Introduction

In many areas of western North America, the melodious song of the Black-headed Grosbeak is a familiar harbinger of spring. This species breeds from subalpine forests to desert riparian zones throughout western North America from southwestern Canada to southern Mexico. Relatively tolerant of human disturbance, it breeds in yards and gardens if adequate cover for nesting and feeding is available. Along river corridors in the Great Plains, the range of the Black-headed Grosbeak overlaps that of the closely related Rose-breasted Grosbeak (Pheucticus ludovicianus), and the two species are known to hybridize.

The Black-headed Grosbeak is sexually dimorphic and socially monogamous. Adult males have a flashy black, white, and cinnamon plumage; females are relatively drab buff and brown. Despite their showy plumage, males share about equally with females in incubating eggs and feeding young.

Both male and female Black-headed Grosbeaks sing, and both sexes often do so from the nest. Male song appears to function primarily in territory defense. Female song is generally a simplified version of male song and appears to function in communication between mates and in maintaining family groups once the young fledge. Occasionally, females sing full “male” song, apparently to deceive mates about the presence of intruders and force greater nest attentiveness.

One interesting feature of the Black-headed Grosbeak is that males do not attain definitive nuptial plumage until their second breeding season and vary in appearance from female-like to adult-male-like in their first potential breeding season. Only yearling males that most closely resemble adult males are able to defend a territory and attempt to breed. Extent of plumage development is positively correlated with testes size in yearling males. The subadult plumage worn by yearling males apparently helps deter aggression from older males: yearling males in female-like plumage are subject to less aggression from older males than are yearlings in adult-male-like plumage. Yearling males also arrive later on the breeding grounds than older males, likely to reduce aggressive encounters with more experienced adults.