What kills 5% of the bird population (1 billion birds) in North America each year? Plate glass windows. Birds see trees and sky reflected in them or a possible opening through which to fly, and crash into the glass and die of brain injuries (more here). It's a very serious problem, and today I had two experiences with the issue at work.
Last week I noticed a dead Mourning Dove on the window sill inside the barn where I park. I thought someone had found it and placed it on the ledge, but today I went and looked and could see the impression of feather dust on the window where the bird crashed into it. Apparently, the bird had been feeding in the open barn and tried to fly out through the bright window, only to crash and die on the spot.
Sometimes birds see their reflections in the window, and when they are feeling territorial, may attack their reflection. This afternoon a co-worker called me down to watch a Northern Cardinal repeatedly crash into the window of her office. The bird would perch in a bush near the window, look at the window, and fly directly at its reflection, falling to the ledge before hopping back into the bush and starting over again. While I'd heard about this behavior for years, this was the first time I'd actually seen it.
Both these behaviors can be reduced by cutting down the reflectivity of glass windows. It can easily be done with screening or by placing dots every 2 x 4 inches across the outside of the window (hawk or falcon shapes on the window don't really do it unless covering the window every 2 x 4 inches like the recommended dots). However, as long as people enjoy their clear window views, anything to break up the view is hard to sell. Interestingly, if bird populations were the stock market, how long would we accept 5% declines each year? Indeed, how long can we?