The Young Cowbird: Average or Optimal Nestling?

Abstract

We studied whether the young of the Brown-headed Cowbird are more successful than those of nonparasitic passerines when raised by other species. Eggs and nestlings of seven species were placed in Barn Swal- low and House Sparrow 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 four of the five nonparasitic species did as well as the cowbird nestlings. On the other hand, the three foreign species placed in the House Sparrow nest, including the cowbird, all failed to survive through the nestling stage. In addition, a Red- winged Blackbird, a catbird, and a cowbird were observed immediately after departing from their Barn Swallow host nests; only the cowbird elicited feed- ings from its host parents. Thus 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, a hypothesis that awaits further investigation. The young of many avian brood parasites possess striking adaptations to their unusual rearing situations. The eggs, nestling ap- pearance and mouth markings of several parasitic widowbirds (Vidua spp.), for ex- ample, are almost indistinguishable from each species' respective host (Nicolai 1974). In addition, the begging postures and vocal- izations of each widowbird species match those of its host species. Newborn cuckoos (Cuculus spp.) of parasitic species eject the eggs or other nestlings from the nest by ma- neuvering them onto their slightly concave back and pushing them out. A newly hatched Lesser Honeyguide (Indicator mi- nor) has a mandibular hook with which it attacks and kills its nestmates (Skutch 1976). The Brown-headed Cowbird (Molothrus ater) is known to use more host species than any other brood parasite: its eggs have been found in the nests of 217 species and young