Gapes of sexually dimorphic blackbird nestlings do not show sexually dimorphic growth

Within broods of many altricial birds, one or more nestlings is often predictably smaller than the rest. This occurs both because of hatching asynchrony within broods and also because one sex rapidly becomes larger than the other in sexually dimorphic species. If parents rely on competitive, size-based cues from nestlings in deciding which to feed, this size asymmetry presents a major predictable problem to younger or smaller nestlings. A recent study of sexually dimorphic Yellow-headed Blackbirds (Xanthocephalus xanthocephalus) suggested that the gape, a prominent feature of the begging signal of hungry young, may grow more rapidly in female nestlings than males. In this paper, I report similar data on nestling gape growth in another sexually dimorphic species, the Red-winged Blackbird (Age-laius phoeniceus), and show that the apparent relatively faster gape growth of females in both Yellow-headed and Red-winged blackbirds is an artifact of the analytical method used in the comparison. Gapes of males and females grow in exactly the same relation to their mass and, to the extent that the mass of females is less at a given age, females will have smaller gapes. I further analyze gape growth in a group of undernourished nestling Red-winged Blackbirds to evaluate the related prediction that, when resources for growth are limited, nestlings should conserve the growth of those morphological characteristics such as gapes that directly influence feeding. The data yield little support for this hypothesis. The specialized and prominent nestling gapes, however, may have important functions beyond feeding competition.