A phenological comparison of grizzly (Ursus arctos) and polar bears (Ursus maritimus) as waterfowl nest predators in Wapusk National Park

Relatively little is known about the feeding ecology of grizzly bears ( Ursus arctos ) in Wapusk National Park. Other ursids, polar bears ( Ursus maritimus ), are well known predators of waterfowl nests in the area, and grizzly bears could feasibly make use of such resources. However, since the arrival of polar bears on land in the spring is largely dependent on date of sea-ice breakup, the timing of predation by each bear species may differ. We investigated the timing of bear predation in common eider ( Somateria mollissima sedentaria ) and lesser snow goose ( Anser caerulescens caerulescens ) colonies from 2012 to 2018 using nest cameras. We observed grizzlies consuming eggs of both species in all years except 2013 and confirmed that at least two individual grizzlies occupied the park in 2016. In the 3 years when both grizzly and polar bears were detected, grizzlies were observed earlier in the year than polar bears and had greater overlap with an estimated availability index of incubating waterfowl. We hypothesize that grizzly bears could have earlier access to waterfowl eggs in the park, potentially reducing availability of these terrestrial foods to polar bears.