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Like many seabirds, auks spend most of the year in offshore areas. Information on which oceanic areas they rely on throughout the winter is therefore important in understanding their population dynamics and establishing appropriate conservation measures. The breeding populations of Thick-billed Murres (Uria lomvia), Common Murres (Uria aalge) and Razorbills (Alca torda) in Iceland have been reported declining for the last 30 years. Thick-billed Murres have shown the most alarming rate of decrease, while Razorbills have declined the least. To help understand these changes, we collected information about the non-breeding distribution of these three species by using light-based geolocation. Geolocators were deployed on breeding adults in three different colonies in Iceland in 2013 and 2014. Data showed that the three species’ wintering areas differed substantially. Thick-billed Murres wintered off the west coast of Greenland and East Greenland/Northern Iceland, Common Murres favoured areas around Iceland/East Greenland and to the southwest along the Mid-Atlantic Ridge, and Razorbills were mostly distributed around Iceland. Although some intraspecific variation was evident, we conclude that the population development of Thick-billed Murres in Iceland is likely to be largely influenced by environmental conditions in west Greenland, while Common Murres and Razorbills are more dependent on the oceanic area around Iceland. The results may therefore prove to be an important platform for understanding the population dynamics of these three species in Iceland and informing conservation actions. [ABSTRACT FROM AUTHOR]

ABSTRACT Aim Location Taxa Methods Results Main Conclusions Ecologically similar species living in sympatry are expected to segregate to reduce the effects of competition where resources are limiting. Segregation from heterospecifics commonly occurs in space, but it is often unknown whether such segregation has underlying environmental causes. Indeed, species could segregate because of different fundamental environmental requirements (i.e., ‘niche divergence’), because competitive exclusion at sympatric sites can force species to either change the habitat use they would have at allopatric sites (i.e., ‘niche displacement’) or to avoid certain areas, independently of habitat (i.e., ‘spatial avoidance’). Testing these hypotheses requires the comparison between sympatric and allopatric sites. Understanding the competitive mechanisms that underlie patterns of spatial segregation could improve predictions of species responses to environmental change, as competition might exacerbate the effects of environmental change.North Atlantic and Arctic.Common guillemots Uria aalge and Brünnich's guillemots Uria lomvia.Here, we examine support for these explanations for spatial segregation in two closely‐related seabird species, common guillemots (Uria aalge) and Brünnich's guillemots (U. lomvia). For this, we collated a pan‐Atlantic data set of breeding season foraging tracks from 1046 individuals, collected from 20 colonies (8 sympatric and 12 allopatric). These were analysed with habitat models in a spatially transferable framework to compare habitat preferences between species at sympatric and allopatric sites.We found no effect of the distribution of heterospecifics on local habitat preferences of the focal species. We found differences in habitat preferences between species, but these were not sufficient to explain the observed levels of spatial segregation at sympatric sites.Assuming we did not omit any relevant environmental variables, these results suggest a mix of niche divergence and spatial avoidance produces the observed patterns of spatial segregation. [ABSTRACT FROM AUTHOR]

Density-dependent prey depletion around breeding colonies has long been considered an important factor controlling the population dynamics of colonial animals. 1–4 Ashmole proposed that as seabird colony size increases, intraspecific competition leads to declines in reproductive success, as breeding adults must spend more time and energy to find prey farther from the colony. 1 Seabird colony size often varies over several orders of magnitude within the same species and can include millions of individuals per colony. 5,6 As such, colony size likely plays an important role in determining the individual behavior of its members and how the colony interacts with the surrounding environment. 6 Using tracking data from murres (Uria spp.), the world's most densely breeding seabirds, we show that the distribution of foraging-trip distances scales to colony size0.33 during the chick-rearing stage, consistent with Ashmole's halo theory. 1,2 This pattern occurred across colonies varying in size over three orders of magnitude and distributed throughout the North Atlantic region. The strong relationship between colony size and foraging range means that the foraging areas of some colonial species can be estimated from colony sizes, which is more practical to measure over a large geographic scale. Two-thirds of the North Atlantic murre population breed at the 16 largest colonies; by extrapolating the predicted foraging ranges to sites without tracking data, we show that only two of these large colonies have significant coverage as marine protected areas. Our results are an important example of how theoretical models, in this case, Ashmole's version of central-place-foraging theory, can be applied to inform conservation and management in colonial breeding species. [Display omitted] • The distribution of foraging trips around murre colonies scaled to colony size0.33 • Colony size can be used to predict foraging areass over a large geographic scale • Foraging areas of most North Atlantic murre breeding populations are not within protected areas Seabird colony size can vary over several orders of magnitude. Colony size likely plays an important role in determining individual behavior. Using tracking data from murres, the world's most densely breeding seabirds, Patterson et al. show how the distribution of foraging-trip distances scales to colony size, consistent with Ashmole's halo theory. [ABSTRACT FROM AUTHOR]