Foraging decisions of American robins: somatic and reproductive tradeoffs

Summary

We examined the behavior of a central place forager, the American robin (Turdus migratorius), with emphasis on the self-feeding component of foraging bouts. Among individuals the mean percentage of all prey captured that was consumed by adults varied from 10% to 60% (x ; 31%). Size of prey appeared important in determing whether the prey was eaten; adults generally consumed small prey and fed larger prey items to their young. In addition, females consumed more prey per bout than males, although rates of prey consumption were similar between sexes. For those robins in which a correlation was demonstrated between bout duration and amount of prey captured, amount of prey eaten by adults influenced bout length more strongly than size of the load. Attempts to model the robins' foraging process indicated that prey size, quantity of prey in the bill, and time away from the nest were important in determining when to terminate a bout. The rate at which robins foraged in their territory increased with age of the nestlings. In addition, mated pairs partitioned territories roughly in half, presumably as a mechanism for increasing foraging efficiency in a fairly homogeneous environment. Foraging activity declined as distance from the nest increased, but not in the manner predicted by optimality models.