Factors affecting variation in the primary song of North American solitary vireos (Aves: Vireonidae)

Abstract

Tape recordings of the primary song of eight subspecies of solitary vireo were examined sonagraphically. Within one population all birds sing a small number of song phrases in common but each bird has a slightly different repertoire of additional phrases. Populations closest together share the most phrases, but even among widely separate groups or subspecies a few of the same phrases may be found. Song learning appears to maintain local uniformity of song while, with drift and recombination, providing opportunities for the development of new song phrases. Only in eastern North America are primary song phrases pure in tone with a gradual shift of frequency-modulated phrases in both grayish and yellowish subspecies in the west. The presence of pure tone phrases in eastern North America probably contributes to and is maintained by the need for species isolation from yellow-throated vireos and does not indicate that eastern and western birds may be separate species. Climate and habitat appear to have little influence on song variability among subspecies.