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Bird–window collisions
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Bird–window collisions
Bird–window collisions (also known as bird strikes after the aviation term or as window strikes) are a problem in both low- and high-density areas worldwide. Birds strike glass because reflective or transparent glass is often invisible to them. Previous research estimated that between 100 million and 1 billion birds are killed by collisions in the United States annually, but a 2024 study on the survival rate of bird-building collision victims indicates that previous research was vastly underestimating the number of deaths caused by collisions, and in actuality well over 1 billion birds die from collisions in the United States every year. An estimated 16 to 42 million birds are likewise killed each year in Canada.
The issue of bird-window collisions has become more prevalent as wild habitat is lost. It has intensified as landscaping and exterior glass continue to become more popular. However, due to differences within the taxon, built environments, time of year, and other effects, there is great variation in the nature and frequency of collisions.
Studies analyzing window collisions across greater spatial scales reveal interesting trends in species composition, indicating that some birds are more vulnerable to collisions than others. This most likely depends on differing morphology and physical flight characteristics of birds, but more subtle differences between groups are also thought to contribute to differences in vulnerability. Examples include differences in vision, degree of flocking, flight behaviors, and more specific life history traits, such as provisioning of young.
Species of warblers, thrushes, sparrows, hummingbirds, and vireos are among the most susceptible, with Bay-breasted Warblers, Ovenbirds, Red-eyed Vireos, and Blackpoll Warblers being the most notable. The reason for these species' vulnerability is not well understood, but it is speculated that species-specific behaviors are a likely contributor, as other factors like flight altitude differ greatly between these groups. Many of these birds have been documented as being especially attracted to lit structures. Warblers, thrushes, and vireos are known to make quick flight movements through densely vegetated areas, and are thought to be heavily guided by light in flight, which could account for this susceptibility to light disruption. Further, some of these species, such as thrushes and ovenbirds, spend more time near the ground, which is another characteristic shared among many common window-strike victims. Species like Cedar Waxwings, which make up a disproportionately high amount of window collisions in the fall and winter, are thought to be susceptible due to their flocking behaviors. During these months, waxwings forage in large flocks to more efficiently search for berries. It is thought that this seasonal increase in collisions is due to their increased concentration of movement, and perhaps because flocking birds are less attentive to their surroundings, opting to follow the lead bird in the flock.
There are also patterns of species mortality across different building types, which are most likely due to differences in flight behavior. For instance, Golden-winged Warblers and Canada Warblers are most at risk at low-rises and high-rises, Painted Buntings at low-rises, Worm-eating Warblers at high-rises and Wood Thrushes at residences.
It has been observed that many species which are very high in abundance in urban areas, such as House Sparrows, are killed at relatively low rates, further indicating that species mortality is not dependent on density.
The number of observed bird fatalities caused by any given building varies greatly across a spatial scale. There is a positive correlation between the number of collisions which occur at a building and the amount of the building surface area which is covered with windows. This is heavily evidenced by high levels of mortality at large commercial buildings. Further, buildings located in more developed areas experience fewer collisions than those in less-developed areas, due to effects of proximity to forested patches. This is most noticeable in residences across a rural-urban gradient, where per-building mortality rates are higher in rural areas. However, despite causing the lowest total mortality, more recent studies reveal that high-rise buildings have the highest median annual mortality rates.
The presence and height of vegetation surrounding a building is also positively correlated with bird mortalities. This is because highly reflective windows create an illusion of vegetation that birds can fly into, and birds are unable to recognize the cues of a window the way that humans do. A study conducted in Manhattan found support for the hypothesis that most collisions occur during daytime hours, when birds are foraging for food, due to the high number of collisions that occurred at windowed exteriors incorporating vegetation.
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Bird–window collisions
Bird–window collisions (also known as bird strikes after the aviation term or as window strikes) are a problem in both low- and high-density areas worldwide. Birds strike glass because reflective or transparent glass is often invisible to them. Previous research estimated that between 100 million and 1 billion birds are killed by collisions in the United States annually, but a 2024 study on the survival rate of bird-building collision victims indicates that previous research was vastly underestimating the number of deaths caused by collisions, and in actuality well over 1 billion birds die from collisions in the United States every year. An estimated 16 to 42 million birds are likewise killed each year in Canada.
The issue of bird-window collisions has become more prevalent as wild habitat is lost. It has intensified as landscaping and exterior glass continue to become more popular. However, due to differences within the taxon, built environments, time of year, and other effects, there is great variation in the nature and frequency of collisions.
Studies analyzing window collisions across greater spatial scales reveal interesting trends in species composition, indicating that some birds are more vulnerable to collisions than others. This most likely depends on differing morphology and physical flight characteristics of birds, but more subtle differences between groups are also thought to contribute to differences in vulnerability. Examples include differences in vision, degree of flocking, flight behaviors, and more specific life history traits, such as provisioning of young.
Species of warblers, thrushes, sparrows, hummingbirds, and vireos are among the most susceptible, with Bay-breasted Warblers, Ovenbirds, Red-eyed Vireos, and Blackpoll Warblers being the most notable. The reason for these species' vulnerability is not well understood, but it is speculated that species-specific behaviors are a likely contributor, as other factors like flight altitude differ greatly between these groups. Many of these birds have been documented as being especially attracted to lit structures. Warblers, thrushes, and vireos are known to make quick flight movements through densely vegetated areas, and are thought to be heavily guided by light in flight, which could account for this susceptibility to light disruption. Further, some of these species, such as thrushes and ovenbirds, spend more time near the ground, which is another characteristic shared among many common window-strike victims. Species like Cedar Waxwings, which make up a disproportionately high amount of window collisions in the fall and winter, are thought to be susceptible due to their flocking behaviors. During these months, waxwings forage in large flocks to more efficiently search for berries. It is thought that this seasonal increase in collisions is due to their increased concentration of movement, and perhaps because flocking birds are less attentive to their surroundings, opting to follow the lead bird in the flock.
There are also patterns of species mortality across different building types, which are most likely due to differences in flight behavior. For instance, Golden-winged Warblers and Canada Warblers are most at risk at low-rises and high-rises, Painted Buntings at low-rises, Worm-eating Warblers at high-rises and Wood Thrushes at residences.
It has been observed that many species which are very high in abundance in urban areas, such as House Sparrows, are killed at relatively low rates, further indicating that species mortality is not dependent on density.
The number of observed bird fatalities caused by any given building varies greatly across a spatial scale. There is a positive correlation between the number of collisions which occur at a building and the amount of the building surface area which is covered with windows. This is heavily evidenced by high levels of mortality at large commercial buildings. Further, buildings located in more developed areas experience fewer collisions than those in less-developed areas, due to effects of proximity to forested patches. This is most noticeable in residences across a rural-urban gradient, where per-building mortality rates are higher in rural areas. However, despite causing the lowest total mortality, more recent studies reveal that high-rise buildings have the highest median annual mortality rates.
The presence and height of vegetation surrounding a building is also positively correlated with bird mortalities. This is because highly reflective windows create an illusion of vegetation that birds can fly into, and birds are unable to recognize the cues of a window the way that humans do. A study conducted in Manhattan found support for the hypothesis that most collisions occur during daytime hours, when birds are foraging for food, due to the high number of collisions that occurred at windowed exteriors incorporating vegetation.
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