They must have been cleaned with an advanced formula of Windex and then sprayed with scent from one of their species in order to lure them to their unwitted doom.

Probably, it's bright outside and the birds are seeing a reflection of the sky.

maybe they have a mental disability

1) Birds are sometimes bathing in dirt, but I don't thing they can fly covered with curtains... 2) What dirty is for you, is not necessarily dirty for birds. 3) Here an interesting thread about this issue. Among others submission, I found this: "I wondered why there was bird poop all over the outside mirrors of my parked car until I noticed a bird fighting his reflection for hours on end. It's common territorial behaviour. A couple of socks over the mirror solved the problem. A black cutout of a small hawk taped to the window will likely fix your problem." "So the guy who won the Ig Nobel a couple years ago for observing necrophilia in ducks has currently been studying a similar situation in Rotterdam with a single individual bird that has been repeatedly flying into the same window for a couple years now. The guy's name is Kees Moeliker, you might send him an email. Even as a professional ornithologist, he said he hasn't figured out why this is happening. It happens year round, clean window or dirty, ornamented or not. The one variation was that he did it less during the season when he was mating or had young, when he was busy with those more productive activities." "I don't see that anyone else would be reading the thread this late, but I made a transparency of a silhouette of a large bird and put it on the window today. Now the cardinal is attacking that spot with even more intensity." Source and further information: http://ask.metafilter.com/26033/What-causes-a-bird-to-fly-into-my-window-repeatedly-with-reckless-abandon

MOst of the time the birds see the reflection of the trees and sky and light in their side of the glass and think it is safe to fly through or to. It doesn't matter if you place curtains on the inside because that doesn't change the outside reflection.

Because they're dumb. I love all these explanations that involve the birds having reasoning capabilities. They're birds. They're dumb. Ergo, they do dumb things. Like fly into windows.