Aspen stands and riparian areas are important to breeding birds in the southwestern U.S. because they provide resources such as food and shelter. We investigated how this importance varies throughout the year for both resident and migratory birds. We sampled birds in 96 sites, half in small isolated aspen stands and half in the ponderosa pine forest in northern Arizona during the summer of 1996, and a subset of those plots during fall of 1996 and the spring of 1997. Bird species richness and abundance varied seasonally. During the summer there were more birds and more bird species in aspen stands. This relationship appears to be driven by an affiliation between Neotropical migrants and aspen trees. During fall, residents were associated with riparian areas. We demonstrate the importance of small inclusions of aspen to Neotropical migrants in the Southwest during the breeding season and we show that preference for habitat types among migratory groups can vary seasonally.