Causes of Collisions

Conditions Affecting Bird Collisions

Imperatives for Bird Protection



Conditions Affecting Bird Collisions

For both new and existing buildings, undertaking bird-safe best practices requires an assessment of a range of macro and micro conditions. These include evaluation of the region and the site; bird demographic chronology and habitat use; building height; glass coverage and glazing characteristics; and building operational criteria for exterior and interior illumination. Conditions affecting bird collisions include: proximity to stopover locations, building height, glass coverage and glazing characteristics, building orientation and massing features, proximity to feeding grounds, and local meteorological conditions.

Broad-front Songbird Migrations

Billions of migratory birds travel across North America each spring and fall to seek breeding or wintering grounds in locales that offer abundant food, shelter, and adequate climates. Songbirds travel primarily at night in what can best be described as "broad-front" migration. These migrations, in which weather plays a significant role, can have seasonal and annual variations in songbird numbers and concentrations and in timing. During fall migration, birds travel from summer breeding grounds in the temperate or arctic northern hemisphere to wintering grounds in the equatorial tropics or temperate zones of the southern hemisphere, making the reverse trip the following spring. These historic routes follow major rivers, coastlines, mountain ranges, and lakes. Along the way densely built urban areas have become migration danger zones.

Local Meteorological Conditions

Regions that are prone to haze, fog, mist, and/or low-lying clouds may see more frequent bird-kills, especially if the area contains tall buildings over 500 feet that are highly illuminated. Generally, there are fewer birds aloft during precipitation; however, inclement weather can reduce their navigational awareness forcing them to fly at lower altitudes in search of visual clues. Heavily illuminated buildings in their path can serve as a deadly attractant.

Proximity to Stopover Locations

Historically birds have made stopovers in waterfront, coastal, wetland, wooded, and weedy environments that are now America's most densely populated urban areas. Scientists estimate that migrating birds have a 70% chance of encountering at least one major metropolitan area during migration from breeding to wintering grounds and vice versa. Sites located within these urbanized regions are likely to be zones of greater danger, especially to birds landing and taking off from stopover sites. During their rest intervals, they are exposed to hazards in the immediate urban context while they forage for food. Building sites located near bird feeding areas, waterfront habitat, or patches of urban vegetation experience increased risk of bird collisions.

Proximity to feeding grounds and habitat area

Building sites near water bodies and wetlands - no matter how small -put both resident and migrant species at risk. Sites bordering parkland, pocket parks, habitat patches, green roofs, and street-tree corridors present bird-vulnerable facades, since birds forage these areas for food. Suburban building sites with proximity to natural landscapes also present a range of hazards and can be as dangerous to birds as urban settings.

Immediate Building Context

While bird collisions with buildings occur in locales of all densities, from rural to suburban to urban, there are some overarching conditions that may affect the mortality rate in a particular setting. These include the density of the site's surrounding urban fabric, the degree of sky visibility from the site footprint, the street width and proximity of streetscape vegetation, and the height of the surrounding buildings. In general, at-risk conditions are those that obstruct flight paths and place birds in close proximity to glazed facades.

Building Orientation and Massing Features

Since migratory routes are broad districts and bird flight patterns vary day to day, one cannot simply address building facades that face an assumed direction of migration. The impacts of all facades, with special emphasis on those adjacent to landscapes or other features attractive to birds must be considered. Buildings with exterior and/or interior landscaped courtyards create additional hazards, as do glazed areas. As tight enclosures, they can make it very difficult for birds to escape safely.

Building Height

  • Lower levels: The most hazardous areas of all buildings, especially during the day and regardless of overall height, are the ground level and bottom few stories. Here, birds are most likely to fly into glazed facades that reflect surrounding vegetation, sky and other features attractive to birds.
  • Moderate height: Buildings between 50 and 500 feet tall pose hazards since migrating birds descend from migration heights in the early morning to rest and forage for food. Migrants also frequently fly short distances at lower elevations in the early morning to correct the path of their migration, making moderate-height buildings a prime target, especially if they have large expanses of reflective or transparent glass, or if they are highly illuminated.
  • Tallest: While the exact height of birds' migratory paths varies depending on species, geography, season, time of day/night, and weather conditions, radar tracking has determined that approximately 98% of flying vertebrates (birds and bats) migrate at heights below 500 meters (1640 feet) during the spring, with 75% below that level in the fall. Today, many of the tallest buildings in the world reach or come close to the upper limits of bird (and bat) migration . Storms or fog, which cause disorientation, put countless numbers of birds at risk during a single evening. Any building over 500 feet tall then--approximately 40-50 stories--is an obstacle in the path of avian nighttime migration and must be thoughtfully designed and operated to minimize its impact.

Glass Coverage and Glazing Characteristics

A major determinant of potential strikes is the sheer percentage of glass used on the building facade. In general, collisions will occur wherever glass and birds coexist. The ground level and lowest stories are the major collision zones. At these levels large expanses of monolithic glazing should be minimized, glazing reflectivity (especially when adjacent to landscapes) reduced, and situations where glazing promotes the false vision of unobstructed passage limited. One proven technique is to maximize a fašade's "visual noise", or the readily visible differentiations of material, texture, color, opacity, or other features that help to fragment glass reflections and reduce overall transparency. "Visual noise" at the scale of the building and at the level of the individual glass unit should be incorporated.

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