Parental consumption of nestling feces: good food or sound economics?

Parent birds generally eat their nestlings' feces when the nestlingsare young and carry the feces from the nest as the nestlingsget older. This change in behavior may be due to the declinein energy content of the feces as the nestlings' digestive systembecomes more efficient, or because the cost to the parent fromeating nestling feces increases with the volume and number offecal sacs eaten. In tree swallows (Tachycincta bicolor), red-wingedblackbirds (Agelaius phoenice-us), and American robins (Turdusmigraiorius), parents ate a smaller proportion of fecal sacsas their nestlings got older, even though the energy densityof fecal sacs remained constant with nestling age. The increasein size of fecal sacs with nestling age explains the declinein parental consumption much better than does energy content.These results better support the hypothesis that eating fecalsacs is an economic alternative to carrying them from the nestand is not done so that parents use the sacs as a source ofenergy. The benefits of not carrying a fecal sac from the nestmay be that parents can remain at the nest longer for otherpurposes (e.g., brooding) and that they avoid the transportationcosts associated with fecal sac disposal. Understanding thecosts and benefits of fecal sac consumption may explain bothintersexual and interspecific differences in this behavior.Behav Ecol 1991, 2: 69–76]