Habitat Suitability for Williamson's Sapsuckers in Mixed-conifer Forests

Williamson's sapsuckers (Sphyrapicus thyroidem) have narrow habitat requirements and are sensitive indicators of change in intensively managed forest habitats of western North America. Thus, we studied habitat suitability for Williamson's sapsuckers at 99 4-ha sites (33 nest sites, 66 non-use sites) in mixedconifer forests in Arizona during 1991. Nesting success of sapsuckers was high in this habitat (93.2% nest success, 0.0014 daily mortality, n = 724 nest days), and they preferred to nest in tall (P < 0.05) aspen snags (P < 0.001) near the bottom (P = 0.012) of snow-melt drainages with 0-20% of the canopies dominated by aspen. Sapsucker nest sites had particularly large (P < 0.05) live aspen and aspen snags in the surrounding area. Nest sites also had high (P < 0.05) snag densities (a = 7.65 snags/ha) in the surrounding area, and these snag densities exceeded those commonly used in forest management plans. Effective snag management should concentrate snags in groups within low-lying areas and conserve large-sized snags. A Habitat Suitability Index (HSI) correctly predicted that Williamson's sapsuckers should generally prefer drainages over ridgetops, but the model could not distinguish between use and non-use sites within drainages. Future HSI models for Williamson's sapsucker should continue to stress snag density, but should consider aspen snag density separately from density of other snags, incorporate height and diameter of aspen snags, and use a more liberal definition of aspens contributing to overstory canopy cover. J. WILDL. MANAGE. 57(2):322-328 Forest managers are faced with trying to maintain viable populations of vertebrates while simultaneously managing for other resource needs such as timber production. This delicate balancing act depends upon knowing the effects of land management practices on habitat suitability for wildlife. The first step in attaining this knowledge is to identify habitat features that influence habitat suitability. Moreover, identification of habitat features that directly affect reproduction and survival is the most effective means of indexing habitat suitability because maintenance of viable populations depends on sustaining these fitness components (Martin 1992). Cavity-nesting birds are a particularly appropriate group of species for examining effects of land management on habitat suitability because they are often affected by land management practices (Gysel 1961, Haapanen 1965, Hunter 1990). For instance, snags are less abundant in managed than unmanaged forests (Cline et al. 1980), and availability of snags commonly limits populations of cavity-nesting birds by lim' Present address: Department of Natural Resources Science, 210B Woodward Hall, University of Rhode Island, Kingston, RI 02881. iting nesting opportunities (for review, see Li and Martin [1991], Martin and Li [1992]). In response, explicit policies for snag conservation have been implemented for managed stands (Hunter 1990). Yet, management for presence of snags is not sufficient; size, species, and age of snags, plus distribution of nest trees and vegetation in surrounding patches can affect presence and even reproductive success of birds (Flack 1976, Raphael and White 1984, Sedgwick and Knopf 1990, Li and Martin 1991). Key habitat features that influence habitat suitability, particularly with regard to reproduction or survival, need to be identified for individual