Climate-mediated phenological shifts can cause species to lose access to their primary prey while increasing opportunities for alternative-prey encounters. Species that are able to capitalize on alternative resources could potentially profit from prey-switching should the benefits of procuring these alternative resources outweigh their acquisition costs. Polar bears, Ursus maritimus, use sea ice as a platform to hunt seals, and individuals inhabiting the southern-most extent of their range rely on accumulated fat reserves to sustain themselves during the increasingly lengthy ice-free season. In response to declining access to their primary prey through documented sea ice loss, some polar bears are foraging on the eggs of birds in lieu of hunting opportunities on ice, as their onshore arrival is increasingly overlapping with birds’ breeding schedules. To gain a better understanding of the energetic consequences of this behaviour, we used aerial drones to record polar bears foraging on sea duck eggs (common eider, Somateria mollissima) on Mitivik Island, Nunavut, Canada. Using these data, we examined variation in individual polar bear foraging behaviours and estimated the energetic benefits and costs associated with foraging on common eider eggs. Because of low costs associated with nest searching and consumption, the energetic cost of foraging remained relatively constant throughout the 2-week observation period. However, we found that as the common eider breeding season progressed, polar bears consumed eggs at a lower rate as they depleted the nesting colony and spent proportionally more time searching for nests. Collectively, this foraging pattern led to an overall declining trend in the net energy gained from egg consumption. Foraging on common eider eggs during increasingly lengthy ice-free seasons is apparently beneficial for polar bears, but only during a limited window of opportunity. By coupling energetic estimates with detailed behavioural data collected through aerial videography, this study provides a quantification of both the benefits and costs of egg consumption for polar bears.

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北极熊在普通绒毛上觅食：估计气候介导的行为转变的能量后果

气候介导的物候变化可能导致物种无法接近其主要猎物，同时增加与替代猎物相遇的机会。如果获得这些替代资源的收益超过其获取成本，那么能够利用替代资源的物种可能会从猎物转换中获利。北极熊 Ursus maritimus 使用海冰作为捕猎海豹的平台，而居住在其分布范围最南端的个体则依靠积累的脂肪储备在越来越长的无冰季节维持生计。由于记录在案的海冰消失导致无法接近其主要猎物，一些北极熊正在以鸟类的蛋为食，而不是在冰上狩猎，因为它们的陆上到达与鸟类的繁殖时间表越来越重叠。为了更好地了解这种行为的能量后果，我们使用空中无人机记录了北极熊在加拿大努纳武特米蒂维克岛上觅食海鸭蛋（普通绒鸭，Somateria mollissima）的情况。使用这些数据，我们检查了个别北极熊觅食行为的变化，并估计了与在普通绒鸭蛋上觅食相关的能量收益和成本。由于与巢穴搜索和消耗相关的成本较低，在整个 2 周的观察期内，觅食的能量成本保持相对稳定。然而，我们发现随着普通绒鸭繁殖季节的推进，北极熊消耗鸡蛋的速度较低，因为它们耗尽了筑巢群，并相应地花费了更多的时间寻找巢穴。总的来说，这种觅食模式导致从鸡蛋消费中获得的净能量总体呈下降趋势。在越来越长的无冰季节，以常见的绒毛蛋为食显然对北极熊有益，但仅限于有限的机会之窗。通过将能量估计与通过航空摄像收集的详细行为数据相结合，这项研究提供了对北极熊食用鸡蛋的收益和成本的量化。但仅限于有限的机会之窗。通过将能量估计与通过航空摄像收集的详细行为数据相结合，这项研究提供了对北极熊食用鸡蛋的收益和成本的量化。但仅限于有限的机会之窗。通过将能量估计与通过航空摄像收集的详细行为数据相结合，这项研究提供了对北极熊食用鸡蛋的收益和成本的量化。