Coevolutionary arms race between the brood parasitic Shiny Cowbird and one of its hosts, the Rufous Hornero.

REBOREDA, J.C.; MASSONI, V.; DI GIACOMO, A.G.

Cornell University, USA

Congreso; 12th Biennial Congress of the International Behavioral Ecology Society; 2008

Rufous hornero ejects shiny cowbird eggs narrower than its own eggs. It has been proposed that this antiparasitic defense may have selected for an increase of parasite´s egg size in some areas of its distribution. We measured egg and body size of host and parasite and conducted artificial parasitism experiments with small and large mimetic parasite eggs and with control host eggs in two host populations that differ in the frequency of parasitism: Buenos Aires (2%) and Formosa (59%). We found that: 1) rufous horneros were larger and their eggs wider and longer in Buenos Aires than in Formosa, 2) shiny cowbirds were larger in Buenos Aires than in Formosa, but their eggs were wider and more rounded in Formosa than in Buenos Aires; 3) parasite and host eggs did not overlap in width in Buenos Aires but they overlapped widely (50% of the eggs) in Formosa; 4) rufous horneros ejected small parasite eggs and accepted control eggs at both places, but they ejected large parasite eggs in Buenos Aires and accepted them in Formosa; 5) Accepted shiny cowbird eggs in naturally parasitized hornero nests in Formosa were wider than those in nests of sympatric hosts that do not eject parasite eggs using egg size as a cue. Our results indicate that rufous horneros have selected for wider parasitic eggs in Formosa but not in Buenos Aires. We discuss how selection for life history traits, like body or clutch size, may constrain coevolutionary processes between hosts and parasites.