The relation between species occurrence and the structure or composition of habitat can be complex and often varies in a species-specific manner. Sometimes, species–habitat relations are defined by thresholds, or abrupt nonlinear responses to a habitat gradient. Threshold responses are expected when certain habitat features are required for species occurrence. For example, primary cavity-nesting woodpeckers often typify the threshold concept because in the absence of appropriate substrates (decadent wood) large enough for nest cavities, woodpeckers will not occur. In such cases, identifying thresholds is important to ensure that management activities meet minimal (or maximal) habitat requirements of target species. The Red-headed Woodpecker (Melanerpes erythrocephalus) is a species of national conservation concern, and loss of suitable nesting habitat has been suggested as a primary factor in its population declines. Threshold responses may partly explain why this species has gone locally extinct in areas where land-use change resulted in a transition from suitable to unsuitable habitat. Here, we tested whether a regionally important population of Red-headed Woodpeckers in northern New York, near the periphery of the species' range, exhibited threshold responses in their selection of nesting habitat. We used boosted regression trees (BRT) to identify nest habitat thresholds in relation to multiple habitat variables at multiple spatial scales (nest tree, forest patch, and forest stand). We identified nest tree thresholds related to tree decay class (≥33% decadent canopy), cumulative dead limb length (≥4 m), and tree diameter (≥34 cm dbh). Forest patch (vegetation surrounding nest tree; 0.04 ha) thresholds were related to cumulative dead limb length (≥17 m), woody understory height (<12 cm), mean tree diameter (30 cm dbh), and large tree (≥30 cm dbh) density of ≥4. Forest stand thresholds were related to overall stand decadence (on average trees have ≥1% crown decay) and woody understory height (<12 cm). Red-headed Woodpeckers exhibited scale-dependent and multi-metric nest habitat thresholds, which indicate that a lack of suitable nesting habitat (i.e. habitats meeting or exceeding multi-metric and multi-scale criteria) may be limiting population growth near the periphery of the species' range. In particular, the abundance of decadent (dead and decaying) tree resources appears to limit the distribution of suitable habitat. Our objective threshold criteria can be used to identify habitats of high conservation value for this species, or to identify specific habitat features that require management or restoration to increase suitable habitat for this threatened species.
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Vol. 117 • No. 2