While I nodded, nearly napping, suddenly there came a tapping,
As of some one gently rapping, rapping at my chamber door.
“’Tis some visitor,” I muttered, “tapping at my chamber door—
Only this and nothing more.”
- Edgar Allan Poe, The Raven
There was a time when most zoo animals were kept behind bars. Technology was what it was at the time, and bars seemed like the most effective way to hold animals in while allowing visitors to get close to them. Carl Hagenbeck popularized moated enclosures at the turn of the last century, but many zoo directors still preferred the bars. Then came new advances in technology. Harp-wire... stainless-steel mesh... and glass. Well, okay, glass isn't new by any means, and it had always been used in reptile houses and small mammal houses. But new tempered glasses, as well as acrylics and Plexiglas, were developed which could hold back bears, big cats, and other large carnivores.
Bars came down, glass came up, and exasperated keepers began spending lots of time with a bottle of Windex in one hand, a rag in the other.
Glass, unfortunately, has a downside, besides being a pain in the butt to clean. It's a great, nearly invisible barrier for visitors (unless kids rub their hands all over it). It's also a terrible, nearly invisible barrier for birds. Not so much zoo birds*. I'm talking about wild ones.
The American Bird Conservancy estimates that a billion birds a year - all species, all sizes - die from collision with glass windows in this country. Many are killed in cities with giant, glass-covered skyscrapers.
Now, the solutions are doable. For one thing, windows can be shaded to make the glass more visible to birds. For another, reflective tape or images can be put up to clue birds in to the fact that there is a solid barrier ahead of them. Many zoos put flying bird decals on some of their windows as a warning to flying birds. There are transparent films which allow people on one side of a window to see out, but make the view opaque or dark from the other, sort of like the tinted windows of limousines.
Or, we could just use less glass outdoors. For zoo settings, that's my favorite option. There was a time when glass was the only alternative to, say, chain-link or other ugly wires, but now there are so many nearly-invisible meshes, many of which are not only cheaper than glass, they a) don't break or scratch, b) don't provide an annoying medium for banging on, and c) allow visitors to hear and (yes) smell the animal more easily, while not impeding viewing or photographs.
Many zoos take this steps to reduce bird fatalities on their own campuses. It's even more important to work to communicate this problem, and its solutions, to the visitors. Very, very few of those billion-odd birds that die every year are killed in zoos. They die in homes, in cities, and offices... many of which are lived in or staffed by people who visit zoos and aquariums. By providing answers of how to prevent or reduce window-strike, zoos can help educate their visitors and save millions of feathered live a year.
* Which isn't to say that glass can't be a problem for zoo animals also. When a new animal is released into an enclosure, especially one with a glass or wire fencing barrier, it's customary to put a visual barrier over it - paper tapped to the glass, cloth zip-tied to the fence, etc - to help familiarize the animal with the barrier. That way, it doesn't fly or run straight into it. As the animal settles in, the visual barriers are gradually peeled back.