Habitat availability for bottomland hardwood forest birds: the importance of considering elevation

Estimating habitat availability at large spatial scales is critical for identifying conservation and management priorities for birds. Given the effects of patch size on habitat selection and productivity of many bird species, large-scale approaches often focus on identifying and enumerating patches large enough to support sustainable populations. Declines in the availability of bottomland hardwood forests have made this approach imperative for species that depend on these forests. However, most remaining bottomland forests are relatively low-elevation sites that were difficult to convert to agriculture. For species that require densely vegetated understories or well-developed litter layers, such as Swainson's Warblers (Limnothlypis swainsonii), these relatively flood-prone forests may not provide suitable habitat and, consequently, current prioritization methods may overestimate habitat availability. We examined the effect of elevation on estimates of habitat availability for Swainson's Warblers in a 65,000-ha bottomland hardwood forest (White River National Wildlife Refuge) in eastern Arkansas. We detected Swainson's Warblers at 88 of 2222 sampled points, and Swainson's Warblers only used relatively high elevations in the refuge. Based on our estimates, about 25% of the refuge is at a suitable elevation for Swainson's Warblers. Without considering elevation, this refuge would support an estimated 7000 to 7600 pairs, but, when elevation is considered, the estimated number of pairs drops to 1500-2000 pairs. In reality, because of a lack of suitable seral stages at high elevations in the refuge, this area likely supports only 75-100 pairs. For Swainson's Warblers and other understory-dependent bottomland species, elevation should be incorporated into conservation planning to obtain accurate estimates of habitat availability. In addition, management should be focused on these high-elevation areas to maximize habitat availability for these species of concern.