Pierre Legagneux

As sea-ice cover is shrinking, polar bears ( Ursus maritimus, Phipps, 1774) face decreased access to seals, their primary prey, resulting in a greater dependence on terrestrial food sources. Whether polar bears can benefit from these terrestrial food sources, however, depends on their ability to find and capture prey items without expending more energy than is acquired. Here, we report one of the northernmost observations of polar bear predation on adult birds. The bear used a dive-hunting technique, which consisted of submerging itself, approaching underwater, and catching flightless greater snow geese ( Anser caerulescens caerulescens (Linnaeus, 1758)) from beneath the surface of a tundra pond. After evaluating energy expenditures during swimming and energy intakes from consuming geese, we estimated that this rarely documented dive-hunting technique could be energetically profitable for a certain range of pursuit durations. This observation highlights the behavioral plasticity that polar bears can deploy to punctually exploit land-based food sources.

Reproduction of long-distance migrants can be affected by local conditions on the breeding grounds as well as those encountered during the nonbreeding season through carry-over effects. We show that this is true in Long-tailed Jaegers (Stercorarius longicaudus) because individuals that spent less time flying at sea during winter had a high breeding propensity and a reduced pre-laying interval, but breeding propensity and nesting success were also positively associated with food abundance at the breeding site. This seabird switches from a marine to a terrestrial lifestyle to breed in summer in the Arctic, where it primarily feeds on lemmings. We monitored jaeger reproduction and lemming densities on Bylot Island in the Canadian Arctic for 16 years, and we used geolocator to study annual movements. We assessed whether movement parameters (travel distance, migration duration, phenology, and number of flying bouts inferred by saltwater immersions) during the nonbreeding season affected the breeding propensity, phenology, and success of individuals. We also examined whether cyclic lemming fluctuations influenced Long-tailed Jaeger reproduction and whether nesting success affected the phenology of their outbound migration. We found that increased time spent flying during winter and early arrival at the breeding site reduced breeding propensity. Moreover, spending less time flying during winter shortened the pre-laying period, and advancing laying date increased nesting success. Birds may thus face a trade-off to minimize the relative costs associated with arriving too early and breeding too late. Local food availability had a strong effect on reproduction because breeding propensity and nesting success increased sharply with lemming abundance. Failed breeders advanced their outbound migration by 10 days on average compared to successful ones, but migration duration was similar. Therefore, the unpredictability of the highly seasonal Arctic environment, especially fluctuating food abundance, appears to be a strong driver of reproduction that can modulate the strength of carry-over effects.