Can you overfeed wild birds?

Probably a dumb question, but will pose it anyway!

I started putting out a 'fat ball' feeder in my garden about 3 years ago and, perhaps coincidentally, the number of sparrows and tits has grown enormously. There are now a large number living in the eaves and the hedgerow opposite the house.

I am (they are) getting through around 3 fat balls a day. Which is fine by me, as long as I'm doing them no harm. I have water for them too.

I can't easily feed them anything else due to a pretty large crow/jackdaw population - my current feeder is crow proof.