Age-related differences in diet and foraging behavior of the critically endangered Mariana Crow (Corvus kubaryi), with notes on the predation of Coenobita hermit crabs

Knowledge of foraging behavior across life stages of endangered species is important for identifying potential drivers of age-dependent mortality. Juvenile mortality is a primary threat to the persistence of the single remaining Mariana Crow (Corvus kubaryi) population, which is found on Rota, Commonwealth of the Northern Mariana Islands. Therefore, variation in foraging behavior among fledglings, sub-adults and adults may highlight different needs or susceptibilities that could inform age-specific management strategies. During observations of 36 Mariana Crows, we found that of all food captures, 14% were plant-based, 56% were insects or their larvae and eggs and 30% were non-insect animal prey. Two food categories, fruits/seeds/plants and ants/termites/larvae, which were procured and processed with simple behaviors, were taken more frequently by fledglings. Crabs, which were processed using complex behaviors, were captured more frequently by adults. Adults acquired more food items from the ground than did fledglings and sub-adult birds, a result that was driven by the former's high level of crab predation. We did not detect differences in foraging behavior between wet and dry seasons, suggesting that Mariana Crows maintain a similar diet year-round. Overall, our results highlight age-related differences in foraging behavior; however, future studies should identify whether these differences drive age-dependent variation in survivorship. Finally, we suggest that complex trophic interactions between non-native snails and Coenobita hermit crabs may have modified Mariana Crow foraging behavior, increasing their vulnerability to feral cat predation.