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Population distribution, density and habitat preference of the Cerulean Warbler ( Setophaga cerulea ) in the Delaware Water Gap National Recreation Area

– The breeding range of the Cerulean Warbler has expanded into second-growth forest and converted agricultural land in the northeastern United States where, according to the North American Breeding Bird Survey, the population is increasing. Despite this expansion in one part of its range, the population as a whole is still in rapid decline implying that habitat quality supersedes habitat expansion in importance. The Delaware Water Gap National Recreation Area (DEWA) in New Jersey and Pennsylvania, USA, is representative of recently occupied northeastern habitat. We found Cerulean Warblers breeding at relatively low densities, compared to their core range in West Virginia, in most of the southern and central portions of the park to which the species was confined. On one particular study site, the Freeman Tract Road, breeding densities did rival those of the species’ core range. Sampling plots within Cerulean Warbler territories were characterized by a higher than expected prevalence of black walnut (Juglans nigra) and American sycamore (Platanus occidentalis) while northern red oak (Quercus rubra) and evergreen tree species were less prevalent than on randomly placed plots. Territorial plots also exhibited a more developed and taller shrub layer. Recently occupied breeding habitat, such as DEWA, may thus serve as a population stronghold in the future and as a study site for efforts to further understand Cerulean Warbler habitat requirements in the northeastern U.S.