Relationships between Forest Bird Declines and Hemlock Woolly Adelgid Prevalence in the Eastern United States

Julie E. Blum, Gettysburg College

Environmental Studies Honors Thesis

Faculty Advisor: Professor Andrew Wilson


The eastern hemlock (Tsuga canadensis) is a vital foundation tree species throughout the eastern United States, providing essential structural diversity and habitat for more than 120 different animal species. Within the past few decades, T. canadensis has undergone significant declines that are largely associated with the hemlock woolly adelgid (HWA; Adelges tsugae), an exotic, aphid-like insect native to East Asia. From the 1970s to present day, the HWA has spread throughout southern New England, large portions of the Mid-Atlantic region, and parts of Tennessee and the Carolinas. Research has shown that loss of the eastern hemlock is drastically altering forest community structures, potentially impacting a wide variety of forest fauna, including avian populations strongly associated with hemlock forests. Here I present research investigating the correlation between HWA prevalence and recent declines of hemlock-associated forest birds in the Eastern US. Using ArcMap 10.1, I utilized North American Breeding Bird Survey (BBS) data, US Forest Service HWA data, and USGS land cover data to analyze the population trends of hemlock-associated and forest generalist species in association with the arrival of HWA, taking hemlock density into account. Results suggest that there is significant correlation between the time of HWA arrival along BBS routes and the pattern of decline among some hemlock specialists. The Black-throated Green Warbler and the Blue-headed Vireo exhibited significant decline along survey routes after HWA arrival, while populations of forest generalists were unaffected.