Habitat of Breeding Blackpoll Warblers at a Persistent Extralimital Breeding Site in Pennsylvania

Abstract

Setophaga striata (Blackpoll Warbler) is a boreal forest breeder that inhabits an expansive breeding range, with its southern limit in the northeastern US. The Pennsylvania breeding population is small and isolated but has persisted since its discovery in 1993, with the nearest breeding population about 150 km northeast in the Catskill Mountains of New York. Little information is available on the structure and composition of the vegetation where Blackpoll Warblers establish territories and how these vegetative parameters compare with those present in the core of their range. In 2016, we quantified the understory and canopy structure and composition, as well as the groundcover of active Blackpoll Warbler breeding territories (n = 15). Blackpoll Warblers occupied areas that were dominated by spruce (Picea rubens [Red Spruce] and P. mariana [Black Spruce]; 75% canopy cover) with a relatively low mean canopy height (6 m) and a mean diameter at breast height of 13 cm. Overall, the structure and composition of the vegetation within territories of Blackpoll Warblers in the Pennsylvania population are similar to those found in core portions of their breeding range. While the Pennsylvania population has bred exclusively within 1 small, confined area, similar Red Spruce and Black Spruce communities can be found to the east in the Pocono Mountains of Pennsylvania, suggesting there may be an opportunity for range expansion. However, there are increasing concerns that populations of boreal species at the southern edge of their range are especially vulnerable to climate change as warming and weather extremes decrease the suitability of isolated locations.