Well, SOME birds are pretty dumb, that's for sure. Some of them obviously don't have the brain power to figure out that they're attacking their own reflection. Either that, or their instinct temporarily overwhelms normal thought...
I have heard, but not seen, that if turkeys look up when it's raining, they'll drown. And we have the expression silly as a goose; haven't witnessed any reason for that, myself, but it must have come from somewhere. I think the term birdbrain actually originated from how small a proportion of a bird's body the brain is.
Here's a link that offers another opinion on bird brain power:
http://www.pbs.org/wnet/nature/animalmind/intelligence.html