A novel egg-rejection method in a host of the screaming cowbird: antiparasitic defense, nest sanitation behaviour or both?

URSINO, C.A.; DE MÁRSICO, M.C.; GLOAG, R.; REBOREDA, J.C.

Lund

Congreso; 14th International Society of Behavioral Ecology Congress; 2012

ISBE

Exploitation of parental care by obligate avian brood parasites is costly to hosts and should select for defences. A well-known antiparasitic defence is the rejection of parasite eggs. Hosts larger than the parasite usually remove parasite eggs by grasping them with their bills, while smaller hosts puncture the parasite egg before removing it. We studied host behaviours towards parasite eggs in the baywing (Agelaioides badius), the primary host of the screaming cowbird (Molothrus rufoaxillaris). Most baywing nests are multiply parasitized (90-100%), and 30% of all parasitic eggs are typically laid in advance of hosts? egg-laying. Baywings show intriguing egg rejection habits; they eject all parasitic eggs laid before they start their own laying, but accept all those laid subsequently, except when multiple parasitism results in clutch sizes double or more the usual complement of eggs (i.e. 6-8 vs. 3-4 eggs). In the latter cases, they reject the entire clutch, including their own eggs, and begin again. Using microcameras placed within baywing nests, we recorded for the first time egg-laying by screaming cowbirds and baywings? behaviour towards cowbird eggs. We found baywings consistently ejected eggs by rolling them out of the nest using their feet, a behaviour which to our knowledge has not previously been reported in any host of a brood parasite. We propose that this singular rejection method is likely an exaptation, derived from nest sanitation behaviours widespread among birds. While baywings benefit from egg rejection, their technique presumably does not allow them to selectively reject parasitic eggs, and explains their all-or-nothing egg rejection strategy.