Causes of Collisions

Conditions Affecting Bird Collisions

Imperatives for Bird Protection



Birds & the Built Environment

In recent decades, sprawling land-use patterns and intensified urbanization have degraded the quantity and quality of bird habitat throughout the globe. Cities and towns cling to waterfronts and shorelines, and increasingly infringe upon the wetlands and neighboring woodlands that birds depend upon for food and shelter. The loss of habitat forces birds to alight in city parks, streetscape vegetation, waterfront business districts, and other urban green patches, where they encounter the nighttime dangers of illuminated structures and the daytime hazards of dense and highly glazed buildings.

The increased use of glass poses a distinct threat to birdlife.
From urban high-rises to suburban office parks to single-story structures, large expanses of glass are now routinely used as building enclosure. Energy performance improvements in transparent exterior wall systems have enabled deep daylighting of building interiors, often by means of floor-to-ceiling glass expanses. The aesthetic and functional pursuit of still greater visual transparency (integrating indoors with outdoors) has spurred the production of low-iron glass, eliminating the greenish cast.

The combined effects of these factors have led scientists to determine that bird mortality caused by building collisions is a "biologically significant" issue. In other words, it is a threat of sufficient magnitude to affect the viability of bird populations, leading to local, regional, and national declines.
Researchers and volunteers have documented hundreds of thousands of building collision-related bird deaths during migration seasons. Included in this toll are specimens representing over 225 species, a quarter of the species found in the United States. Songbirds already imperiled by habitat loss and other environmental stressors are especially vulnerable during migration to nighttime collisions with buildings and daytime glass collisions as they seek food and resting perches among urban buildings.

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