Migratory Strategy and Seasonal Patterns of Bird Diversity in Relation to Forest Habitat

Ci.—Aspen stands and riparian areiis are important to breeding birds in ihe southwestern U.S. because they provide tesources such as food atid shelter. We investigated how this importance varies throughoul the year for bolh resident and migratory birds. We sampled hitds in 96 sites, half in small isolated aspen stands and half in the ponderosa pine forest in northern Ariirona thiring tlie sumiuer of 1990, and a snbset of those plots during fall of 1996 and ihe spring of 1997. Bird species richness and ahundance viiried seasonally. During the summer there were more birds and more bird species in a.spen stands. This relationship appears to he driven by an afhliation 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 tiie hreeding season and we show that preference for habitat types among migratory groups can vary seasonally.