T. P. Hahn and Kelsey (unpubl.) describe a fascinating pattern of geographic variation in flight calls (Fig. 3). These authors have documented the existence of 4 acoustically distinct types of flight call, with each type predominant in a particular geographic region and uniform in structure across its region. Type 1 calls (Fig. 3A) predominate in much of the western portion of the Evening Grosbeak's range; Hahn and Kelsey have heard and/or recorded Type 1 calls in Washington, Oregon, Idaho, Montana, Wyoming, and the northernmost portion of California. Type 2 calls (Fig. 3B) predominate in the Sierra Nevada of California. Type 3 calls (Fig. 3C) are characteristic of the eastern portion of the species' range, and Type 4 calls (Fig. 3D) seem to be restricted to the s. Rockies, especially Colorado. Call types are sometimes recorded "out of range" (e.g., Type 1 calls in the Sierra Nevada and in Colorado), but these records typically come from a small number of individuals amidst a larger population that is using the locally expected type.
Hahn and Kelsey suggest that the geographically distinct call types may correspond to distinct evolutionary units within Evening Grosbeaks. The regions delimited by the different call types match closely the geographic ranges occupied by formerly recognized Evening Grosbeak subspecies. Type 2 calls seem to be restricted to the range of the former C. v. californicus, Type 4 calls appear to be limited to the range of the former C. v. warreni, and Type 2 calls are found in the range originally described for C. v. brooksi (current nomenclature subsumes all 3 of these taxa within C. v. brooksi). Type 3 calls predominate across the range of the current Eastern Evening Grosbeak, C. v. vespertinus. (The other currently recognized subspecies, C. v. montanus, occupies s. Arizona and Mexico. Birds in those locations might be expected to use a fifth type of flight call. Although the extensive recording necessary to confirm this prediction has not been completed, a small sample of recordings from s. Arizona [Fig. 3E] suggests that the region does indeed have a distinctive call type.)
The data gathered by Hahn and Kelsey suggest that the now-defunct taxonomic scheme that recognized 5 subspecies correctly identified Evening Grosbeak subpopulations that tend to remain within particular (if large) regions. Hahn and Kelsey speculate that, if the flight call dialects do reflect population structure, the structure could be maintained by assortative mating by flight call. In this scenario, Evening Grosbeaks might use the distinctive flight calls to recognize potential mates, as appears to be the case in Red Crossbills (Groth 1993).
