Short-term foraging costs and long-term fueling rates in central-place foraging swans revealed by giving-up exploitation times

Foragers tend to exploit patches to a lesser extent farther away from their central place. This has been interpreted as a response to increased risk of predation or increased metabolic costs of prey delivery. Here we show that migratory Bewick's swans (Cygnus columbianus bewickii), though not incurring greater predation risks farther out or delivering food to a central place, also feed for shorter periods at patches farther away from their roost. Predictions from an energy budget model suggest that increasing metabolic travel costs per se are responsible. Establishing the relation between intake rate and exploitation time enabled us to express giving-up exploitation times as quitting harvest rates (QHRs). This revealed that net QHRs were not different from observed long-term net intake rates, a sign that the birds were maximizing their long-term net intake rate. This study is unique because giving-up decisions were measured at the individual level, metabolic and predation costs were assessed simultaneously, the relation with harvest rate was made explicit, and finally, short-term giving-up decisions were related to long-term net intake rates. We discuss and conceptualize the implications of metabolic traveling costs for carrying-capacity predictions by bridging the gap between optimal-foraging theory and optimal-migration theory.