Passerine Subadult Plumages and the Deceptive Acquisition of Resources: Test of a Critical Assumption

Abstract

The subadult plumage of year-old males of certain temperate passerines has been hypothesized to be a female-mimicry feature which deceives older males, enabling young males to settle in good habitat. Adult male red-winged blackbirds seem to be deceived, despite the fact that subadult red-winged blackbirds are poor female mimics. Adults responded very aggressively to mounts of older males and with much less aggression to mounts of subadult males that best resembled females. Bright subadults were treated with intermediate levels of aggression. A system of converting multiple measures of aggression into a single variable by using factor scores with a varimax rotation of principal axes is described. The 1st rotated axis elucidated an interpretable pattern in the character coefficients; this interpretation was confirmed by a test performed by presenting adult males with a mounted female.