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

Article excerpt

ABSTRACT.-

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.

INTRODUCTION

Resources vary throughout space and time and we should expect birds to vary in their use of these resources based on life history requirements. Within a season, birds that use the same strategies for migration might be expected to choose similar habitat types based on timing and availability of resources in relation to their migration. For instance, migrants may select breeding sites based on cues different from those used by residents because they are able to exploit highly seasonal resources (Rabenold, 1993) and they may be dependent on highly specialized breeding sites to maintain reproduction (Sherry and Holmes, 1995). Residents are on, or near, the breeding sites longer and might be less dependent on seasonal flushes in production. Between breeding and non breeding seasons we may see the same migratory guild selecting components of the habitat differently; for example, home ranges and habitat breadth tend to expand during non breeding seasons due to decreased resource concentration (Rolando, 1998; Wiktander et al., 2001). Additionally, resource requirements can change based on season; for example, species that are typically granivores or frugivores during the winter often consume insects during the breeding season to obtain protein for nestlings.

In a pilot study, we examined bird abundance and species richness of resident, short distance migrant and Neotropical migrant species during different seasons in small aspen (Populus tremuloides) stands and compared these measures to those in the ponderosa pine (Pinus ponderosa) forest matrix in northern Arizona. Additionally, we examined the abundance and species richness of residents and short distance migrants between the summer breeding season and the following fall and spring. We examined correlations between bird abundance and diversity and environmental variables during these periods.

METHODS

STUDY SITE

We studied bird communities in small quaking aspen stands and the surrounding forest matrix in the Coconino National Forest of northern Arizona during the summer of 1996 (June-July), fall of 1996 (September) and spring of 1997 (April). The forest matrix was primarily ponderosa pine and ponderosa pine-Gambel oak ( Quercus gambelii). Elevation of our study sites ranged from about 2060 to 2480 m. We selected aspen stands >0.1 ha and that were surrounded by forest matrix on at least two-thirds of the stand's edge. Aspen stands were small, averaging 13 ha (median 4 ha), and comprised a very small percentage of the landscape at elevations between 1900 and 2600 m. These stands are described in greater detail by Griffis-Kyle and Beier (2003).

SAMPLING

We placed one plot in each aspen stand and a second plot in the ponderosa pine forest 275 m to 950 m straight line distance away from the edge of each aspen stand. Plots within the aspen stands were located randomly; whereas each pine plot was located to match the paired aspen plot in elevation, slope, aspect and topographic setting. …