Niche construction through a Goldilocks principle maximizes fitness for a nest-sharing brood parasite

Generalist brood parasites that share nests with host nestlings can optimize resource acquisition from host parents by balancing the benefits that host nest-mates provide, including attracting increased provisions to the nest, against the costs of competing with the same host young over foster parental resources. However, it is unclear how parasitic chicks cope when faced with more nest-mates than are optimal for their survival upon hatching. We suggest that, in the obligate brood parasitic brown-headed cowbird (Molothrus ater), chicks use a niche construction strategy and reduce larger, more competitive host broods to maximize the parasites' survival to fledging. We experimentally altered brood sizes to test for Goldilocks principle patterns (i.e. a 'just right' intermediate brood size) of cowbird survival in nests of prothonotary warbler (Protonotaria citrea) hosts. We found that intermediate brood sizes of two host nestlings maximized cowbird fledging success relative to 0 or 4 host nest-mates at hatching. Specifically, cowbird nestlings lowered host brood sizes towards this optimum when placed in broods with more host nestlings. The results suggest that cowbirds reduce, but do not eliminate, host broods as a niche construction mechanism to improve their own probability of survival.