Autumn migration phenology of polar bears (Ursus maritimus) in Hudson Bay, Canada

Migratory species may shift phenology with climate change so understanding the energetic consequences of behavioural plasticity may provide insight into how migrants respond to warming. We examined the autumn migration of 151 adult female polar bears (Ursus maritimus) in Hudson Bay, Canada from 1991 to 2020 using satellite telemetry. We examined trends in sea ice freeze-up experienced by bears as adults and dependent juveniles to assess if maternal behaviour influenced migrating adults. To quantify vulnerability to increasing fasting, we examined the condition and starvation threshold of 136 females with offspring. There were no temporal trends in departure date or sea ice, but bears departed onto sea ice further north over time by approximately 7 km/year (SE 12 km). Freeze-up date was delayed 0.4 (SE 0.8) days/year for the time when juveniles were dependent on their mothers. Departure date varied across individuals each year by a mean difference of 14 (SE 1.5) days. Inter-annual variation in the on-ice departure date was explained by freeze-up date, delaying in years with later freeze-up, and individual age, with younger bears departing earlier. Condition of females with first-year cubs declined over time, and we estimate that 67% of females with offspring have ceased lactation by on-ice departure or risk starvation while 20% still risk starvation having ceased lactation. Delayed freeze-up over the juvenile period may explain the population's northward shift in migration suggesting learned behaviour. Declining energetic stores available to females with cubs during this critical stage in their life history is a developing conservation issue.

Automated summary

This study on the autumn migration of polar bears in Hudson Bay, Canada reveals that climate change can affect their migration and reproductive behavior. It shows that the delay in sea ice freeze-up leads to a northward shift in polar bear migration, causing fasting issues for adult female bears and their dependent offspring. This research has important implications for the conservation of polar bear habitat and survival capabilities.