Mass Fluctuations Suggest Different Functions of Bimodal Foraging Trips in a Central-place Forager

The function of a foraging trip (self-feeding or provisioning) and the spatial distribution of food resources play an essential role in foraging decisions of central-place foragers. Theory predicts that if foraging patches optimal for self-feeding and provisioning are spatially separated, a bimodal foraging strategy should often be favored. A bimodal foraging strategy involves the alternation of short feeding trips (ST) to collect food for offspring with long trips (LT) for self-feeding, as previously described in some procellariiform seabirds. Using an automated passage recording and weighing system, we tested whether the bimodal trip pattern observed in little auks (Alle alle), a small Arctic alcid, reflects a corresponding functional difference of bimodal foraging trips. We found that the body mass trajectory of little auks was closely related to their bimodal trip pattern. Little auks weighed significantly more returning from a LT and lost an equivalent amount of mass during subsequent ST. The decision to start a LT was not directly related to body mass but seemed to depend on the individual strategy of a bird. During LT, birds likely utilized distant foraging areas suitable for self-feeding while relying on near-shore areas during ST. These results support predictions of recent foraging models and indicate that the function of long and short trips of little auks is similar to procellariiforms independent of large differences in the temporal and spatial scale of feeding trips among these taxa. We suggest that bimodal foraging might be more widespread among central-place foragers than previously thought.