Experts at the Cornell Lab of Ornithology and the National Audubon Society said the most likely cause was a deliberate poisoning of the troublesome grackles, which is more common than people think. It's also legal, with local permits, said Greg Butcher, director of bird conservation at the Audubon in Washington.
http://www.bismarcktribune.com/articles/2007/01/08/ap/us/d8mh9nvg1.txt
I was in downtown San Antonio a few weeks ago. Hundreds of birds were flying where ever I went. I thought they were pretty. They were loud though and I could see businesses and hotels having concerns.