Can the Hermit Warbler ( Setophaga occidentalis ) serve as an old-forest indicator species in the Sierra Nevada?

Changing fire regimes in western North America have raised the possibility of widespread loss of forest cover, making forest restoration a major priority. In one such ecosystem, the Sierra Nevada in California, the implications of forest management policy have been evaluated primarily via their potential effects on the California Spotted Owl ( Strix occidentalis occidentalis ). Yet the owl’s cryptic life history, large home range, and declining population all make it difficult to study. The Hermit Warbler ( Setophaga occidentalis ) may be a valuable proxy species for the Spotted Owl because the two have similar associations with older forest habitat, but the former could enable researchers to achieve higher statistical power when studying changes to key habitats. We conducted passive acoustic surveys across the entire west slope of the Sierra Nevada between May and July 2021, identified both Hermit Warbler and Spotted Owl vocalizations in the resulting audio using the BirdNET algorithm, and used single-season occupancy models to examine the relationship between Hermit Warbler occupancy and six remotely sensed variables representing key attributes of older forests as well as Spotted Owl presence. Hermit Warblers were observed at all sites at which Spotted Owls were present, but those sites represented just 30.5% of the Hermit Warbler’s total occupied range. Hermit Warbler site occupancy was positively associated with mean tree diameter and the presence of Spotted Owls (model weight = 0.97). The Hermit Warbler is more appropriate as a proxy for habitat beneficial to the California Spotted Owl than as a proxy for the owl itself. As such, monitoring and studying the Hermit Warbler could be a means of understanding the effects of forest restoration on important old-forest habitat.