eWestern Birds
The Quarterly Journal of Western Field Ornithologists
A New and Cryptic Call Type of the Red Crossbill
Kenneth Irwin
ABSTRACT: I describe a new call type (type 10) of the Red Crossbill (Loxia curvirostra complex) associated with Sitka Spruce (Picea sitchensis) in Humboldt County, California. As with other types of the crossbill’s flight calls, the birds using this type of call apparently constitute a subset of the species that is cohesive socially, behaviorally, and morphologically. The patterns of frequency and amplitude modulation of flight calls of type 10 are similar to those of the second half of type 4 but change in frequency more slowly and are given at a higher pitch. The flight calls of type 10 vary among individuals and within an individual’s repertoire, perhaps to a greater extent than in other call types. Most type 10 birds gave toop calls distinctly different from those of all other call types, but a few were similar to those of types 2 and 4. Likewise, the chitter calls of type 10 differed from those of the three call types (2, 3, and 4) found most commonly near type 10. The song repertoires of types 10 and 4 differed as well. Type 10 crossbills are intermediate in size between types 3 and 1. Large numbers of type 10 were resident in Sitka Spruce forests from 2001 to 2010, whereas the few type 4 birds recorded in spruce stands remained only briefly. Morphological and behavioral evidence indicates that type 10 is specialized for foraging on seeds in Sitka Spruce cones.