Scale-dependent responses to loblolly pine trees in two ground-nesting warblers
Forest fragmentation and forestry management practices influence the habitat suitability of forests for migratory warblers. Here, we focused on habitat use of two species of ground-nesting warblers, Ovenbirds and Worm-eating Warblers, on the Eastern Shore of Maryland. Few studies have examined the habitat requirements of these two species on the Atlantic Coastal Plain. In other parts of their range, Ovenbirds favor large tracts of upland and moderately sloped forests, while Worm-eating Warblers tend to nest on the steep hillsides of deciduous or mixed deciduous forests. The lowland forests of the Eastern Shore are mostly small and highly fragmented with little-to-no topographic variation. We sought to determine patterns in 1) landscape-level occupancy and 2) habitat-patch nesting site preferences for these two ground-nesting warblers. We established 103 point count locations in forested habitat across the Eastern Shore. In addition, we located and monitored Ovenbird and Worm-eating Warbler nests at three study sites. We collected vegetation data at the point count locations and at nests and control locations within the three study sites . Vegetation measures included: tree species and DBH, understory composition and density, and leaf litter composition and depth.
Looking at landscape-level patterns, ground-nesting warblers, including Ovenbirds and Worm-eating Warblers, were more likely to be found in forests with fewer pine trees. At the habitat-patch level, both Ovenbirds and Worm-eating Warblers tended to select nest locations with greater leaf litter depth compared to control locations. However, while Ovenbird nest sites did not differ from control sites in the proportion of pine needles in the leaf litter, Worm-eating Warblers selected nest sites that contained more pine needles in the leaf litter compared with control sites. Similarly, Ovenbird nest sites mirrored controls in terms of the size of pines (DBH) in the forest, whereas Worm-eating Warblers selected nest locations with larger pine trees compared with control locations. Paradoxically, Worm-eating Warbler nests tended to be more likely to be subject to nest parasitism by Brown-headed Cowbirds at nest sites with a higher percentage of pine in the leaf litter. These results highlight the complexity of understanding natural and management factors that influence both density and nest success of ground-nesting warblers.