Several things can happen when a bird approaches a building. Some birds approach and, seeing either a reflection of the grass, trees, and sky behind them or the grass, trees, and sky on the other side of a visual pass-through, simply collide with the window at whatever speed they were flying. Many of these birds (Dan Klem estimates about 50% of them) will suffer a severe injury and die immediately. Others might be stunned, knocked out, or otherwise impaired to the point that they cannot fly. Of these that don’t die immediately, some will recover fully and some will die some time later, either because they are easy pickings for a neighborhood cat while in their dazed state or because they’ve suffered too much brain damage for “time” to repair their wounds. Some birds will be dazed from the initial collision and able to fly to some more protected perch nearby where they will either recover on their own or ultimately die, but at least they are better protected from predators if they can make it to the relative safety of a nearby tree. Klem estimates that 50% of those that die from windows succumb to their injuries sometime after they leave the site of the collision.
I find these injured-but-not-dead-birds rather frequently: 56 individuals since I started paying attention during the fall of 2009. As of August 2012, I’ve also been recording the location I first encountered each of these live birds. Some of them look pretty rough when I find them, but if when I grab the bird it makes a fuss and tries to escape from me, I consider that it’s got a fighting chance to recover from whatever injury was so severe that it allowed me to catch it by hand. These feisty birds I carry away from the building and place in some dense shrubs or low branches. This way, the bird is out of the direct sun and has some better cover from predators while it regains its wits. If the bird is, when I leave it, alert and able to perch strongly, I usually will not count it as a “casualty”; I’ll assume that it’s one of the ones that would have survived its ordeal.
Some birds, however, are neither feisty or alert. Their bodies droop, their eyelids sag, they might be panting heavily, they can neither hold onto a perch or stand up straight in the palm of my hand. The prognosis for recovery is very poor for these birds, and I have euthanized 3 or 4 of them in this condition, and counted them as casualties.
There is another category, however. Some birds are found in fine physical condition: active, volant, and apparently not dazed in the least. They are, however, out of place, like the Eastern Wood-Pewee I found this morning perched on the ground, on a sidewalk, in an exterior alcove of the Noble Research Center. These birds are “trapped” in the sense that they are in inappropriate habitat and, like a classic funnel trap, for some reason unable to figure out that they simply need to fly back the way they came for 100 m or so so they can clearly see how to fly around or over the building. Instead, they might spend hours flying back and forth against the back wall of the alcove. In this way, I suspect that some birds die from the stress and exhaustion of this experience, and that they might never actually strike the window with enough force to cause injury. It is the shape of the building as much as its windows that makes it deadly for migrant songbirds. This is one reason that the dead birds that I find are so often in immaculate shape: they are dying from cardiac arrest from exhaustion as opposed to cranial bleeding from collision.
So I consider trapped birds to be either of the following:
* a live bird that I am able to catch and move to a secluded location where, based on my assessment of its condition, I judge that it can survive the experience,
* a live bird that I am able to “shoo” or herd away from the building.