Seasonal Changes in the Habitat Distribution of Transient Insectivorous Birds in Southeastern Arizona: Competition Mediated?

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Abstract
The distribution and abundance of 26 migratory insectivorous bird species were recorded over an elevational habitat gradient in the Chiricahua Mountains, Arizona [USA] for the spring and fall migratory seasons. Most of the species used this area only during migratory passage, and 54% exhibited significant shifts in the habitats occupied from spring to fall. The majority (69%) of species also exhibited significant changes in density within habitats between seasons. Using pairwise correlations of bird densities from 7 habitat types and 2 seasons, 5 groups that contained species whose seasonal distributional patterns were similar to one another but independent and distinct from members of the other 4 groups were identified. Despite independence among groups in the seasonal patterns of habitat distribution, the combined density of all species was significantly positively correlated with a measure of food availability taken from each of the habitat types in each migratory season. The spring-to-fall change in insect density within each habitat also was significantly correlated with the seasonal change in bird density over each of the habitat types. The hypotheses that best explain these correlations include that in which competitive adjustments among the migratory birds enable a close match to food resource availability and that whereby noncompetitive adjustments occur in response to the diversity (itself correlated with food abundance) of food types available.