1 Answer

Birds Attacking Windows: One fine spring day without provocation or warning, a cardinal or robin begins flying into a window, over and over again. If we could witness the scene from the bird's point of view, the real cause of the bird's behavior would be clear. We would see what the bird sees: its reflection in the windowpane.

Territorial Behavior: When a bird decides on a nest site, the surrounding area becomes its territory and it aggressively protects its territory by driving other birds away. Northern Mockingbirds sometimes display similar behavior when they are protecting a food source, such as a shrub full of berries.

Most birds have a strong sense of territory, especially during the breeding season. It is nature's way of minimizing competition for food and nest sites. The singing of adult male birds is a familiar manifestation of nesting territoriality in birds. A male's characteristic song, usually given from several preferred perches in its territory, says, in effect, "This is where my mate and I plan to nest; others of my species respect my boundaries and stay out."

Territorial Behavior at Reflective Surfaces: When a territorial bird, accidentally sees its image in a reflective surface such as a window, or in the mirror or bumpers of a car, it perceives its reflection as a rival in its territory and it tries to drive the other bird away. Birds apparently do not have the ability to reason or comprehend the concept of reflection. To a male cardinal, its own reflection is as much of a rival as real cardinal would be.

Size of Territory:  The size of territory varies with the time of year (breeding or non-breeding season) and the availability of food in the area. An abundance of food means a bird can survive in a smaller territory, while a shortage of food means that birds have to search for food over a larger area, and therefore defend a larger territory.

Territories also vary in size depending on the bird species; a typical suburban song bird such as an American Robin needs only a small backyard for its territory while a pair of Black-capped Chickadees will chase off trespassers in an area anywhere from 8 to 17 acres.

Species That Display This Behavior
Territorial behavior directed at windows and automobiles, occurs most often in species that frequent suburban yards and look for nesting sites in the trees and shrubs that surround houses. The species that most frequently display this behavior are: American Robin and Northern Cardinal, and occasionally Northern Mockingbird, American Goldfinch, Wild Turkey, and Ruffed Grouse.

Can They Harm Themselves?
Despite the violent appearance of this behavior, from the other side of the window, the birds very rarely kill themselves, all though there could be injury to the bird's beak if the behavior continues.

How Long Does the Behavior Last?
The breeding season, the time in which birds mate, build nests, lay eggs, and raise young, is the time when birds are most territorial. For robins and cardinals, the breeding season may total seven or eight weeks; and the "window fighting" may continue off and on between April and August as two or three broods are produced during that time. Quite often the behavior stops during the month of May, possibly because the bird has chosen a new nesting territory.

SITUATIONS AND SOLUTIONS

Window Attacks
Reason would suggest that covering reflective surfaces would eliminate the behavior, but this is not always the case. When the outside of the glass is covered, the bird may go searching for its imaginary rival at other windows. Not surprisingly, it will find the intruder at every window it encounters. There are reports of robins attacking as many as fifteen windows on both the first and second stories of homes. For this reason, it is probably best to be patient and do nothing. If bedroom windows are being attacked and the occupants are being awakened at dawn, cover just those windows.

If people chose to cover the windows, it is the outside that must be completely covered. A medium-weight, plastic painter's drop cloth, available at hardware stores and home centers, works well. The plastic is clear enough to allow light into the room, but cloudy enough to eliminate reflections. Attach the plastic to the top of the outside of the window and allow it to hang freely over the window. The bird will no longer be able to see itself in the window, and the movement of the plastic, as it blows in the wind, should frighten it.

Do not waste money on fake owls or rubber snakes, which reportedly frighten birds. The rival bird will still be visible in the window, and the territorial birds quickly learn that these artificial critters pose no threat.

Birds Attacking Cars
Cars have many reflective surfaces: side mirrors, bumpers, windows, and chrome grills. Place opaque plastic grocery bags over side mirrors, and cover other reflective surfaces with tarps or sheets of opaque plastic. Moving the vehicle to a different location may also discourage the behavior.

Birds and the Law
Most birds are protected by federal laws under the "Migratory Bird Treaty Act of 1918," making it illegal to destroy or possess birds, their nests, or their eggs. In Massachusetts it is also illegal to relocate wildlife. The only exceptions to the Migratory Bird Treaty Act are non-native: House Sparrows, European Starlings, and Pigeons. Trained and licensed wildlife rehabilitators, who have passed a federal and/or state-administered test, are permitted to care for injured or orphaned wildlife.

http://www.massaudubon.org/Nature_Connection/wildlife/index.php?subject=Birds:%20Behavior&id=17

Since windows are reflective, and there probably is not a lot inside your home that a bird may find enticing, the general school of thought is that birds see the wide open outside world reflected in your picture window and believe they have a clear path of flight. Therefore, when they fly up to your window, they think they are flying to more sky, trees and flowers.

f the bird flies up to the window and can see through to another window, she may believe she can fly on through your living room or kitchento the other window. She does not realize that there is a barrier between herself and the yard she sees through your home. You may have even experienced this yourself if you have ever walked into a sliding glass door, believing it to be open.