Parental investment and primary sex ratio of brood parasitic shiny cowbirds in two hosts that differ in body size

TUERO, D.T.; FIORINI, V.D.; MAHLER, B.; REBOREDA, J.C.

Cornell University, USA

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

The shiny cowbird, Molothrus bonariensis, is a sexually dimorphic brood parasite (males are 20-30% larger than females). This sexual dimorphism could arise from differences in size of eggs determined as males or females and/or develop during the nestling period. Besides, sexual differences in competitive ability of parasitic chicks may result in differential survival of males and females depending on host body size. In this case, we expected that parasitic females invest differentially in a particular sex depending on fitness of sons and daughters raised in different hosts. We studied parental investment of female shiny cowbirds when they parasitize hosts smaller (house wren, Troglodytes aedon) and larger (chalk-browed mockingbird, Mimus saturninus) than the parasite. For each host we analyzed: 1) size of male and female cowbird eggs, 2) growth of male and female cowbird chicks, and 3) primary sex ratio. There were no sexual differences in size of cowbird eggs or weight at hatching of cowbird chicks, but eggs were larger and chicks at hatching were heavier in mockingbird that in wren nests. In both hosts, the asymptotic weight of the parasite chick was higher in males than in females. In both hosts sex ratio was not skewed. Our results indicate that female shiny cowbirds can adjust parental investment in eggs depending on host characteristics but they do not skew primary sex ratio or invest differentially in males or females.