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Monday, 14 July 2014

The Auk, July 2014 : Volume 131, Issue 3. Contents and Abstracts

The Auk

Published by: The American Ornithologists' Union

Table of Contents

July 2014 : Volume 131 Issue 3


Winter body condition in the Collared Flycatcher: Determinants and carryover effects on future breeding parameters 
Rita Hargitai, Gergely Hegyi, Márton Herényi, Miklós Laczi, Gergely Nagy, Balázs Rosivall, Eszter Szöllősi, and János Török
pg(s) 257–264
Factors that determine the condition of migratory birds at their wintering sites are poorly known. Age, sex, and morphological characteristics of birds may have an influence on their winter condition by affecting their foraging and competitive abilities. Winter body condition could have long-term consequences on the reproductive success of migratory birds during the subsequent breeding season. Using 3 yr of data from Collared Flycatchers (Ficedula albicollis), we examined the characteristics of winter-grown tail feathers, as indicators of winter body condition, in relation to sex, age, morphological traits, and future breeding variables. Tail feather mass was highly repeatable between years, but feather growth rate was not repeatable, which suggests that the latter trait mainly indicates environmental circumstances during molt, whereas feather mass may more strongly reflect genetic effects. Tail feathers of males and adults showed better quality than those of females and juveniles, possibly because of differences between age classes and sexes in individual quality and foraging skills or because of winter habitat segregation. Birds with longer wings produced better-quality tail feathers, which suggests that wing and tail feather characteristics are similarly affected, presumably by individual genetic quality. Smaller Collared Flycatchers grew their tail feathers faster during the winter molt, possibly because they had better foraging ability due to better flight maneuverability. Tail feather quality showed no relationship with laying date; however, females that had produced heavier tail feathers during winter laid larger clutches during the following breeding season, which suggests that tail feather mass potentially reflects intrinsic individual quality.

On the “real estate market”: Individual quality and the foraging ecology of male Cory's Shearwaters 
Antje Chiu Werner, Vitor H. Paiva, and Jaime A. Ramos
pg(s) 265–274
An extended reproductive period and high variability in food resource availability at sea make good quality nest sites particularly important for the survival of pelagic seabird chicks. Despite high philopatry during the early pre-laying period, males compete strongly for nests, making this period a unique opportunity to independently assess the influence of nest-site characteristics and individual quality on the foraging behavior of Cory's Shearwater (Calonectris diomedeaborealis) individuals. We found significant differences in the at-sea foraging behavior of males and females at temporal (trip duration) and spatial (foraging areas, trip distance, and trip sinuosity) scales, both of which are greater in females. Furthermore, we suggest that nests of higher quality are deeper and closer to the nest of a conspecific neighbor because both variables were associated with males foraging closer to the colony. Finally, we showed that during the early pre-laying period the influence exerted by nests on males' behavior at sea is independent from the individual's quality. Our study links nest-site features with the at-sea behavior of pelagic male seabirds during a period of nest competition and suggests that nest-site characteristics are important to explain foraging patterns of central-place foraging birds.
Carotenoid coloration predicts escape performance in the House Finch (Haemorhous mexicanus
Fernando Mateos-Gonzalez, Geoffrey Hill, and Wendy Hood
pg(s) 275–281
Carotenoid coloration has been repeatedly shown to serve as a sexually selected signal of individual quality. Across different species, individuals showing brighter carotenoid-based signals have been found to have superior foraging abilities, to recover faster from diseases and, in general, to enjoy a better body condition. Experiments with birds have also shown that carotenoid supplementation can enhance flight performance, allowing birds to take off faster from the ground. Healthy, agile individuals should be better prepared to avoid predators, so it could be expected that individuals displaying brighter carotenoid-based coloration would show a higher escape ability from predator attacks. To test this prediction, we measured the escape ability of male House Finches (Haemorhous mexicanus) from a human with a net in a large aviary and related the escape ability of each individual to its breast coloration. Males with redder feathers showed a higher individual ability to escape than duller individuals. The superior flight performance of redder birds would be an important asset in escape from predators, as well as when foraging or maintaining a territory. In the specific case of the House Finch, the higher escape ability of redder individuals could be the reason for their higher overwinter survival rate.


Type specimens in modern ornithology are necessary and irreplaceable 
A. Townsend Peterson
pg(s) 282–286
Recent years have seen a series of new species descriptions in which no type specimen or fragmentary type specimen material was provided as documentation. These descriptions have been controversial, but the Code of Zoological Nomenclature makes clear that such nondiagnostic types are not acceptable specimen documentation. A more appropriate approach is documentation of the discovery, but without formal naming of the species, until suitable specimen documentation can be assembled.


Previous success and current body condition determine breeding propensity in Lesser Scaup: evidence for the individual heterogeneity hypothesis 
Jeffrey M. Warren, Kyle A. Cutting, John Y. Takekawa, Susan E. De La Cruz, Tony D. Williams, and David N. Koons
pg(s) 287–297
The decision to breed influences an individual's current and future reproduction, and the proportion of individuals that breed is an important determinant of population dynamics. Age, experience, individual quality, and environmental conditions have all been demonstrated to influence breeding propensity. To elucidate which of these factors exerts the greatest influence on breeding propensity in a temperate waterfowl, we studied female Lesser Scaup (Aythya affinis) breeding in southwestern Montana. Females were captured during the breeding seasons of 2007–2009, and breeding status was determined on the basis of (1) presence of an egg in the oviduct or (2) blood plasma vitellogenin (VTG) levels. Presence on the study site in the previous year, a proxy for adult female success, was determined with stable isotope signatures of a primary feather collected at capture. Overall, 57% of females had evidence of breeding at the time of capture; this increased to 86% for females captured on or after peak nest initiation. Capture date and size-adjusted body condition positively influenced breeding propensity, with a declining body-condition threshold through the breeding season. We did not detect an influence of age on breeding propensity. Drought conditions negatively affected breeding propensity, reducing the proportion of breeding females to 0.85 (SE = 0.05) from 0.94 (SE = 0.03) during normal-water years. A female that was present in the previous breeding season was 5% more likely to breed than a female that was not present then. The positive correlation between age and experience makes it difficult to differentiate the roles of age, experience, and individual quality in reproductive success in vertebrates. Our results indicate that individual quality, as expressed by previous success and current body condition, may be among the most important determinants of breeding propensity in female Lesser Scaup, providing further support for the individual heterogeneity hypothesis.
Genetic and morphometric diversity in the Lark Sparrow (Chondestes grammacus) suggest discontinuous clinal variation across major breeding regions associated with previously characterized subspecies  
Jeremy D. Ross and Juan L. Bouzat
pg(s) 298–313
Quantifiable geographic variation in DNA or morphology is often used to gauge past and present levels of population interchange and has thus helped define taxonomic boundaries, resolve evolutionary histories, and develop effective conservation strategies to preserve evolutionary diversity. We examined rangewide patterns of genetic (mtDNA and microsatellites) and morphological diversity within the Lark Sparrow (Chondestes grammacus), focusing on diversity patterns across potentially independent Western, Central, and Eastern USA regional breeding zones. Phylogenetic analysis of mtDNA sequences did not support phylogeographic patterns associated with previously characterized subspecies, though the presence of regionally specific clades suggests incipient evolutionary diversification. Regional differentiation was evidenced by significant differences in morphological traits, significant levels of genetic differentiation, reduced estimates of migration among regions, and the characterization of 2 distinct populations through Bayesian clustering. Morphometric and genetic variation distinguished Western Lark Sparrow populations historically characterized as subspecies C. g. actitus from conspecifics across a secondary cline. By contrast, regional variation between Central and Eastern populations, encompassing subspecies C. g. strigatus and C. g. grammacus, was less pronounced and was consistent with a primary cline across the American Great Plains. Our results indicate that clinal variation among populations of long-distance migratory birds may reflect incipient evolutionary divergence, secondary contact zones, and local adaptation of populations to continuously variable environments.
Do nestling Tree Swallows (Tachycineta bicolor) respond to parental alarm calls? 
Emma McIntyre, Andrew G. Horn, and Marty L. Leonard
pg(s) 314–320
Nestling birds use a conspicuous begging display, which includes loud begging calls, to solicit food from their parents. Although these calls are important in communicating offspring need, predators can use begging calls to locate nests. Parents may, however, counteract nestling vulnerability by giving alarm calls to silence calling nestlings when predators are nearby. This defense has been observed in grass- and reed-nesting species, whose nestlings beg to vibrational cues of a parent's arrival, even in the absence of the parents. It is considered less likely to occur, however, in cavity-nesting species, because the rigid substrate of the nest does not provide vibrational cues, and nestlings instead wait for a parental food call before begging. Cavity-nesting Tree Swallows (Tachycineta bicolor) use food calls to solicit begging, which suggests that parental alarm calls are not required to silence them. Older nestling Tree Swallows, however, beg in the absence of their parents, which suggests that responding to parental alarm calls with silence might be adaptive after all, at least at older ages. The goal of our study was to determine whether nestling Tree Swallows alter their begging behavior in response to parental alarm calls and, if so, whether this response varies with age. We found that older nestlings (15 days posthatch) reduced calling and crouched in the nest in response to playback of parental alarm calls, unlike younger nestlings (5 and 10 days posthatch). Our results suggest that parent Tree Swallows might reduce predation risk caused by calling nestlings, especially older nestlings, by giving alarm calls when predators are near their nests. Counter to our prediction, nestlings of cavity-nesting species may indeed respond to alarm calls, particularly if they beg in the absence of parents.


A call for the preservation of images, recordings, and other data in association with avian genetic samples, and the introduction of a solution: OMBIRDS 
Ildiko Szabo, Grant Hurley, Stephanie Cavaghan, and Darren E. Irwin
pg(s) 321–326
Much current and historical research in ornithology employs catch-and-release methods, resulting in a variety of data and materials from birds for which whole-body specimens have not been collected. Often, a genetic specimen (e.g., blood or feathers) is collected along with “media specimens” such as images and/or sound recordings, providing a rich source of research material as well as an opportunity to use each type of specimen as a source of validation of the other. Despite the abundance of these datasets and their potential use in future research, the preservation of such data and associated materials is currently a task that each researcher must confront individually, which results in the loss of these research materials over time. To promote the long-term utility of information collected from the thousands of birds that are captured and released each year, we present a protocol and database template (OMBIRDS; the Online Museum of Bird Images, Recordings, and DNA Samples) for organizing and preserving images, recordings, and data associated with genetic samples. This protocol can be used by individual researchers and institutions to organize their own collections, and it also facilitates submission of records to international data repositories such as VertNet. By contributing OMBIRDS to the research community as a free database tool that can be downloaded and adapted by researchers and institutions, we hope to encourage the collection of media along with genetic samples and to facilitate the archiving of these materials for their use in future research.


Peripheral androgen action helps modulate vocal production in a suboscine passerine 
Matthew J. Fuxjager, Jonathan B. Heston, and Barney A. Schlinger
pg(s) 327–334
Androgenic activation of intracellular androgen receptors (AR) influences avian vocal production, though this has largely been investigated at the level of the brain. We investigated the influence of predominantly peripheral AR on vocal output in wild Golden-collared Manakins (Manacus vitellinus). In this suboscine species, males court females by performing acrobatic displays and by producing relatively simple chee-poo vocalizations. To assess whether peripheral AR influences the acoustic structure of these vocal signals, we treated reproductively active adult males with the peripherally selective antiandrogen bicalutamide and then measured phonation performance. Inhibiting AR outside of the central nervous system increased the duration of the chee note and decreased the fundamental frequency of the poo note. This treatment caused no discernable change to chee-poo frequency modulation or entropy. Our results show that activation of peripheral AR mediates note-specific changes to temporal and pitch characteristics of the Golden-collared Manakin's main sexual call. Thus, our study provides one of the first demonstrations that androgenic action originating outside of the brain and likely on musculoskeletal targets can modulate avian vocal production.
A falconid from the Late Miocene of northwestern China yields further evidence of transition in Late Neogene steppe communities 
Zhiheng Li, Zhonghe Zhou, Tao Deng, Qiang Li, and Julia A. Clarke
pg(s) 335–350
Although the family Falconidae, which includes extant falcons and caracaras, has a long evolutionary history, most previously reported fossils referred to this family are isolated single elements. We report a new species, Falco hezhengensis sp. nov., represented by a nearly complete and articulated skeleton from the Late Miocene deposits of Linxia Basin in northwestern China. The new fossil shares an array of derived morphologies with the genus Falco, and analysis of the largest morphological dataset for Falconidae, sampling most genera, identifies the specimen as a new stem kestrel. The phylogeny shows a high degree of congruence with published molecular phylogenies and time trees supporting a Miocene radiation of Falconidae. The species provides a new calibration for the divergence of extant kestrels from other Falco. Remains of a small mammal, a jerboa (Dipodidae), are preserved in the abdominal region of the specimen. Integrated with data from other avian remains from the Linxia Basin, the new fossil provides further support for changes in the open steppe environment of Central Asia since the Late Miocene. Changes in falconid ecology and diet, shifts in small-mammal abundances, as well as the extinction of the Central Asian ostrich may be involved in community turnover in the Late Neogene.
Influence of climate on annual survival of Barn Swallows (Hirundo rustica) breeding in North America 
Belén García-Pérez, Keith A. Hobson, Gretchen Albrecht, Michael D. Cadman, and Antonio Salvadori
pg(s) 351–362
Population dynamics of migratory birds are influenced by both local weather and larger-scale patterns in climate that can operate at various stages of their annual cycle. We investigated correlations between (1) annual climatic indices and weather during the breeding season and (2) the annual survival of Barn Swallows (Hirundo rustica) breeding at 2 sites in North America. Mark–recapture data collected during a 10-yr period for each of the 2 colonies in eastern and western North America were analyzed to model annual survival probabilities. Annual survival rates of Barn Swallows breeding in Seattle, Washington, USA, were higher in years preceded by El Niño Southern Oscillation (ENSO) winters and higher in years with more positive North Atlantic Oscillation (NAO) values. ENSO was expected to primarily influence wintering conditions through rainfall amount, and NAO was expected to influence climate on the breeding grounds; thus, climatic conditions on both breeding and wintering grounds likely affected the survival of these Seattle-breeding birds. By contrast, annual survival of swallows breeding in southern Ontario, Canada, remained constant over time and were not affected by any of the climatic parameters studied, which suggests that NAO did not have a strong effect on climatic conditions there and/or that these birds winter in regions where ENSO is not strongly correlated with local weather conditions. Alternatively, there may be less geographic variation in wintering-ground locations for Barn Swallows breeding in Seattle, resulting in stronger ENSO effects on survival for the Seattle population. Our results demonstrate how correlations between climate patterns on wintering grounds and annual survival can provide information on migratory connectivity at continental scales and underline the importance of local weather conditions throughout the annual cycle on survivorship and population dynamics of aerial insectivorous birds.
Can wheatears weather the Atlantic? Modeling nonstop trans-Atlantic flights of a small migratory songbird 
Marc Bulte, James D. McLaren, Franz Bairlein, Willem Bouten, Heiko Schmaljohann, and Judy Shamoun-Baranes
pg(s) 363–370
Oceans represent extreme ecological barriers for land birds. Yet the Northern Wheatear (Oenanthe oenanthe leucorhoa), a 25-g songbird, negotiates the North Atlantic Ocean twice yearly between Canadian natal and sub-Saharan wintering grounds. Each autumn, these migrants appear to have 2 options: (1) a detour via Greenland, Iceland, and/or Europe to reduce the extent of open-ocean flights or (2) an astonishing nonstop flight of 4,000–5,000 km without resting opportunities between eastern Canada and northwestern Africa. We assessed the feasibility and reliability of nonstop trans-Atlantic migration of Northern Wheatears from Canada to Africa using an individual-based model incorporating flight costs and autumnal wind data from 1979 to 2011. Prevalent wind conditions were supportive of nonstop migration, especially at high altitudes and when winds at departure were favorable. For modeled individuals with high fuel loads, flying at altitudes of ∼3,000 m, successful nonstop trans-Atlantic flights reached Africa on 62% of departure days. On 24% of unsuccessful departure days, individuals could have first stopped in Europe before continuing to Africa. Durations of successful flights varied between 31 and 68 hr, with significantly shorter flights after mid-September. It remains unclear whether natural selection might favor nonstop ocean crossings by O. o. leucorhoa between North America and Africa, but we conclude that reliably supportive winds en route and potentially huge time savings render it a feasible migration strategy.


Searching for consensus in molt terminology 11 years after Howell et al.'s “first basic problem” 
Jared D. Wolfe, Erik I. Johnson, and Ryan S. Terrill
pg(s) 371–377
Howell et al. (2003) published an innovative augmentation to terminology proposed by Humphrey and Parkes (1959) that classified bird molt on the basis of perceived evolutionary relationships. Despite apparent universal applicability, Howell et al.'s (2003) proposed terminological changes were met with criticism that cited a failure to verify the evolutionary relationships of molt and an inability to recognize homologous molts even within closely related taxa. Eleven years after Howell et al. (2003), we revisit arguments against a terminological system of molt based on evolutionary relationships, suggest an analytical framework to satisfactorily respond to critics, clarify terminology, and consider how to study molt variation within an evolutionary framework.


No evidence of food limitation during the breeding season of a freshwater marsh-nesting tern 
David A. Shealer
pg(s) 378–387
Food availability is considered an important limiting factor in the breeding performance of marine birds, which exhibit restraint in reproductive life-history characteristics (e.g., delayed maturation, small clutch size, slow growth). Less well understood, however, is the extent to which taxonomic analogue species that breed in freshwater habitats are similarly regulated by food availability. Marsh-nesting Forster's Terns (Sterna forsteri) were studied from 2004 to 2008 at Horicon Marsh, a freshwater colony site in Wisconsin, USA, where reproductive success has been chronically poor. The adequacy of the food base to support a breeding colony of terns was evaluated (1) indirectly, through measures of breeding performance correlated with food availability during the egg-laying and incubation stages; and (2) directly, through a supplemental feeding experiment, conducted in 2004 and 2006, to determine whether nestling growth was limited by food availability. Clutch size, egg size, and adult body condition did not differ significantly among years, despite considerable annual fluctuation in environmental conditions and the rapid and extensive colonization of the wetland complex during the study period by a potential food competitor, American White Pelican (Pelecanus erythrorhynchos). Growth rates of chicks were ∼10% higher, on average, in 2006 than in 2004, but no difference was found in mean daily growth rates between food-supplemented and control chicks, nor did chick growth differ according to hatching order in the brood or hatching date. These results suggest that food availability is not a limiting factor during the breeding season for Forster's Terns at Horicon Marsh, the only actively managed breeding site remaining for this species in Wisconsin.
Offspring sex ratio varies according to nest location within a colony of Great Cormorants 
Piotr Minias, Katarzyna Wojczulanis-Jakubas, and Krzysztof Kaczmarek
pg(s) 388–395
Offspring sex ratio in birds is adjusted according to the relative fitness payoff of producing sons and daughters, which is known to depend on parental quality. Therefore, spatial patterns in offspring sex ratio should be consistent with the distribution of pair quality within avian colonies. In many colonial birds, central parts of colonies provide greater safety against predators and thus are occupied by high-quality pairs, who relegate conspecifics of poorer quality to the peripheral zones of colonies. For this reason, we expected that offspring sex ratio was likely to follow similar central–periphery gradients. This hypothesis was tested in a colony of tree-nesting Great Cormorants (Phalacorcorax carbo sinensis) in central Poland, where 204 nestlings from 53 broods were molecularly sexed. We found a clear central–periphery pattern in offspring sex ratio within the studied colony. There was a considerable bias toward male offspring in the broods of centrally nesting pairs, while edge pairs invested more in female progeny. We also found that broods with a higher proportion of male offspring were associated with higher nesting densities. A nonrandom distribution of offspring sex ratio within the studied colony of Great Cormorants indicates that special care should be taken by researchers to randomize sampling of broods when studying sex allocation in colonial species.
Compensatory growth in nestling Zebra Finches impacts body composition but not adaptive immune function 
Tess L. Killpack, Dan Nan Tie, and William H. Karasov
pg(s) 396–406
Compensatory strategies have evolved in birds to minimize the effects of poor rearing conditions on future survival and fitness. Undernourished nestlings may accelerate growth and/or development relative to age, termed compensatory growth, to achieve normal asymptotic size at the same time as well-fed chicks. While compensatory growth allows birds to fledge at a suitable size and time, it may have persistent detrimental effects on the development of complex body systems. Our study examined compensatory growth following short-term food restriction of caged groups of Zebra Finches (Taeniopygia guttata; adult breeding pair and their brood) during the nestling period, and tested for postrestriction effects on tissue maturity (indexed by body tissue water content) and adaptive immune function. Food restriction reduced nestling body mass, tarsus, and culmen growth. Rapid compensatory growth in body mass, but not in tarsus and culmen lengths, was observed upon return to ad libitum feeding of the food-restricted group. Nestlings that were raised in food-restricted treatment cages showed no significant difference in adaptive antibody response to a model antigen compared with those raised in control cages with ad libitum feeding. Reductions in tissue maturity were observed in nestlings that experienced food restriction, indicating a decoupling of chronological age and physiological age. These data provide evidence of capacity for rapid, compensatory body mass growth in nestling Zebra Finches, and suggest that the energetic costs of body growth and development are relatively larger than the costs of development of the humoral immune system.
Resource configuration and abundance affect space use of a cooperatively breeding resident bird 
Richard A. Stanton, Jr., Dylan C. Kesler, and Frank R. Thompson, III
pg(s) 407–420
Movement and space use of birds is driven by activities associated with acquiring and maintaining access to critical resources. Thus, the spatial configuration of resources within home ranges should influence bird movements, and resource values should be relative to their locations. We radio-tracked 22 Brown-headed Nuthatches (Sitta pusilla) and related their space use and home range sizes to available resources while taking nest site locations into account. We developed utilization distributions (UDs) from nuthatch locations, and treated the area of each 95% isopleth as home range size and the height of the UD as relative probability of use. We fit models relating home range size to mean resource measures within home ranges, and used lognormal regression to relate intensity of use to resource metrics at random points by ranking linear mixed models. Nuthatch home ranges typically had two centers of activity. Areas of high use were associated with the density of recently killed snags (likely a foraging resource), recent prescribed fire, pine dominance, low tree stocking rates, and grassy herbaceous cover. Home ranges were generally large (median: 7.1 ha; range: 0.3–47.6 ha), and smaller home range sizes were associated with pine dominance and higher nest snag density. Predicted home range sizes decreased by 77% and 69%, respectively, when percent pine and nest snag density were maximized. Our results illustrate that movement decisions within home ranges are driven by both the availability and spatial distribution of resources, while ongoing savannah-woodland management is providing resources that are used by Brown-headed Nuthatches.
Vocal distinctiveness of the Red Crossbill (Loxia curvirostra) on the island of Newfoundland, Canada 
Douglas P. Hynes and Edward H. Miller
pg(s) 421–433
Ten vocal types of Red Crossbill (Loxia curvirostra) are recognized in North America. Little is known about this species' vocalizations on the island of Newfoundland, where the subspecies L. c. percna (Bent 1912) was described on the basis of study skins collected in a single year from one area; subsequently a distinctive vocal type was proposed on the basis of one sound recording. We made field recordings (∼1000 min) of Red Crossbill vocalizations in Newfoundland to describe vocalizations and compare them with vocal types recognized elsewhere in North America. One class of call (“flight call”; class I call hereafter) was distinctive, and discriminant analysis distinguished 98% of Newfoundland class I calls from mainland North American and European samples. Class I calls of 5 (out of 83) Newfoundland birds resembled recognized vocal types from mainland North America. Other call classes (“excitement/alarm” and “chitter”) of Newfoundland birds also differed from mainland North American samples. We conclude that several vocal types of Red Crossbill, one of which may represent L. c. percna, occur in Newfoundland. Samples from other areas are needed to determine whether the distinctive vocal type in insular Newfoundland (a) represents L. c. percna and (b) is restricted to Newfoundland or also occurs in other areas such as Cape Breton Island.
Resource partitioning in three congeneric sympatrically breeding seabirds: Foraging areas and prey utilization 
G. S. Robertson, M. Bolton, W. J. Grecian, L. J. Wilson, W. Davies, and P. Monaghan
pg(s) 434–446
Morphologically similar sympatric species reduce competition by partitioning resources, for example by occupying different dietary niches or foraging in different areas. In this study, we examine the foraging behavior of Arctic (Sterna paradisaea), Common (Sterna hirundo), and Roseate terns (Sterna dougallii) breeding on Coquet Island, northeast England, using colony-based observations and coincident at-sea visual tracking of foraging birds to quantify interspecific overlap in prey selection and foraging areas. Although visual tracking methods have been used in previous studies, our study is the first example of this method being used to quantify multi-species overlap in foraging areas and the first time Roseate Tern foraging locations have been conclusively identified using a visual tracking method. Percentage overlap in foraging areas varied among species with Arctic and Common terns sharing a higher percentage of their foraging range with each other (63%) than either species did with Roseate Terns (Common = 41% and Arctic = 0%). Arctic and Common terns utilized similar foraging areas and partitioned resources by diet while Roseate Terns differed from other species in both diet and foraging area. Arctic and Common terns varied provisioning rate, prey length, and foraging areas with increasing brood age, while Roseate Terns fed similar prey and foraged consistently inshore. Although there were some similarities in areas utilized by these species, there were sufficient differences in behavior to minimize interspecific competition. Our study further demonstrates the successful use of a visual tracking method to show how morphologically similar sympatric seabird species partition resources by diet, foraging area, and response to increasing brood age.

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