Plentiful food means a smaller territory and less food means birds have to search for food over a larger area. Birds defend their territory most aggressively during the breeding season when they are protecting a nest site. Territories also vary in size depending on the bird species, a typical suburban songbird 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. Nonetheless, the chickadee is just as committed to the defense of its territory as is hawk. Unlike a real bird, which will normally retreat from aggressive behavior, the rival in the glass appears to fight back as often as it is attacked.
Birds have no ability to reason or to comprehend the concept of reflection.
To a male cardinal for instance, its own reflection is a rival and he has no choice but to attack it until it leaves. Which Display Behavior Territorial behavior at windows occurs most often in birds that frequent yards and have nest sites in trees and shrubs near houses. The birds mostly frequently reported displaying the behavior are the American robin and northern cardinal, but occasionally a northern mockingbird, American goldfinch and ruffed grouse. During their search for a nest site these birds probably catch a glimpse of their reflection in a window, and the battle begins. Despite the violent appearance of this behavior, from the other side of the window, the birds rarely kill themselves.