Increased strength of Shiny and Brown-headed Cowbird eggshells prevents damage during laying.

FIORINI VD; LOPEZ AV; PEER B; ELLISON K

Congreso; VI North American Ornithological Conference; 2016

Brood parasites lay eggs with shells that are thicker than expected, which resists host puncture-ejection. These strong eggshells may also benefit the parasite by resisting damage when (1) the parasitic egg is laid onto another egg from an elevated position, (2) when the parasitic egg has an egg land on it during host laying, and (3) when the egg is jostled while a host attacks the parasite during laying. We tested these three possibilities using the eggs of the parasitic Shiny Cowbird (Molothrus bonariensis) and its hosts the House Wren (Troglodytes aedon) and Chalk-browed Mockingbird (Mimus saturninus) in South America and the eggs of the parasitic Brown-headed Cowbird (M. ater) and its hosts the House Wren and Red-winged Blackbird (Agelaius phoeniceus) in North America. We experimentally dropped parasite eggs onto host eggs to simulate laying by the parasite, host eggs onto parasite eggs to simulate host laying, and stirred eggs in the nest to simulate the jostling that occurs when a host attacks the parasite during laying. In general, we found that small and large host eggs were significantly more likely to be damaged when struck by a parasitic egg compared to when a host egg struck the parasite egg. There was minimal damage to eggs during the jostling experiments. These findings indicate that thick-shelled cowbird eggs are resistant to damage when laid from an elevated position and that these eggs damage host eggs in the process of being laid.