Video recordings of Brown-headed (Molothrus ater) and Shiny (M. bonariensis) cowbirds reveal oviposition from an elevated position: Implications for host-parasite coevolution

Abstract

Molothrine cowbirds are obligate brood parasites that lay eggs within 5-10 s in the minutes before sunrise. Robust analysis of their position at laying has therefore only been made possible with the development of high-resolution and low-light video recording technologies. From video recordings, we observed the moment of oviposition clearly in 8 and 31 instances of egg laying by Brown-headed (Molothrus ater) and Shiny (M. bonariensis) cowbirds, respectively. Both species laid from an elevated position such that their eggs dropped into nests from an estimated height of 4-8 cm. We propose this elevated laying position has selected for high eggshell strength in cowbirds, because stronger eggshells would be less likely to be damaged during egg laying. Stronger eggshells might also damage other eggs when dropped into the nest, thereby further reducing nestling competition from nest mates and drive selection for cowbird eggshell strength. A role for laying position in the evolution of cowbird egg strength is not mutually exclusive of the view that thickly shelled eggs have evolved as a defense to egg rejection by hosts and/or egg puncture behavior of cowbirds.