The western purple martin (Progne subis arboricola) is North America’s largest swallow. These birds historically nested in the cavities of standing dead trees (snags) in upland forest habitat. However, the western purple martin suffered major population declines in the 1960s and 1970s, likely due to a combination of nesting habitat loss and degradation and use of insecticides. Consequently, the western purple martin is a species of conservation concern throughout its range.
So, what is upland forest habitat? Highly productive forestland, which makes up about 80% of Western Oregon. The land is a checkerboard of federal public agency and private ownerships. Wildfire and timber management are the two most common disturbances on the landscape. Many species depend on fire for survival and the western purple martin is one of them. As cavity-nesters and aerial insectivores, they require nesting structures in open habitat historically created by wildfire.
Moderate to severe fires create open, post-disturbance (or “early seral forest”) habitat by clearing out the understory and killing trees in the overstory. In the wake of wildfire, dead trees become nest snags for woodpeckers, which excavate nest cavities into decaying wood. Eventually, the purple martins move in as a secondary cavity nester.
Due to decades of fire suppression, purple martins started primarily using snags left in clearcuts. Researchers believe the availability of nesting habitat in the upland forest may be a major limiting factor for populations due to snag removal during timber harvest and salvaging of snags, combined with a paucity of wildfire in previous decades. To understand how to improve or create additional nesting habitat, multiple stakeholders spearheaded a purple martin habitat study in Oregon in 2016 and 2017.
The habitat study revealed more mated pairs were found nesting on private timber lands than on land owned by the U.S. Bureau of Land Management. They nested most frequently in Douglas-fir (Pseudotsuga menziesii) snag cavities created by the northern flicker (Colaptes auratus). Unexpectedly, nest snags had on average lower snag height and cavity height than snags not used for nesting. This may be due to the limited availability of large snags left in upland forest habitat, leaving them to nest in small diameter snags.
To accommodate a clutch of four to six nestlings, a nest cavity must be of a certain size. The unexpectedly low cavity height of nest snags may be attributed to snag diameter limitations to accommodate the appropriate size nest cavity. The area of open habitat around the nest snag was found to be one of the most important predictors for use of a snag by nesting purple martins.
How can you create western purple martin habitat on your woodland?
To create western purple martin habitat, leave medium to large snags at harvest and leave them far away from the mature forest edge. Research shows snags intended for use by purple martins should be at least 100 meters away from the edge of mature, closed-canopy forest. Artificial housing attached to snags can bridge the gap between snag creation and excavation of cavities by woodpeckers. Leaving downed wood creates insect habitat, which is purple martin food.
