Effects of Prescribed Burns on Wintering Cavity-Nesting Birds

Primary cavity-nesting birds play a critical role in forest ecosystems by excavating cavities later used by other birds and mammals as nesting or roosting sites. Several species of cavity-nesting birds are non-migratory residents and consequently subject to winter conditions. We conducted winter bird counts from 1998 to 2000 to examine the abundance and habitat association of cavity-nesting birds in prescribed burned and unburned ponderosa pine (Pinus ponderosa) stands. Even though bird diversity indices did not differ between burned and unburned stands, species-specific bird abundance was associated with habitat variables in three burned and four unburned stands. Total cavity-nesting bird abundance was greater in burned stands. Most cavity-nesting birds were observed in mixed-species flocks. Individual species of these flocks were associated with different habitat variables within stands. Numbers of woodpeckers were significantly greater in burned stands and numbers of chickadees were significantly greater in unburned stands. Bark foragers such as woodpeckers (Picoides spp.) and pygmy nuthatches (Sitta pygmaea) were associated with fewer small trees and recently decayed snags and logs. Foliage gleaners such as the chickadees (Poecile spp.) were associated with small diameter snags. The juxtaposition of burned and unburned stands is important for individual birds reliant upon other members of a mixed-species flock and habitat heterogeneity within stands is important for maintaining a diverse cavity-nesting bird assemblage. Author to whom correspondence should be addressed. Email: [email protected] Introduction Primary cavity-nesting birds, such as woodpeckers (Picoides spp.) and nuthatches (Sitta spp.), play important roles in forest ecosystems and are considered keystone species because they excavate cavities that, once vacated, provide nesting and roosting sites for other birds and mammals (Johnsson 1993). In addition, woodpeckers are the primary vertebrate that prey on wood-boring beetles (Otvos 1965, Koplin 1972). Within ponderosa pine (Pinus ponderosa) forests, foraging for beetles by woodpeckers in the genus Picoides may promote fungal invasion, thus promoting snag decay and providing substrate for excavation of additional cavities (Farris et al. 2004). Several species of cavity-nesters are resident birds (Scott 1977) and rely on snags for roosting, and foraging during the winter. Due to limited food and cover resources (Chambers and McComb 1997), winter can be a critical season determining survival of primary cavity-nesters and success of avian communities in general (Smith 1971, DellaSala et al. 1996). Understanding factors associated with cavity-nesting birds’ use of snags during winter provides insight into forest management approaches that might promote overwinter survival of these keystone species. Cavity-nesters have evolved in forests containing a substantial amount of decaying wood (Dickson et al. 1983) and are associated with habitats of high snag densities for nesting, roosting, and foraging (Mannan et al. 1980, Dickson et al. 1983, McComb and Muller 1983, Zarnowitz and Manuwal 1985, Chambers et al. 1997, Chambers and McComb 1997). There may be competition for snags containing cavities (Nilsson 1986) suggesting that snags may be a limiting resource for cavity-nesting bird populations in conifer forests (Cline et al. 1980, Mannan et al. 1980, Zarnowitz and Manuwal 1985). In unmanaged forests, snag densities are maintained through natural fire, insects, and disease. For example, historical snag densities in ponderosa pine forests east of the Cascade Mountains in Washington were estimated to be between 14.5 and 34.6 snags ha (Harrod et al. 1998). Within managed forests, silvicultural practices typically decrease snag densities (Cline et al. 1980, Zarnowitz and Manuwal 1985). Prescribed burning