Environmental component of morphological differentiation in birds.

Geographic character variation in birds is usually attributed to natural selection for phenotypes that reflect locally adapted genetic differences. However, experimental transplants of red-winged blackbird eggs between nests in northern and southern Florida, and from Colorado to Minnesota, show that in this species a significant proportion of the regional differences in nestling development is nongenetic. If natural selection is maintaining the clines of character variation that are observed in adult phenotypes, the genetic and nongenetic components of phenotypic variation must covary.