Habitat shift and geographical variation in North American tanagers (Thraupinae: Piranga).

Relationships of habitat structure among North American tanagers were studied in large areas of their breeding ranges in the USA. In the large areas sampled, the four species differ significantly in multivariate scores of habitat structure. In Scarlet and Summer Tanagers (for which the most data was collected) there is intraspecific geographical variation in habitat features. Variation in habitats of Summer Tanagers is more pronounced than that of Scarlet Tanagers, and is related to changes of macrohabitats in low elevations from oak-pine and mixed woods in the southeast to riparian vegetation in the dry southwest. In the southeast these two species often breed in the same localities, and establish territories next to each other. In six localities in which I found both of them, the difference between the two species in habitat structure is significantly greater than in allopatric areas in the southeast. These results, and those of another study on song variation and behavioral responses to song suggest that coexistence of Scarlet and Summer Tanagers in sympatry can be explained both by partial habitat shift, and by interspecific aggressive division of space. It is concluded that quantitative sampling of habitats in large areas of distributional ranges of species is needed before any conclusions on species-specific preference and interspecific competition can be made.