Resistance is futile: prohibitive costs of egg ejection in an obligate avian brood parasite host

Most hosts of the parasitic brown-headed cowbird, Molothrus ater, demonstrate an astonishing lack of defences against parasitism, typically explained by evolutionary lag. We investigated antiparasite strategies of the dickcissel, Spiza americana, whose apparent acceptance of parasitism is unlikely to be explained by lag because its historic centre of abundance overlaps with that of the cowbird. Cowbirds parasitized almost half of dickcissel nests (343 of 767 nests), and dickcissels suffered significant costs when attempting to eject cowbird eggs. Our predicted responses indicated that acceptance of parasitism would lead to the loss of 0 hosts eggs, attempted ejections would lead to the loss of 1.2 host eggs and successful ejection of cowbird eggs would lead to the loss of 1.6 host eggs. There was no significant cost of raising a single cowbird nestling, but parasitized nests had 1.1 fewer host eggs due to removal by female cowbirds or when the thick-shelled cowbirds eggs struck the host eggs during laying. After accounting for damaged eggs that still hatched, acceptance of parasitism yielded a loss of 1.1 eggs/nestlings, those that attempted to eject the cowbird egg lost 1.8 eggs/nestlings and those that ejected the cowbird egg lost 2.0 eggs/nestlings. The prohibitive costs of egg ejection combined with the relatively low costs of raising a cowbird nestling may explain why most dickcissels (64%) accepted parasitism or stopped trying to eject cowbird eggs. However, some birds persisted in their ejection attempts, so there are likely additional carryover fitness effects on hosts of raising and sharing nests with cowbirds. Because of the difficulty in ejecting cowbird eggs, dickcissels would benefit from a strategy that emphasizes frontline defences to prevent parasitism from occurring in the first place.