Causes of CollisionsThe magnitude of bird collisions in any one area depends on local and migratory avian populations, densities and species composition; diurnal (daytime) or nocturnal (nighttime) migration characteristics; resting and feeding patterns; habitat preferences; time of year; prevailing winds; and weather conditions. Understanding the conditions that contribute to bird collisions with buildings is the first step towards planning bird-safe environments.
DAYTIME: Since birds do not perceive glass as an obstacle to their flight path, collisions and mortality occur at any place where birds and glass coexist . Daytime building collisions occur on windows of all sizes at every building type, from single-story to high rise structures; in all seasons and weather conditions; and in every type of environment, from forested and rural settings to dense urban cores. Glass in buildings is an indiscriminate killer regardless of species, sex, age, size, migration characteristics, or level of adaptation to the built-environment . After colliding with a glass surface, the majority of birds either die instantly or shortly thereafter from brain injuries or fall prey to scavengers. Two conditions contribute to bird's inability to see glass:
" Glass Reflectivity: Mirror Effect From outside most buildings, glass often appears highly reflective, increasingly so when seen from an oblique angle. Almost every type of architectural glass under the right conditions reflects the sky, clouds, or nearby trees and vegetation, reproducing habitat familiar and attractive to birds.
" Glass Transparency: Fly Through During daylight hours, birds strike transparent windows as they attempt to access potential perches, potted plants, water sources and other lures inside and beyond the glass. The trick of transparency is exacerbated when windows are installed on opposite sides of a building directly across from one another or at a corner, because birds perceive an unobstructed passageway and fly towards the glass with no awareness of an obstacle.
Nighttime: Beacon Effect The illumination of buildings at night, and in the early morning and evening, creates conditions that are particularly hazardous to nighttime migrating birds. Typically flying at heights over 500 feet, especially if weather conditions are favorable, nocturnal migrants depend heavily on visual reference to maintain orientation. During inclement weather, these migrants often descend to lower altitudes, possibly in search of clear sky celestial clues or magnetic references and are liable to be attracted to illuminated buildings or other tall structures. Heavy moisture (humidity, fog or mist) in the air greatly increases the illuminated space around buildings, regardless of whether the light is generated by an interior or exterior source. Birds become disoriented and entrapped while circling in the illuminated zone and are likely to succumb to exhaustion, predation, or lethal collision.
Glass Reflectivity: Mirror Effect
Glass Transparency: Fly Through
Nighttime: Beacon Effect