The leading cause of death for birds in the United States is collisions with buildings. (See table 2 in this paper.) As much as cats love killing birds, cat-related bird deaths are less than 1/5 as common as collisions with buildings.
The problem is not that birds can’t navigate around large stationary objects: they are actually quite good at doing so. The problem is that we use a lot of reflective glass, and there is no such thing in nature as a vertically-oriented mirror. So birds see a reflection of sky and try to fly straight through a window, or they see a reflection of trees and fly at the window to try to land, and so on.
It turns out that it’s quite easy to reduce the number of birds that fly into glass-walled buildings: put a pattern on the glass to make it a visible surface rather than a mirror. As of next year, that will be required in New York City for new buildings. So a small victory that will hopefully spread to window replacements in general and to other cities.
The odd thing to me is that this law is even required. Architects – in theory, and in practice among the members of the profession that I know – want their buildings to be seen. Wrapping the building in glass greatly reduces the visual impact of the building and therefore, to me, weakens the appearance of the design. (The reduced visual impact is, of course, part of the cause of the bird problem.) And “reflecting the sky” or “crystalline form” were design clichés more than fifty years ago. Glass is really not the best material to use for entire facades, but somehow energy conservation regulation didn’t kill the all-glass facade; maybe mandatory markings on the glass will reduce the popularity of that particular cliché.
