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Wednesday, 12 August 2015

The Auk (AOU) July 2015, Volume 132, Issue 3 - Abstracts

The Auk
Published by: The American Ornithologists' Union

July 2015 : Volume 132 Issue 3 


New ecological information for the Black Tinamou (Tinamus osgoodi hershkovitzi)
Pablo Jose Negret, Oscar Garzón, Pablo R. Stevenson and Oscar Laverde-R.

The Black Tinamou (Tinamus osgoodi) is a rare and endangered bird with two geographically disjunct subspecies. Very little pertinent information exists due to its secretive habits and cryptic coloration. Observations from a one-year study at Alto Fragua Indi Wasi National Park in southern Colombia have provided new ecological information for T. o. hershkovitzi. This subspecies vocalizes mostly between March and April, suggesting that the breeding season occurs during the first half of the year. Detections by camera traps indicate that this tinamou is more active in late morning, a pattern also found in other lowland tinamous. The subspecies was found in the entire study area, but more commonly at middle altitudes (1,400–1,600 m). We estimated a density of 13.47 birds km−2, which is relatively high compared with the abundance of other tinamous of similar size. Despite the locally observed high density of this subspecies of Black Tinamou, high rates of logging and hunting in the area make this population vulnerable to rapid decline in the future.

Western Veeries use an eastern shortest-distance pathway: New insights to migration routes and phenology using light-level geolocators 
Keith A. Hobson and Kevin J. Kardynal

We investigated movements of a western population of Veeries (Catharus fuscescens) breeding in the Okanagan region of British Columbia, Canada, in 2013–2014 using light-level geolocators. We tracked 9 individuals and incorporated a state-space Kalman filter model approach to estimate movement parameters. During migration, Veeries traversed the Rocky Mountains, Great Plains, Gulf of Mexico, and Caribbean Sea with stopovers generally closer to the shorter orthodromic (great circle) route than a loxodromic (straight line) route between breeding and first wintering grounds, particularly on fall migration. Birds initially settled in the south-central portion of the Amazon basin in Brazil at sites that were 666 ± 299 km apart, suggesting low migratory connectivity. Intra-tropical movements were observed for 8 of 9 (88.9%) birds, with second wintering sites an average of 1,447 ± 472 km to the northwest (initial bearing x̄ = 316 ± 16°). Veeries typically followed a pattern of loop migration at the Gulf of Mexico, with more birds using the Yucatan Peninsula to stop and reorient toward destinations on spring migration (n = 7) vs. fall migration (n = 2). Western Veeries follow a presumed ancestral (eastern) migration route, but this route is also the shortest (great circle) route between breeding and wintering grounds, even though this route was only ~100 km shorter than the straight line route. Eight Veeries (88.9%) underwent a post-breeding, pre-migratory movement up to 628 km (x̄ = 263 ± 152 km) away from breeding territories, possibly to molt. We encourage researchers utilizing light-level geolocators to apply similar state-space modeling approaches to reduce the influence of observers and erroneous location estimates on analysis and interpretation of geolocator data.

Feather-chewing lice and Tree Swallow biology
Michael P. Lombardo, Patricia Drake, Amber Olson, Sango Otieno, Lena Spadacene and Patrick A. Thorpe

Feather-chewing lice (Order Phthiraptera, Suborder Ischnocera) commonly infest birds and may affect their survival and reproduction. From 1993 to 2005, we examined several aspects of the biology of breeding Tree Swallows (Tachycineta bicolor) potentially associated with holes in wing and tail feathers caused by feather-chewing lice. Most individuals had <10 feather holes, but 90% of second-year (SY) females, 68% of after-second-year (ASY) females, and 80% of males had ≥1 feather hole. ASY females had significantly fewer feather holes than SY females and males. There was evidence of positive assortative mating for feather hole number between ASY females and their mates. SY females and their mates did not differ significantly in the number of feather holes, but ASY females had significantly fewer holes than their mates. Males with fewer feather holes were heavier and had longer right wings. Feather hole abundance was not significantly associated with reproductive performance. Feather hole abundance was not associated with whether females bred 1 time or >1 time at our study site, but males with fewer holes were more likely to breed >1 time. Mean feather hole abundance differed significantly among years for SY females and males that bred 3 and 4 times, respectively, but not for ASY females that bred 4 times. Collectively, these data suggest that feather-chewing lice, as estimated by the damage they cause to wing and tail flight feathers, have little effect on Tree Swallow fitness.


Are prenatal maternal resources more important in competitive than in benign postnatal environments? 
Miloš Krist, Martin Janča, Anaïs Edme and Rudolf Dzuro

According to theoretical models, the optimal solution of the life-history trade-off between the number and size of offspring depends on the quality of the environment. Offspring size should be more important for their fitness in more competitive environments. This idea was rarely experimentally tested in taxa with prolonged periods of parental postnatal care, such as in birds. Here we manipulated the offspring rearing environment by enlarging or reducing brood size. Enlarged broods suffered greater mortality rates and raised smaller fledglings. Egg size had a significant positive effect on fledging mass and length of tarsus and a nonsignificant effect on wing length. These effects were similar in enlarged-sized as well as reduced-sized broods. We only found a tendency for the predicted interaction between treatment and egg size in the case of nestling mass where egg size had a positive effect in enlarged broods but none in reduced broods. In contrast, in one year we found an opposite interaction where egg size positively affected offspring survival only in reduced broods. More studies that manipulate the offspring rearing environment and follow offspring over the long term are needed to draw general conclusions about context-dependence of early maternal effects.

The coevolution of building nests on the ground and domed nests in Timaliidae
Zachary J. Hall, Sally E. Street, Sam Auty and Susan D. Healy

Despite the accumulation of structural descriptions of bird nests and considerable diversity in these structures across species, we know little about why birds build the nests that they do. Here we used phylogenetic comparative analyses to test one suggested explanation, specifically for Old World babblers (Timaliidae): that building a domed nest coevolved with building a nest on the ground. We show that babblers that build domed nests build them at a lower height than do babblers that build cup-shaped nests, and that in this radiation the evolution of domed nests depended on the transition to building a nest on the ground. Our results are consistent with the hypothesis that babblers add a roof to the nest in order to confer protection against increased predation risk on the ground. We believe that this is the first formal identification of evolutionary pathways that have led to the diversity in nest structure and location that we see today.

The presence of a female influences courtship performance of male manakins
Julia Barske, Barney A. Schlinger and Leonida Fusani

Coordinated courtship displays are a common feature of species forming long-term pair bonds. In lekking species, on the contrary, there are no stable pair bonds because partners meet only to copulate, and males indicate their quality and/or attractiveness to females by displaying morphological and behavioral traits. In some cases, females interact with these displaying males, but little is known about the role of the females in these encounters. In the Neotropical bearded manakins, females join males in their acrobatic courtship displays in the final phase of mate choice. We hypothesize that females participate in the courtship dance to better assess male motor skill by observing male responses to female signals. We filmed at high speed the courtship displays of 2 species of bearded manakins and compared the displays performed by males alone, where the female is absent from the arena, with those performed together with a female. In addition, we compared the movements of the male with those of the female and analyzed the display coordination. We found that when a female is present in the arena, males increase the speed or frequency of several performance parameters that are strongly correlated with courtship success. Additionally, males seem to pace their movements to those of the female as she takes the lead in the duo dance. Our results suggest that before choosing a mate for copulation, female manakins challenge the motor skills of prospective males.

Nest sanitation in cooperatively breeding Carrion Crows
Diana Bolopo, Daniela Canestrari, José M. Marcos and Vittorio Baglione

A clean nest is important for successful breeding in most bird species, because parasites and bacterial infestations can reduce nestling survival. In cooperatively breeding species, caregivers may either share nest and chick sanitation activities or specialize in different tasks related to chick care, such as food provisioning, nest sanitation, and territory and nest defense. We used video-recorded observations at nests of cooperatively breeding Carrion Crows (Corvus corone corone) to describe sanitation behaviors and analyze the factors that influenced individuals' contributions to each particular task. We then combined data on sanitation and chick provisioning to investigate whether labor was divided within the group and whether individuals specialized in any particular task. Finally, we asked whether nest sanitation was flexible and responded to current conditions—in particular, food availability in the territory. In a controlled experiment, we supplemented 11 territories with food during the breeding season, reducing the costs of caring for the young. We found that in the cooperative groups, breeding females carried out the vast majority of sanitation tasks (i.e. nest and chick cleaning and fluffing of the nest's inner layer), whereas other group members contributed relatively little to these activities, with the exception of fecal-sac removal, which was done by any adult present at the nest during excretion. We suggest that the greater contribution of females occured because a clean nest is beneficial not only for the brood, but also for breeding females, which perform all incubation and brooding and are therefore more exposed to nest parasites. Nest sanitation also proved to be condition dependent and decreased in experimentally food-supplemented territories, where birds may have been in healthier condition and therefore less likely to transmit parasites to the brood. We suggest that extending the study of division of labor to different tasks other than chick provisioning is important to widen our understanding of the evolution and maintenance of cooperative breeding.

High functional complexity despite an extremely small repertoire of calls in the Spotted Crake (Porzana porzana)
Paweł Ręk

Vocal learning and nonlearning birds frequently use the same habitats and are subject to similar selection pressures. However, although the repertoires of learning species are usually more complex, we know much less about how nonlearners encode information in acoustic signals. The present study describes the use of 3 basic mechanisms of acoustic encoding in signals of a vocal nonlearner: in repertoire, in acoustic structure, and in the temporal distribution of sounds. The study is based on observations of a simulated territorial intrusion in the Spotted Crake (Porzana porzana), a cryptic rail from Western and Central Eurasia. Males produced 5 types of calls. The loudest, the whitt call, consisted of 2 independent sounds with different fundamental frequencies: a soft and low F call and a loud and high G call. Before the playback, all males produced calls that consisted of both frequencies (F+G calls), and most of the birds still produced such calls after the playback. Only birds that approached the speaker during the playback produced F calls, F+G calls with a muffled G fundamental, and structurally distinctive soft rumble calls, whereas only birds that did not approach the speaker used G calls. These data suggest that males' engagement in aggressive interaction was associated with muting G or F fundamentals. The acoustic structure of whitt calls varied significantly between preplayback and postplayback recordings, and between approaching and nonapproaching males. However, certain acoustic parameters retained a high potential for individual recognition despite the playback. Finally, males lengthened their between-call intervals as they approached the speaker, which suggested that there was a link between the temporal distribution of calls and the aggressive motivation of males. Although they have a small repertoire of calls with innately programmed structures, Spotted Crakes modify their calling on many different planes and produce a high diversity of signals.

Differential contributions of endogenous and exogenous nutrients to egg components in wild Baltic Common Eiders (Somateria mollissima): A test of alternative stable isotope approaches
Keith A. Hobson, Kim Jaatinen and Markus Öst

The relative importance of nutrients derived from feeding on breeding vs. nonbreeding grounds to the formation of eggs is crucial for predicting how the breeding success of migrating birds responds to changes in food availability during any part of their annual cycle. Eiders have been considered a classical capital breeder, but this assumption has rarely been tested. The measurement of naturally occurring stable isotopes in egg components, together with those in endogenous and exogenous nutrient endpoints, allow the estimation of the relative sources of nutrients to eggs, but these mixing models rely critically on appropriate isotopic discrimination factors that link egg isotope values with their source. A recent captive study using Spectacled Eiders (Somateria fischeri) provided estimates of these isotopic discrimination factors for income breeding. We applied these discrimination factors for investigating nutrient allocation strategies in Common Eiders (Somateria mollisima) breeding in the northern Baltic (Tvärminne, Finland) and wintering in Danish waters, sourced during 2009–2012. Our overall estimates of protein sources using isotopic mixing models were mixed for egg yolk (median: 44.5–56.5% endogenous) and overwhelmingly exogenous for egg albumen (0.4–0.7%). We tested our conclusions also with a single (δ15N) model and with a more parsimonious δ13C discrimination factor between diet and egg albumen, and both supported little to no endogenous reserves being used for egg albumen. A strong positive correlation between egg lipid δ13C and lipid-free yolk δ13C suggests similar metabolic pathways between diet sources and these egg macromolecules. The applicability of isotope discrimination factors used in nutrient allocation studies derived from captive populations needs to be tested in wild populations. Our results support the idea that potential food limitation not only at the wintering areas, but also at the breeding grounds, can limit breeding success of Baltic Common Eiders, which are currently declining.

Natal territory size, group size, and body mass affect lifetime fitness in the cooperatively breeding Florida Scrub-Jay
Ronald L. Mumme, Reed Bowman, M. Shane Pruett and John W. Fitzpatrick

Early rearing conditions can have profound short- and long-term effects on survival and reproduction of vertebrates. In cooperatively breeding birds, where variable social conditions interact with other sources of environmental variation, fitness consequences of the natal environment are of particular interest. We used data from a long-term study of the Florida Scrub-Jay (Aphelocoma coerulescens) to examine how characteristics of the natal brood, territory, and social group interacted to influence future survival and reproduction. Nestling and juvenile body mass were significant positive predictors of survival from fledging through the first year of life. The area of oak scrub in the natal territory correlated positively with nestling and juvenile body mass and had two long-term consequences: it was positively related to the probability of becoming a breeder and negatively related to age at first breeding, particularly for males. Effects of natal group size were complex and partially dependent on territory size. Although positively associated with nestling mass and postfledging survival, the presence of helpers was negatively related to juvenile body mass, but only in territories with <8 ha of oak scrub, suggesting that helpers become competitors for food as juveniles in small territories reach independence. Although early-life environmental conditions had strong effects on nestling and juvenile body mass, survival to yearling stage, and acquisition of a breeding territory, early conditions were not significant predictors of survival or reproduction once breeding status was obtained. This result suggests that low-quality jays from poor natal environments are winnowed out of the pool of potential breeders by intense intraspecific competition, ultimately leading to low heterogeneity in breeder quality. Our findings help to resolve the long-standing question of why Florida Scrub-Jays defend such unusually large territories for their body size, revealing a key selective advantage for cooperative territorial defense in the evolution of cooperative breeding.

Geographic variation in morphometrics, molt, and migration suggests ongoing subspeciation in Pacific Golden-Plovers (Pluvialis fulva)
Joop Jukema, Johan G. van Rhijn and Theunis Piersma

Breeding Pacific Golden-Plovers (Pluvialis fulva) cover 140 longitudinal degrees of Arctic tundra. Having examined 557 museum skins from across this huge distributional range, we conclude that Pacific Golden-Plovers breeding in Alaska are structurally larger than those breeding in Siberia, especially in wing length. Birds from Alaska also have more pointed wings and almost always postpone the initiation of primary molt until they reach their winter quarters, whereas many Siberian birds start primary molt in the breeding areas. These differences could have been favored by the longer transoceanic flights followed by the Alaskan populations to nonbreeding destinations in the Pacific Islands. We propose that the Alaskan and Siberian breeding birds be distinguished as distinct flyway populations to be used in conservation assessments by the international conservation community.

Fast and efficient: Postnatal growth and energy expenditure in an Arctic-breeding waterbird, the Red-throated Loon (Gavia stellata)
Daniel J. Rizzolo, Joel A. Schmutz and John R. Speakman

Environmental conditions can exert a strong influence on the growth and energy demands of chicks. We hypothesized that postnatal growth in a cold, aquatic environment would require a high level of energy metabolism in semiprecocial Red-throated Loon (Gavia stellata) chicks. We measured body-mass growth and daily energy expenditure (DEE) of free-ranging chicks in the Arctic. We used daily gains in body mass and DEE to estimate daily metabolizable energy (DME, kJ day−1) and total metabolizable energy (TME, kJ chick−1). Chicks gained body mass quickly, with a logistic growth rate constant 57% greater than the allometric prediction, yet were at only 60% of adult body mass at fledging. Males grew at a rate similar to that of females but for a slightly longer duration and so reached an asymptotic body mass 23% greater, and tarsus length 8% longer, than that of females. Chick growth performance was similar between first- and second-hatched chicks within broods of 2, which suggests that food availability was not limited. DEE increased in proportion to body mass, and DME peaked at 1,214 kJ day−1 on day 25 posthatching. Over the average 49-day postnatal period, TME was 49.0 MJ, which is within the range of error of the allometric prediction. Parents provided 58.6 MJ as food to meet this energy requirement. Given this chick energy requirement and the range of energy content of prey observed in the chick diet, selecting prey with higher energy content would greatly reduce adult provisioning effort. Red-throated Loon chicks did not have a high postnatal energy requirement, but rather grew quickly and fledged at a small size—with the effect of reducing the length of the postnatal period and, consequently, parental energy investment in chicks.

Effects of weather variation on the timing and success of breeding in two cavity-nesting species in a subtropical montane forest in Taiwan
Ming-Tang Shiao, Mei-Chen Chuang, Hsiao-Wei Yuan and Ying Wang

Biological processes at low latitudes are often associated with seasonal rainfall, while those at high altitudes are also influenced by low temperature and high humidity. Thus, the breeding phenology of montane species at low latitudes requires clarification. We monitored the timing of nesting for two species of secondary cavity-nesting passerine, the Green-backed Tit (Parus monticolus) and the Rufous-faced Warbler (Abroscopus albogularis), in a montane cloud forest in subtropical Taiwan between 2010 and 2014. To determine the effects of weather and extreme events on nesting success, we used program MARK to model daily nest survival rate as a function of weather variables. The results showed that March temperatures, but not spring (February–March) rainfall, affected the mean laying date of Green-backed Tits. Females shifted their laying dates to lay earlier in response to earlier warm spring weather. The Rufous-faced Warbler exhibited a similar but weaker relationship between March temperature and mean laying date. Spring temperature and rainfall did not affect the length of the laying season of either species. The reduction in the limiting effect of spring rainfall on biological processes may be attributable to the perhumid climate of the study area. Both species were negatively affected by heavy seasonal rainfall during the nesting period. Nest survival declined under extremely heavy rainfall (daily precipitation >20 mm), but did not vary linearly or nonlinearly with daily precipitation levels. The daily survival rate of Green-backed Tits was further reduced by cold weather combined with heavy rain. Our results show that the breeding densities of both species declined across the 5-yr study period, indicating that the negative effects of adverse weather may contribute to further decreases in annual productivity, which would accelerate population declines at the current rate of climate change.

Evolution of supraorbital ossification in Charadriiformes
Austin L. Hughes

Phylogenetic analysis of morphological data on 92 species in the order Charadriiformes showed that increased supraorbital ossification (SO) has evolved independently at least 4 times in this clade. Principal component analysis of size-corrected skeletal measures showed that an evolutionary increase in SO was associated with a body plan in which the cranium, humerus, and sternum were relatively large. A mass-corrected measure of the axial length of the eye was positively correlated with a mass-corrected measure of cranium length, both in the raw data and in phylogenetically independent contrasts. By revealing an association between the evolution of SO and increased relative size of the eye and flight-associated skeletal elements, these results support the hypothesis that an increased reliance on visually guided foraging in flight or diving has been a major selective factor favoring the evolution of increased SO in Charadriiformes.

Heterothermic flexibility allows energetic savings in a small tropical swift: The Silver-rumped Spinetail (Rhaphidura leucopygialis)
J. Ryan Shipley, Daniel Y. Gu, Timothy C. Salzman and David W. Winkler

Facultative heterothermy, or torpor, has been demonstrated in different clades of the Apodidae, proposed as a mechanism to reduce energetic demand in response to different physiological cues. In a small tropical Southeast Asian swiftlet species, the Silver-rumped Spinetail (Rhaphidura leucopygialis), we measured the facultative heterothermic response in a pair of experiments. In the first experiment, the mean body temperature of birds immediately after capture was ~38.9°C. Individuals exposed to consistent ambient temperatures of 28.0°C reduced mean body temperatures over time, and these reductions were greater in individuals kept in the dark than in those that were exposed to light during the trial. The second experiment consisted of respirometry trials measuring rates of carbon dioxide production along with internal body temperature in relation to decreasing ambient temperatures. Two different responses to the respirometry trials emerged: Some individuals modulated body temperature in relation to changes in ambient temperature, and other individuals maintained body temperature (~31.6°C) independent of ambient temperature. Our results suggest considerable flexibility among individual heterothermic responses in R. leucopygialis. Our results are in agreement with other studies on swifts, suggesting that these alternative modes for energy conservation are likely context-dependent and based upon individual body condition, life stage, and annual cycle.

Relationship of phenotypic variation and genetic admixture in the Saltmarsh–Nelson's sparrow hybrid zone
Jennifer Walsh, W. Gregory Shriver, Brian J. Olsen, Kathleen M. O'Brien and Adrienne I. Kovach

Hybridization is influential in shaping species' dynamics and has many evolutionary and conservation implications. Identification of hybrid individuals typically relies on morphological data, but the assumption that hybrids express intermediate traits is not always valid, because of complex patterns of introgression and selection. We characterized phenotypic and genotypic variation across a hybrid zone between 2 tidal-marsh birds, the Saltmarsh Sparrow (Ammodramus caudacutus) and Nelson's Sparrow (A. nelsoni) (n = 290), and we sought to identify morphological traits that could be used to classify admixed individuals. Sparrows were sampled from a total of 34 marshes, including 23 sympatric and 11 putatively allopatric marshes. Each individual was scored at 13 plumage traits, and standard morphometric data were collected. We used genotyping analysis at 24 microsatellite loci to categorize individuals into genotypic classes of pure, F1–F2, or backcrossed. Genetic data revealed that 52% of individuals sampled along the geographic transect were of mixed ancestry, and the majority of these were backcrossed. Traits related to the definition of plumage features (streaking, crown, and face) showed less overlap between genotypic classes than traits related to the amount or color of plumage features. Although morphological data performed well in distinguishing between the 2 taxa, pure and backcrossed individuals of each parental type could not be distinguished because of substantial overlap in plumage and morphology. We conclude that the discrimination of pure and hybrid individuals is not possible in the absence of genetic data. Our results have implications for conservation of pure populations, as extensive backcrossing throughout the hybrid zone may present challenges for monitoring pure species identified by morphology alone.

Is brood parasitism related to host nestling diet and nutrition? 
Zachary S. Ladin, Vincent D'Amico, Deb P. Jaisi and W. Gregory Shriver

Food and nutrient limitation can have negative effects on survival, fecundity, and lifetime fitness of individuals, which can ultimately limit populations. Changes in trophic dynamics and diet patterns, affected by anthropogenic environmental and landscape change, are poorly understood yet may play an important role in population regulation. We determined diets of Wood Thrushes (Hylocichla mustelina), a Neotropical migratory songbird species sensitive to urbanization, and explored how brood parasitism by Brown-headed Cowbirds (Molothrus ater) may be related to Wood Thrush nestling diets. Effects of brood parasitism on host nestling diets is an understudied stressor that may help explain observed population declines. We measured carbon (δ13C) and nitrogen (δ15N) stable isotopes of 7 invertebrate food sources (snails, spiders, isopods, earthworms, myriapods, insects, and caterpillars), blood plasma from adult male and female Wood Thrushes and from Wood Thrush nestlings in nests with and without Brown-headed Cowbird nestlings. Wood Thrush diet compositions were largely composed of high calcium (Ca) foods (51–62%, 95% highest density intervals [HDI]), including snails, isopods, and myriapods, as well as spiders (23–33%, 95% HDI). Caterpillars were the least common food item in Wood Thrush diets (0.01–3 %, 95% HDI). Wood Thrush nestling diets in nests without Brown-headed Cowbirds contained greater proportions of Ca-rich foods and spiders compared to the diet of nestlings in parasitized nests. Our data demonstrate that Wood Thrushes preferred Ca- and protein-rich foods, which may have important implications for adult survival and fecundity as well as nestling nutrition and development. Our results suggest that brood parasitism is related to host nestling diet, which could have potentially negative effects on developing nestlings through nutritional stress that may in turn affect survival, fecundity, and ultimately limit population growth.

Diet and reproductive success of an Arctic generalist predator: Interplay between variations in prey abundance, nest site location, and intraguild predation
Gilles Gauthier, Pierre Legagneux, Marc-André Valiquette, Marie-Christine Cadieux and Jean-François Therrien

Under varying prey abundance, generalist consumers should be less affected than specialists due to their more diverse diet. Nonetheless, when prey availability declines, interspecific competition among consumers should increase and could lead to increased intraguild predation. We examined these potential effects in a generalist predator of the tundra, the Glaucous Gull (Larus hyperboreus), over a 7-yr period characterized by large fluctuations in lemming abundance, a potential prey item for gulls. We studied diet by analyzing regurgitated pellets collected at nests and blood nitrogen and carbon stable isotopes, and we monitored annual nesting density and reproductive success on Bylot Island, Nunavut, Canada. Stable isotopes revealed a relatively similar contribution of terrestrial and marine food sources to the gull diet, although the terrestrial contribution increased in the year of high lemming abundance. According to analysis of pellets, diet during incubation was dominated by geese and lemmings, whereas geese were the main prey during chick-rearing. As expected for a generalist predator, annual variation in diet during incubation reflected overall lemming abundance, and the increased consumption of geese during chick-rearing was associated with an influx of goose families into the study area at that time. Gull nest density, laying date, clutch size, and mass gain of chicks did not vary with lemming population fluctuations. Hatching success, on the other hand, was positively related to lemming abundance and was greater for nests located on islets than for those along the shores of ponds and lakes. Intraguild predation on gull eggs by predators such as Arctic foxes, which primarily feed on lemmings during the summer, was probably the main cause of nest failure. Although a generalist predator like the Glaucous Gull has a diversified diet and may feed only opportunistically on lemmings, our results suggest that large cyclical fluctuations in lemming abundance may still affect gull reproductive success through intraguild predation.

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