Differential Growth Patterns of Nestling Brown-Headed Cowbirds and Yellow-Headed Blackbirds

-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 Yellowheaded 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. Received 29 October 1990, accepted 10 January 1992. GROWTH rates vary greatly among animals and, although few grow at the highest potential rate (Needham 1964), they may grow at the maximum potential rate (Ricklefs 1969). Sibling competition may be a major component in the selection for rapid growth (Werschkul and Jackson 1979) and, in some species, may result in brood reduction (Bortolotti 1986). Brood reduction may occur through siblicide (Mock 1984) or asynchronous hatching (Lack 1954, Richter 1982, Mead and Morton 1985). Among sexually dimorphic, altricial birds that also hatch asynchronously, one might expect a high degree of sibling competition resulting in a biased sex ratio. While some investigators of sexually dimorphic birds have found skewed sex ratios of nestlings or fledglings (Howe 1977, Slagsvold 1 Present address: Department of Biology, Fort Lewis College, Durango, Colorado 81301, USA. et al. 1986, Teather and Weatherhead 1989), others have reported that sex ratios were not significantly skewed (Selander 1960, Fiala 1981, Bancroft 1983, Weatherhead 1983). Interestingly, in all the above-listed studies with skewed sex ratios, the sex ratios were female-biased. Explanations for female-biased sex ratios include: depending on hatching sequence, males suffer higher mortality during food shortages (Slagsvold et al. 1986, Teather and Weatherhead 1989); and males are more costly to raise (Howe 1977, Slagsvold et al. 1986). None of the above-listed studies, however, have shown how females may compete with their larger male siblings for food. Richter (1983) reported no significant bias in sex ratios for Yellow-headed Blackbirds (Xanthocephalus xanthocephalus), a sexually dimorphic, asynchronously hatching species. He demonstrated a significant difference between nestling growth rates of males and females, but suggested that there may be little disparity in