As nestlings, male Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) are significantly larger than female Yellow-headed Blackbirds in several measurements, and both male and female Yellow-headed Blackbirds are significantly larger than Brown-headed Cowbirds (Molothrus ater). We investigated the mechanisms by which female Yellow-headed Blackbirds are able to survive in nests with their larger siblings and how cowbirds are able to survive in the nests of hosts much larger than themselves. Growth was monitored of nestling Yellow-headed Blackbirds and Brown-headed Cowbirds, which were experimentally cross-fostered into Yellow-headed Blackbird nests, in Boulder County, Colorado during the 1986 breeding season. Measurements were recorded for weight, tarsometatarsus length, culmen length, gape width (width of bill at loral feathering), and length of ninth (outermost) primary. Nestling weight, tarsometatarsus length, and culmen length were larger in male than female Yellow-headed Blackbirds, and larger in both male and female Yellow-headed Blackbirds than cowbirds. Gape width differed significantly between male and female Yellow-headed Blackbirds throughout much of the nestling period; gape width in both were significantly wider than that of cowbirds. However, cowbirds had a significantly larger gape relative to their weight than did male Yellow-headed Blackbirds, and female Yellow-headed Blackbirds had a significantly larger gape relative to their weight than did males of the species. The relatively larger gape may enable females to compete with larger male siblings and enable cowbirds to compete with foster siblings much larger than themselves. Additionally, feather development was faster in female Yellow-headed Blackbirds than in males and was even more accelerated in cowbird nestlings. We used museum specimens of adults to calculate proportions of adult weight attained by nestlings. Cowbirds attained a greater proportion of their adult weight and adult ninth-primary length by fledging age than did Yellow-headed Blackbirds, and female Yellow-headed Blackbirds attained a greater proportion of their adult weight and adult ninth-primary length by fledging age than did their male siblings.
