Strength and properties of the eggshell in two brood parasitic species and their hosts

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

Cowbirds (Molothrus sp.) are avian brood parasites that puncture eggs at the nests of their hosts. Cowbird eggshells are thicker than expected by allometry. It was proposed that the cowbird eggshell thickness increases the egg?s mechanical strength and protects it from punctures of other cowbirds during events of multiparasitism. We evaluated and compared, through tests of quasi-static punctures, the strength of eggs between: (a) the shiny cowbird (M. bonariensis) and two of its hosts, the chalk-browed mockingbird (Mimus saturninus, a large host) and the house wren (Troglodyte aedon, a small host); (b) the screaming cowbird (M. rufoaxillaris) and its main host, the baywing (Agelaioides badius, similar in size to its parasite). Furthermore, we studied the shell thickness and microstructure of calcite crystals that constitute the eggshell. The strength of the cowbird eggs was higher than the strength of the host eggs. The strength did not differ between mockingbird and baywing eggs, but was higher that the strength of wren eggs. The screaming cowbird eggshell was thicker than the eggshell of the rest of the species. The shell thickness did not differ between shiny cowbird and baywing eggs, but was higher than the shell thickness of chalk-browed mockingbird eggs, which in turn was higher than the shell thickness of house wren eggs. The egg?s strength was not related to the thickness of their shells, but cowbirds had a higher number of crystals of relatively smaller size. This feature could explain the differences among the strength of the eggs of the species.