A Test of Whether Economy or Nutrition Determines Fecal Sac Ingestion in Nesting Corvids Kevin

Parent birds of many species eat the fecal sacs produced by their nestlings. Two hypotheses have been proposed to explain why the parents ingest, rather than simply remove the sacs. (1) The parental nutrition hypothesis proposes that the parent benefits energetically or nutritionally from ingesting the sacs (Morton 1979, Gliick 1988); and (2) the economic disposal hypothesis postulates that parents incur some costs from eating waste products, but the cost of eating them is less than the benefits gained from being allowed to remain at the nest (Hurd et al. 199 1). Behavioral data on nesting Florida Scrub Jays (Aphelocoma c. coerulescens) and American Crows (Corvus brachyrhynchos) support the parental nutrition, and not the economic disposal hypothesis. In both species, when two parents were present at the production of fecal sacs, the most nutritionally stressed parent, the female, ate significantly more sacs than her mate. On occasions where one adult left the nest immediately after fecal sac production and one remained, the departing adult was not more likely to dispose of the sac in either species. In neither species was a departing adult more likely to carry off a fecal sac than eat it.