EGGSHELL STRENGTH IN CUCKOOS AND COWBIRDS

Parasitic cuckoos of the genus Clamator and parasitic cowbirds Molothrus spp. lay eggs that appear to be stronger than those of their hosts. Similar differences in eggshell strength between the eggs of the Cuculus/Cacomantis/Chrysococcyx cuckoos and their hosts are not apparent. We suggest that the advantage of a strong eggshell in the Clamator and Molothrus species is to protect the parasite's egg from damage if the nest is multiply parasitized. This suggestion relies on the observation that Clamator and Molothrus species commonly lay more than one egg per nest and often remove or damage eggs in the process.