The snag’s the limit: Western Purple Martin habitat associations

The snag’s the limit: Western Purple Martin habitat associations
Lorelle Sherman, Oregon State University

The western Purple Martin (Progne subis arboricola), an avian insectivore, is a species of conservation concern throughout the Pacific Northwest. Compared to the well-studied eastern subspecies, we know very little about the life history and biology of the western subspecies. Availability of suitable breeding habitat is believed to be a major limiting factor for western Purple Martin populations, but fundamental information on their current distribution and selection of nesting habitat is deficient. To fill this gap, we compared forest stand and landscape characteristics at sites occupied by snag-nesting Purple Martins in western Oregon to unoccupied sites. Our goal was to identify and quantify key characteristics of suitable upland forest nesting habitat for the western Purple Martin at multiple spatial scales and develop a predictive model that could be used to estimate current and future habitat availability. We tested the ability of snag-level characteristics (i.e., snag size and decay class) and the amount of early seral forest surrounding the nest snag to predict the probability of occupancy. We found that snag-nesting Purple Martins were widely distributed throughout upland forests in western Oregon, but snags in recently disturbed areas (early seral) that encompassed less than about 20 ha (50 acres) had a low probability (less than 50%) of occupancy. The strong positive association of Purple Martin occupancy with recently disturbed forests likely explains why Purple Martins were relatively rare on public forest lands managed under the Northwest Forest Plan to promote development of mature forest.