Shiny and screaming cowbirds prefer to peck weaker eggs

COSSA, N.; LÓPEZ, A.V.; REBOREDA, J.C.; FIORINI, V. D.

New York

Congreso; 15th Conference of the International Society of Behavioral Ecology; 2014

International Society of Behavioral Ecology

When parasitic cowbirds (Molothrus sp.) arrive at a nest they peck the eggs. Pecks sometimes result in an egg?s shell being punctured, which reduces the future food competition for the cowbird?s offspring. On arrival at a host nest, cowbirds may encounter both host and other parasite eggs, and we hypothesized that they may prefer to peck either the largest egg (to eliminate the future strongest competitor), or the weakest egg (as it is easier to break and so guarantees fewer future competitors). We analyzed the egg-pecking preferences of captive females of the generalist shiny cowbird (M. bonariensis) and the specialist screaming cowbird (M. rufoaxillaris) when presented with two-eggs clutches. For shiny cowbirds we used natural eggs of: shiny cowbirds, a large host and a small host. For the screaming cowbirds we used natural eggs of: its main host (Agelaioides badius), similar in size to both parasitic species), shiny cowbirds (which also parasitizes A. badius) and screaming cowbirds. For each type of egg the volume and the resistance was determined. Shiny cowbirds pecked host eggs (i.e. less resistant eggs) more than shiny cowbird eggs, and there were no differences between the eggs of different-sized hosts. Screaming cowbirds pecked host eggs (i.e. less resistant eggs) more than the screaming cowbird egg, but did not peck differently the eggs of similar resistances. We conclude that female cowbirds can recognize weaker eggs and prefer to peck them, increasing the probability of reducing the clutch size and the future food competition for their parasitic chick.