Quantifying the costs and benefits of migration distance is critical to understanding the evolution of long-distance migration. In migratory birds, life history theory predicts that the potential survival costs of migrating longer distances should be balanced by benefits to lifetime reproductive success, yet quantification of these reproductive benefits in a controlled manner along a large geographical gradient is challenging. We measured a controlled effect of predation risk along a 3350-kilometer south-north gradient in the Arctic and found that nest predation risk declined more than twofold along the latitudinal gradient. These results provide evidence that birds migrating farther north may acquire reproductive benefits in the form of lower nest predation risk.

In seasonal environments, breeding events must be synchronized with resource peaks to ensure production and growth of offspring. As changes in climate may affect trophic levels differentially, we hypothesized that a lack of synchrony between chick hatch and resource peaks could decrease growth rates in chicks of shorebirds nesting in the High Arctic. To test this hypothesis, we compared growth curves of chicks hatching in synchrony with peak periods of food abundance to those hatching outside of these peak periods. We also tested for changes in lay dates of shorebirds in the Canadian Arctic using recent and historical data. Mean daily temperatures during the laying period increased since the 1950s by up to 1.5 °C, and changes in lay dates were apparent for three shorebird species, yet differences in median lay dates between 1954 and 2005–2008 were only significant for White-rumped Sandpiper ( Calidris fuscicollis (Viellot, 1819)). During 2005–2008, there was only 1 year of relatively high synchrony between hatch and resource peaks. Asynchrony between hatch and peaks in Tipulidae biomass reduced growth rates in chicks of Baird’s Sandpiper (Calidris bairdii (Coues, 1861)). As anticipated changes in climate may decouple phenological events, the effects of asynchrony on growth rates of arctic-nesting birds warrant further investigation.

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Determining the manner in which food webs will respond to environmental changes is difficult because the relative importance of top-down vs. bottom-up forces in controlling ecosystems is still debated. This is especially true in the Arctic tundra where, despite relatively simple food webs, it is still unclear which forces dominate in this ecosystem. Our primary goal was to assess the extent to which a tundra food web was dominated by plant-herbivore or predator-prey interactions. Based on a 17-year (1993-2009) study of terrestrial wildlife on Bylot Island, Nunavut, Canada, we developed trophic mass balance models to address this question. Snow Geese were the dominant herbivores in this ecosystem, followed by two sympatric lemming species (brown and collared lemmings). Arctic foxes, weasels, and several species of birds of prey were the dominant predators. Results of our trophic models encompassing 19 functional groups showed that <10% of the annual primary production was consumed by herbivores in most years despite the presence of a large Snow Goose colony, but that 20-100% of the annual herbivore production was consumed by predators. The impact of herbivores on vegetation has also weakened over time, probably due to an increase in primary production. The impact of predators was highest on lemmings, intermediate on passerines, and lowest on geese and shorebirds, but it varied with lemming abundance. Predation of collared lemmings exceeded production in most years and may explain why this species remained at low density. In contrast, the predation rate on brown lemmings varied with prey density and may have contributed to the high-amplitude, periodic fluctuations in the abundance of this species. Our analysis provided little evidence that herbivores are limited by primary production on Bylot Island. In contrast, we measured strong predator-prey interactions, which supports the hypothesis that this food web is primarily controlled by top-down forces. The presence of allochthonous resources subsidizing top predators and the absence of large herbivores may partly explain the predominant role of predation in this low-productivity ecosystem.

Apparent competition between prey is hypothesized to occur more frequently in environments with low densities of preferred prey, where predators are forced to forage for multiple prey items. In the arctic tundra, numerical and functional responses of predators to preferred prey (lemmings) affect the predation pressure on alternative prey (goose eggs) and predators aggregate in areas of high alternative prey density. Therefore, we hypothesized that predation risk on incidental prey (shorebird eggs) would increase in patches of high goose nest density when lemmings were scarce. To test this hypothesis, we measured predation risk on artificial shorebird nests in quadrats varying in goose nest density on Bylot Island (Nunavut, Canada) across three summers with variable lemming abundance. Predation risk on artificial shorebird nests was positively related to goose nest density, and this relationship was strongest at low lemming abundance when predation risk increased by 600% as goose nest density increased from 0 to 12 nests ha−1. Camera monitoring showed that activity of arctic foxes, the most important predator, increased with goose nest density. Our data support our incidental prey hypothesis; when preferred prey decrease in abundance, predator mediated apparent competition via aggregative response occurs between the alternative and incidental prey items.

Monitoring bird nests with cameras provides an opportunity to identify the cause of nest failure and record the behavior of individuals. However, leaving an object continuously within sight of a nest could have potential negative effects on nesting success. We compared daily survival rates of nests monitored using cameras and human visitation to nests tracked via human visitation only to test for potential additional effects of camera monitoring on predation rates. From 2006 to 2008, experiments were conducted on Bylot Island (Nunavut) using 80 artificial nests and 53 real nests of Baird's Sandpipers (Calidris bairdii) and White‐rumped Sandpipers (Calidris fuscicollis). Rates of predation on real and artificial nests varied considerably among years. However, survival rates of camera‐monitored nests did not differ from those of nests monitored without cameras. Predators of artificial nests included Arctic foxes (Vulpes lagopus), Glaucous Gulls (Larus hyperboreus), and Long‐tailed Jaegers (Stercorarius longicaudus), whereas Arctic foxes were responsible for all camera‐recorded predation events at real nests. Camera monitoring should be promoted as a viable method for monitoring nests of Arctic shorebirds because our results indicate that placing cameras at nests does not bias estimates of nest survival obtained via nest visits.

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