Shiny and screaming cowbirds prefer to peck weaker eggs

COSSA NATALIA A.; LOPEZ AV; REBOREDA, JUAN C.; FIORINI VANINA D.

New York

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

ISBE

punctured, which reduces the future food competition for the cowbird?s offspring. On arrival at a host nest, cowbirds mayencounter both host and other parasite eggs, and we hypothesized that they may prefer to peck either the largest egg (toeliminate the future strongest competitor), or the weakest egg (as it is easier to break and so guarantees fewer futurecompetitors). 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 usednatural eggs of: shiny cowbirds, a large host and a small host. For the screaming cowbirds we used natural eggs of: its mainhost (Agelaioides badius), similar in size to both parasitic species), shiny cowbirds (which also parasitizes A. badius) andscreaming 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-sizedhosts. Screaming cowbirds pecked host eggs (i.e. less resistant eggs) more than the screaming cowbird egg, but did not peckdifferently the eggs of similar resistances. We conclude that female cowbirds can recognize weaker eggs and prefer to peckthem, increasing the probability of reducing the clutch size and the future food competition for their parasitic chick.