Summary
Parent birds generally eat their nestlings' feces when the nestlings are young and carry the feces from the nest as the nestlings get older. This change in behavior may be due to the decline in energy content of the feces as the nestlings' digestive system becomes more efficient, or because the cost to the parent from eating nestling feces increases with the volume and number of fecal sacs eaten. In tree swallows (Tachycincta bicolor), red-winged blackbirds (Agelaius phoenice-us), and American robins (Turdus migraiorius), parents ate a smaller proportion of fecal sacs as their nestlings got older, even though the energy density of fecal sacs remained constant with nestling age. The increase in size of fecal sacs with nestling age explains the decline in parental consumption much better than does energy content. These results better support the hypothesis that eating fecal sacs is an economic alternative to carrying them from the nest and is not done so that parents use the sacs as a source of energy. The benefits of not carrying a fecal sac from the nest may be that parents can remain at the nest longer for other purposes (e.g., brooding) and that they avoid the transportation costs associated with fecal sac disposal. Understanding the costs and benefits of fecal sac consumption may explain both intersexual and interspecific differences in this behavior. [Behav Ecol 1991, 2: 69–76]