Behavioural dynamics of intraspecific brood parasitism in colonial cliff swallows

Colonial cliff swallows, Hirundo pyrrhonota, in siuthwestern Nebraska commonly broad-parasitize (lay eggs in) nests of neighbouring conspecific. At least 22%, and perhaps as many as 43%, of all nests were estimated to contain one or more parasitic eggs. Colour-marked parasites maintained nests of their own in addition to parasitizing the nests of others. Parasitism usually occurred among close neighbours when nests were left unattended momentarily. Parasites' and hosts' first laying dates tended to be synchronized. Parasitism often occurred early in a host's laying period of 1–3 days prior to the host's start of egg laying. Hosts rejected any eggs added more than 4 days before they began laying. Hosts laid smaller clutches than normal, either as a direct response to being parasitized or because parasites preferentially selected hosts that were to lay small clutches for other reasons. Parasitic egg laying appeared successful, with only about a quarter of parasitic eggs known not to survive. On an annual basis, colour-marked parasites laid more total eggs and fledged more total young than did colour-marked hosts. Parasites, however, were also parasitized themselves, and thus being a parasite was costly. There was little evidence that parasites routinely removed host eggs. Parasitism tended not to occur in extremely small colonies and among solitary nesters, but there was no relationship between incidence of parasitism and colony size for colonies with more than 10 nests. Intraspecific brood parasitism probably serves to minimize risk in uncertain environments by distributing eggs in several nests to ensure that at least one offspring will survive to independence. Incidence of blood parasitism in a colony increased with uncertainty of reproduction.