As a follow up to my first email, a 2nd sparrow was eaten by a Grackle at our feeder and a 3rd has had a close escape. The Grackle had the 3rd one pinned to the ground and was plucking his feathers (they seem to do this before eating) when I noticed it. I had time to get my camera and telephoto and went out out to take a picture (by which point I assumed that the sparrow would be dead). However, when I startled the Grackle, the sparrow took off. I'm amazed that it was still alive. The first two were finished off pretty quickly.
Anyway, while Grackles may well eat other birds on occasion, we've had the feeder up for 7 years and never seen a Grackle attack before. Now we've had 3, that we've seen, in 3 weeks. Maybe I'm using the wrong bird seed.