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

Do foraging decisions tell us something about the cost of foraging? Dutch swan experts at the Netherlands Institute of Ecology (NIOO-KNAW) were interested in the apparent heterogeneity in the use of agricultural fields by migratory Bewick’s swans. In Wieringermeer, a typical Dutch polder that was reclaimed from the sea in 1930, Bewick’s swans stop by during migration to feed on the leftovers of sugarbeets that remain after the farmer’s harvest. The ornithologists were interested in why some fields were used more intensively than others, while all fields seem to offer similar amounts of food right from the start. They wondered: “In case foraging benefits seem to be equal across all fields, there may be variation in foraging costs that may explain the observed spatial heterogeneity in exploitation.” The authors showed that the time spent feeding on a field by a flock of swans can be used as a surrogate of an individual’s energy intake rate (which declines over time due to depletion). This showed that an individual’s intake rate at abandoning a field, which should reflect the foraging cost as experienced by the bird itself, decreased with the distance to roads and increased with the distance to its nighttime roost. The higher intake rates at which fields near roads were given up reflected the chance to be disturbed by humans. The higher intake rates at which fields far from roosts were given up could be explained by the energetic cost of flight, which are substantial for large birds such as Bewick’s swans. When taking these flight costs into account, the authors calculated that the net benefits at which each field was given up corresponded with the net benefits obtained in the long run. The latter was measured via regular visual inspection of the thickness of each bird’s belly! All these insights into foraging costs could be implemented into a model that predicted the number of swans that potentially could stop by at the site. This model accurately and correctly predicted the true number of birds that made use of the stopover. It could thus serve as a valuable tool to manage and protect the relatively small population of Bewick’s swans wintering in NW Europe, which declined from 30,000 to 20,000 birds over the last 15 years.