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

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Abstract
Williamson's sapsuckers (Sphyrapicus thyroideus) 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 mixed-conifer 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 (x̄ = 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.

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