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

URSINO, CYNTHIA; DE MÁRSICO, MARÍA CECILIA; GLOAG, ROS; REBOREDA, JUAN CARLOS

Lund

Congreso; 14th International Behavioral Ecology Congress; 2012

International Behavioral Ecology

A novel egg-rejection method in a host of thescreaming cowbird: antiparasitic defence, nest sanitation behaviour or both? Cynthia A. Ursino1, María C. De Mársico1,Ros Gloag2 and Juan C. Reboreda11Departamento de Ecología, Genética yEvolución, Facultad de Ciencias Exactas y Naturales, Universidad de BuenosAires, Argentina.2 Departmentof Zoology, University of Oxford, South Parks Road, OX1 3PS, United Kingdom Rejectionof foreign eggs by hosts of avian brood parasites is a textbook example of anadaptation evolved in response to the costs of parasitism. Previous studies haveshown that rejector hosts either puncture or grasp eggs to remove them from thenest, but we found that baywings (Agelaioides badius), a host of the screaming cowbird (Molothrus rufoaxillaris) insteadreject eggs by rolling them out of the nest with their feet. We proposethat this novel egg-rejection method is co-optedfrom nest sanitation behaviourswidespread among birds, and has subsequently come under selection pressure asan anti-parasite defence. Exploitation ofparental care by obligate avian brood parasites is costly to hosts and should selectfor defences. A well-known antiparasitic defence is the rejection of parasiteeggs. Hosts larger than the parasite usually remove parasite eggs by graspingthem with their bills, while smaller hosts puncture the parasite egg beforeremoving it. We studied host behaviours towards parasite eggs in the baywing (Agelaioides badius), the primary host ofthe screaming cowbird (Molothrus rufoaxillaris).Most baywing nests are multiply parasitized (90-100%), and 30% of all parasiticeggs are typically laid in advance of hosts? egg-laying. Baywings show intriguingegg rejection habits; they eject all parasitic eggs laid before they starttheir own laying, but accept all those laid subsequently, except when multipleparasitism 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. Usingmicrocameras placed within baywing nests, we recorded for the first time egg-layingby screaming cowbirds and baywings? behaviour towards cowbird eggs. We found baywingsconsistently ejected eggs by rolling them out of the nest using their feet, a behaviourwhich to our knowledge has not previously been reported in any host of a broodparasite. We propose that this singular rejection method is likely anexaptation, derived from nest sanitation behaviours widespread among birds.While baywings benefit from egg rejection, their technique presumably does notallow them to selectively reject parasitic eggs, and explains theirall-or-nothing egg rejection strategy.Keywords: Birds, Cowbirds, Brood parasites