Factors Affecting Migratory Bird-Window Collisions: The Role of Canopy Cover and Scavenger Bias

Window collisions are a major anthropogenic threat to birds, with up to 1 billion mortalities estimated annually in the United States. In addition, migrating birds are more likely to strike windows than residents. Because Norfolk, Virginia is situated within a major migratory flyway, I measured avian window-strike mortality on the Old Dominion University campus. Window strike susceptibility increases with a combination of tall buildings, high glass surface area, and near-building vegetation, all components found on the campus. From 18 September to 01 November 2017, only five individual birds (Mourning Dove [Zenaida macroura], Ovenbird [Seiurus aurocapilla], Northern Parula [Setophaga Americana], Yellow-Rumped Warbler [Setophaga coronata], and Northern Cardinal [Cardinalis cardinalis]) were found dead because of striking windows. Scavengers and other sources of carcass removal, however, likely cause underestimation of window-strike mortality. To assess this potential bias, I used previously frozen specimens of birds that had been salvaged through various means outside of this study. From 23 November to 5 December 2017, I placed large (\u3e30 g) and small (g) bird carcasses outside of the same buildings where I had surveyed for window kills to assess carcass removal rates. All larger carcasses were removed within 48 hours, while smaller carcasses were present after 7 days. Thus, low numbers of carcasses detected at this and other sites could indicate scavenger activity rather than lower window-strike mortality. In addition, many studies report higher window-strike susceptibility in warblers, which may be due to scavenger bias towards larger carcasses rather than innate susceptibility of smaller warblers