Female Responses to Intraspecific Brood Parasitism in the Tree Swallow

We studied female responses to experimental intraspecific brood parasitism (IBP), or egg-dumping, in Tree Swallows (Tachycineta bicolor). Unlike other species of swallows, Tree Swallow nests are rarely parasitized by conspecifics. We experimentally parasitized nests of Tree Swallows to investigate how females respond to uncertain maternity. Host females accepted a parasitic egg if it was added to the nest within 3 days of the host's first egg (62%). In contrast, the host female buried the parasitic egg (24%) or deserted the nest (14%) when the parasitic egg was added 4 or more days before the host's first egg. The acceptance of parasitic eggs close to the host's own laying date is similar to the behavior reported for other species; however, egg burial and nest desertion appear to be rare as responses to intraspecific brood parasitism. We suggest that the low level of IBP in Tree Swallows has evolved as an indirect consequence of females defending their nest cavity against usurpation.