fate in relation to cliff ledge characteristics, murre nesting density, gull attackmode, and wind conditions. In chapter 3, I examine the hypothesis that gull foraging activityand predation rates are positively correlated with windy conditions. In chapter 4, I present adetailed energy budget of gull foraging activities and integrate this with field data of gullforaging behaviour using a dynamic model. I use this model to test the hypothesis that dangerof injury during attack is the main factor influencing the selection of foraging mode by gulls.Chapters 3 and 4 generated the prediction that decreased murre nesting densities decrease therisk of injury for gulls, thereby enhancing their foraging efficiency. Consequently, the impactof glaucous gulls on the reproductive success of thick-billed murres should increase followingany perturbation that decreases murre nesting densities. In chapter 5, I test a prediction of thishypothesis, that glaucous gull predation should be greater at a heavily-harvested and decliningmurre colony (Upernavik in northwest Greenland), than at Coats Island, N.W.T., Canada. Inchapter 6, I briefly summarize the thesis and identify the need for further research on mortalityrisks taken by top predators.10CHAPTER 2EFFECTS OF MURRE NEST DENSITY, CLIFF LEDGE WIDTH, AND WIND ONPREDATION BY GULLSPredation strongly influences the reproductive behaviour of colonial-breeding birds(Wittenberger and Hunt 1985; Burger and Gochfeld, 1994). Nesting in groups may providebenefits through increased vigilance, predator swamping, and group defence (Burger andGochfeld, 1994). Among seabirds, the effectiveness of predator mobbing may increase withgroup size and nesting density as neighbours defend nest sites collectively (Birkhead 1977;Siegel-Causey and Hunt 1981; Spear and Andersson 1989; Spear 1993). It may also vary overthe course of the breeding season, being most effective when most birds in a group have young(Andersson et al. 1980). Colony structure and topography may also influence predatorforaging efficiency because nest site characteristics, such as burrow depth, cliff ledge width,and ledge slope vary within seabird colonies and can affect the accessibility of nest sites toavian predators (Nettleship 1972), and the ability of prey to defend themselves (Siegel-Causeyand Hunt 1981; Young 1994). Therefore, the impact of avian predation is a function of howpredator foraging constraints vary in space and time, and how predators overcome theseconstraints.Glaucous gulls, Larus hyperboreus, are generalist predators that often breed inassociation with waterfowl or seabird colonies (Portenko 1989; Johnson and Herter 1989;Barry and Barry 1989). They commonly prey on thick-billed murre, Uria lomvia, eggs andchicks at colonies in the Arctic (Gaston and Nettleship 1981). Thick-billed murres breed indense colonies on exposed cliff ledges, and the principle habitat features affecting theirreproduction include the slope and width of cliff ledges, the numbers of adjacent rock walls,and the number of breeding neighbours (Gaston and Nettleship 1981). Murres that nest on theinterior of dense groups on broad cliff ledges experience the highest reproductive success11(Gaston and Nettleship 1981; Birkhead et al. 1985; Birkhead and Nettleship 1986; deForest1993), perhaps because they are most successful in avoiding gull predation.In this chapter, I identify some of the foraging constraints of glaucous gulls and examinewhether they vary spatially or temporally. I also explore how the dynamics of gull foragingconstraints affect the vulnerability of murre nest types. I placed eggs experimentally in themurre colony and monitored their fate in relation to cliff ledge width, murre nesting density,timing of murre egg-laying, murre defence, gull foraging mode, and wind conditions. Basedupon previous observations of gull foraging, I predicted that: 1) exposed eggs placed in highdensity nesting areas would survive longer than those in low density areas due to collectivedefence by murres, 2) that eggs placed on narrow ledges would survive significantly longerthan those on broad ledges because the large body size of glaucous gulls, 1.8-2.1 kg, makes itdifficult for them to forage on narrow ledges, and 3) that eggs placed early in the breedingseason would survive for a shorter period than eggs placed following the peak in murre laying.METHODSStudy Area and SpeciesThe study was conducted at a thick-billed murre colony on Coats Island, NorthwestTerritories, Canada (62\u00C2\u00B030\u00E2\u0080\u0099N, 83\u00C2\u00B0OOW) in 1990, 1991, and 1992. Thick-billed murres bredon a vertical cliff up to 65 meters above the sea. Since 1984, 1500-2500 murre chicks havebeen banded each year with metal and colour bands, establishing a sample of birds of knownage. Glaucous gulls nested on ledges within the murre colony or occasionally on turfimmediately above the murres. Glaucous gulls were the primary predators at the Coats Islandcolony, and murre eggs and chicks made up >85% of gull diet for both adults and chicks.Prior to the onset of laying by murres, gulls fed on the carcasses of murre, ringed seal, Phocahispida, and caribou, Rangifer tarundus, present on the sea ice below the colony cliffs.12Egg placement experimentsIn 1990, 1991, and 1992 I placed large chicken eggs painted to mimic murre eggs in themurre colony in several sites and monitored their survival in relation to time of day, gull attacktechnique, murre defence, murre reproductive synchrony, and weather conditions. Inpreliminary studies, experimental eggs were taken readily by gulls and occasionally incubatedby murres. In 1991, I also used genuine murre eggs taken as part of a study examining thick-billed murre reproductive success (deForest 1993).In all studies, eggs were placed by climbing down into the colony from above using fixedropes. Murres often left nesting ledges in response to the disturbance caused during eggplacement, but usually returned within 5 minutes. Climbing equipment allowed me to moveslowly and methodically within the colony, and I was able to avoid significant eggdislodgment. Further, I remained above study plots and deterred gulls from taking exposedeggs until murres had returned to their nest sites. I began to record the survival time ofexperimental eggs after I left the cliff face and entered an observation blind. The survival ofexperimental eggs was monitored continuously for the first four hours after placement, andchecked hourly thereafter. Eggs that survived beyond twenty-four hours, were then checkedevery three hours. I terminated the experiment after 72 hours, and assumed that surviving eggswere either invisible or inaccessible to gulls.In 1990 and 1991, I tested the effects of nesting density and ledge width on the predationof experimental murre eggs. Eggs were placed in four situations: 1) broad ledges with highmurre nesting density; 2) broad ledges with low nesting density; 3) narrow ledges with highdensity; and 4) narrow ledges with low density. High density ledges had >80% of their areaoccupied by breeding murres, whereas low density ledges had <40% of their area occupied.The nesting density of ledges was assessed by eye using a 15-45 power spotting scope andwith the aid of photographs of nesting ledges. Broad ledges supported more than one row ofbreeding sites, whereas narrow ledges supported only one row.13In 1991, I tested the effects of wind speed on the accessibility of narrow ledges to gulls.I conducted the experiment following the peak of murre laying at which time >80% of murreshad eggs. Two eggs were placed on each of fourteen narrow, low-density murre nest sites.One egg was placed under windy conditions (>15 km/h), while the other was placed on thesame site under calm conditions (<5 km/h). I then compared the survival times of the eggsunder the two regimes. Wind speed categories were based upon a 15 km/h wind thresholdabove which glaucous gulls could glide without flapping their wings. This threshold wasdetermined from behavioural observations in the field and from data on flight dynamics of largegulls (Pennycuick 1987). Wind speed was measured to \u00C2\u00B1 1 km/h using a Weather-pro Digitaranemometer mounted 1.5 m from the cliff face. If the wind conditions changed during the firstfour hours of the experiment (e.g., from calm to windy), the trial was abandoned. I predictedthat eggs placed under calm conditions would survive longer than those placed under windyconditions due to the increased maneuverability of gulls in strong winds.In 1992, I used thick-billed murre eggs to test the effect of laying synchrony on eggpredation. For these trials, eggs were placed only when wind speeds were >15 km/hr. Iplaced eggs on broad (N=6) and narrow (N=6) ledges about a week before the peak of egglaying (< 17% of pairs with eggs). These placements were repeated on the same sites 12 dayslater at which time >82% of the pairs had eggs. Survival times were then compared. Layingsynchrony was monitored as part of a project examining the reproductive biology of thick-billed murres (deForest 1993). I predicted that eggs placed at the beginning of the layingperiod would survive for a shorter time, because birds not yet incubating their own eggsshould not contribute to group defence during gull attack.Behavioural ObservationsAfter egg placement, I observed gull foraging activity from blinds and recorded the attackmethods used, classifying them as shown in Table 2.1. In addition, I recorded whether eggswere removed successfully or dropped during the attempt. I also recorded the response of the14murres during attacks by gulls (Table 2.1). When more than one murre responded to an attack,I often recorded several types of behaviour simultaneously.Statistical AnalysisThe survival of eggs relative to ledge width and murre nesting density was comparedusing the Kolmogorov-Smirnov goodness of fit test. The survival of eggs placed on the samenest sites under two different conditions of both wind speed and numbers of neighbors witheggs were compared using the Wilcoxon matched-pairs test.The defensive responses of individual murres nesting on the same ledge could not beconsidered independent. In fact, it is likely that the response of an individual murre to a gullattack depends on the behaviour of its neighbours (Birkhead 1977). For each gull attack, Irandomly selected the response of one individual next to the egg for analysis of the effects ofledge width on murre defence behaviour. For my analysis I used the following categories ofmurre defensive response (see Table 2.1-a): 1) no defence = flush or no response, 2) moderatedefence = orient, and 3) strong defence = lunge. Finally, I used log-likelihood ratio (G) tests toexamine differences in weak, moderate, and strong murre defence behaviour relative to ledgewidth and laying date. I also compared the attack techniques of gulls using a G-test, with theWilliam\u00E2\u0080\u0099s correction for continuity applied because of small sample sizes (Sokal and Rohlf1981).15Table 2.1. a) Attack modes of gulls, and b) murre response to gull attack.Behaviour Descriptiona) Stand lunge Gull stands on murre nesting ledge, head lowered; lunges intonesting group of murres to take eggAerial lunge Gull flying along colony cliff drops into nesting murres to steal eggHover harass Gull flying along colony cliff stops to hover next to murre nest sites;gull then maneuvers into nest site to steal egg without landingb) No response Murres on nesting ledge do not alter behaviour during gull attackOrient Murres elongate their necks and direct their beaks towards attackinggullFlush Murres suddenly fly from nesting ledge avoiding contact with gullLunge Murre runs towards attacking gull standing on ledge; in response toaerial attack, murre drops from ledge and attempts to strike hovering gull16RESULTSNesting DensityOf the 77 experimental eggs that disappeared (n=92), 48 (66%) were taken within thefirst four hours and only 15 of a total of 92 eggs (16.3%) survived for 72 hours. I saw gullstake 58 (75.3%) of the 77 disappearing eggs. As predicted, eggs placed on ledges with highnesting densities remained for significantly longer than eggs placed under low-densityconditions regardless of ledge width (Fig. 2.1). Group defence by murres was important inpreventing gulls from reaching exposed eggs because gulls tried to avoid strikes from nearbymurres. On ledges with high nesting densities, murres collectively defended exposed eggs bylunging at or striking gulls as gulls tried repeatedly, and often unsuccessfully, to reach exposedeggs. On ledges with low nesting densities, gulls could walk among incubating birds whiletaking eggs. As predicted, gulls abandoned foraging attempts more frequently under highnesting density conditions (11 of 25 of high density attempts, 9 of 37 low density attempts;Gadj4.37, dfl, P