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Frequently Asked Questions

Kenneth Herdy 2009

How many birds are killed each year in collisions withwindows?
The question seems like an easy one, but we don’t reallyknow. It is estimated that 100 million to 1 billion birds die inthe U.S.alone every year as a result of window strikes. This is a mortality ratesecond only to habitat destruction. Bird collisions are a source of both significantand indiscriminate mortality for birds, many of which are already in seriousdecline due to other pressures. Unlike some of the other major threats tobirds, such as habitat loss and pesticide use, this is something that citizenscan do something about in their daily lives.

What kinds of birds collide with buildings?
Bird mortality from window strikes has been recorded in morethan half the bird species in the United States. Here in Portland, we have recorded golden-crowned kinglet, rufoushummingbird, Lincoln's sparrow, song sparrow, Swainson's thrush, varied thrush, black-throatedgray warbler,  black-capped chickadee, red-breasted nuthatch, and Cooper’s hawkas window strike casualties.

Over 209 species of birds migrate through Oregon, many of themsmall diurnal songbirds that migrate at night. Some of these are threatenedspecies whose populations already show steep declines. Ironically, common citybirds such as rock doves and English house sparrows are infrequent collisionvictims. This may be due to these species’ adaptation to living amongbuildings.

Birds have been migrating for millennia — whydon’t they learn to avoid buildings?
Birds collide with windows in the daytime when they see the outdoors reflectedin the glass and think they have a clear flight path. Most migrants fly atnight, and the artificial lights in tall buildings confuse them and cause themeither to crash into the structure or to circle it repeatedly until falling to theground in exhaustion. Tall lighted buildings are especially lethal in fog, lowclouds and rain.

As human populations grow, our cities grow withthem: we build more and more structures, and areas that are attractive tohumans — riverbanks, coasts and shorelines — are birds’ traditional migratorypathways. Our built environment is proving to be more and more of a challengeto migrating birds.

If buildings are so dangerous, why don’t we see piles ofdead birds on the sidewalk each day?
Many of the birds that strike windows are killed outrightand fall to the ground where predators — such as gulls, crows, cats and otherscavengers — quickly carry them off, or they are cleaned up by maintenanceworkers. Those that survive an impact may be too injured to fly and findsomeplace to hide as the city wakes up. Those that retain flight ability may beso frightened by the noise and activity of a city in daytime that they fly upand collide with a building again.

How many birds are killed each year in Portland?
We don't know yet! We do know that our Wildlife Care Centeradmits, on average, 200-300 window strike cases per year. We have only been monitoring seasonal routes in Portland since September 2009 in order toestimate the magnitude of strikes. Trained volunteers survey our routes duringspring and fall migration to collect fallen birds. Live birds can bereleased or taken to the Audubon Wildlife Care Center for treatment.Dead birds are collected and examined at the care centerto determine if the injuries are consistent with window strike mortality.

Why should we care if some birds die this way?
As stewards of the environment, everyone can be involved in conserving birds.Birds perform many useful functions, from pollination to seed dispersal toinsect control. In spring, their return signals the renewal of the seasons justas their fall departure precedes the changeover to winter. Across the globe, we can save millions of birds by extinguishing unnecessaryovernight lighting in buildings during fall and spring migration. Migration issuch an arduous business for birds, let’s do our part to make it easier forthem.

Reducing this overnight lighting also helps tominimize ecological light pollution, a phenomenon of increasing study as werealize that our manipulation of natural light-dark cycles has a tremendous andunmeasured impact on the circadian rhythms of plants, wildlife andhumans. Disruption of these cycles impacts migration, reproduction,feeding and predator/prey relationships, and it is only beginning to be quantified.

Audubon Minnesota generously provided some of the above information.

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