The Young Cowbird: Average or Optimal Nestling?

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Abstract
Whether the young of the brown-headed cowbird are more successful than those of nonparasitic passerines when raised by other species was studied. Eggs and nestlings of 7 spp. were placed in barn swallow [Hirundo rustica] and house sparrow [Passer domesticus] nests, and the hatching success and nestling survival rates compared. The hatching success was high for all species in both host species'' nests. In the barn swallow nest, the nestlings of 4 of the 5 nonparasitic species did as well as the cowbird nestlings. The 3 foreign species placed in the house sparrow nest, including the cowbird, all failed to survive through the nestling stage. A red-winged blackbird [Agelaius phoeniceus], a catbird [Dumetella carolinensis] and a cowbird were observed immediately after departing from their barn swallow host nests; only the cowbird elicited feedings from its host parents. The young cowbird fared no better than the other species as a nestling. Although cowbirds can be reared by their most frequent host species, they may not be particularly successful with some of their less common hosts. They may be especially successful as a fledgling.