The Influence of European Buckthorn (Rhamnus cathartica) on Captures of North American Passerines

Abstract

Common buckthorn (Rhamnus cathartica) is an aggressive invasive species in North America. It quickly establishes as a dominant shrub in existing forests, forms a dense, thorny understory that shades existing vegetation, and substantially alters or eliminates the native plant and animal community. We compared the number of birds captured between sites where buckthorn was cleared to sites not cleared of buckthorn at a Monitoring Avian Productivity and Survivorship (MAPS) station. To do this, we first cleared buckthorn from around three net locations in late 2020 and randomly selected three net locations as non-cleared control sites. Then we captured birds on seven dates each summer and examined captures 3 years before and 3 years after buckthorn removal. For this we used a paired approach, calculating the difference between the number of birds captured in the cleared vs. not cleared sites for each date, and then we analyzed the four most captured bird species. Before removing the buckthorn, we captured more birds at sites slated for buckthorn removal than at sites scheduled for no buckthorn removal. After removing the buckthorn, we similarly captured more total birds at sites where the buckthorn was removed than at sites with no buckthorn removal. However, capture rates for individual species were not different between cleared and uncleared sites after buckthorn removal. More yellow warblers were captured at unaltered sites scheduled for buckthorn removal than at unmanipulated sites prior to removal. We found no trends in the abundance of American redstarts, American robins, gray catbirds, or yellow warblers. Research is needed to identify specific vegetative characteristics that yellow warblers selected as well as a longer-