Introduction

Red Crossbills inhabit southern taiga forests from Alaska to Newfoundland, and montane coniferous forests south to Georgia in the Appalachians, Arizona, New Mexico, northern Mexico, and the Sierra Nevada of California. Wandering birds can reach the southern states, where breeding has occurred. The species also occurs in the pine forests of Central America.

The Red Crossbill shows considerable morphological and vocal variation, and classification of the species into races has been as diverse as the many authors involved. Recent progress in the classification of this diversity is the identification of eight discrete types of Flight Calls north of the Mexican border, calls that may play a major role in maintaining reproductive isolation among the groups. These call types may represent discrete species; they are, at least, nomadic populations that are usually reproductively isolated from all other crossbills.

The nomadic movements characteristic of most of these forms are driven by the variable nature of cone production over most of North America. Individual Red Crossbills of various call types may be found far from their usual haunts, sometimes feeding on atypical food sources.

Morphological variation in bill and body size appears to be part of a suite of adaptations for different species and sizes of cones in most forms of this crossbill.

Red Crossbills breed mainly when a group finds an adequate mature cone crop of the appropriate type. In some places more than two call types may breed simultaneously, with little interbreeding, and they may breed repeatedly until food is depleted below a level of profitable foraging. Even though crossbills are often said to breed in all months of the year, a recent study of one call type shows that breeding ceases, even in the presence of a bumper crop, when autumn day length becomes shorter than about 12 hours. Then, after the annual molt, breeding can resume in late December or January at day lengths of 10.5 hours.