Crow Chasing Smaller Bird in Yard

In mid-afternoon today (3/28/2010) I observed a crow chasing a much smaller bird. The chase started somewhere near one of the bird feeders in the front yard and continued up to a mid-height level in the fir trees. The potential victim eventually escaped the crow. The bird that was being chased flew in a curious spiraling pattern, and the possible reason why escaped me at the time. But as I thought about it more I came up with a theory.

First, it’s fairly unusual to see a crow chase another bird unless it is either very young, is sick or disabled or otherwise can’t defend itself.  It’s probably too early in the breeding season to have fully-fledged young around. I think the bird being chased may have been a House finch and that the finch may have had the disease that forms tumors on its face.  That disease is rife amoung the House finch population and was evident where we lived in Texas back during at least the 1990s.  The tumors will eventually result in the bird’s death, either by deforming the beak so that it starves to death or by blinding it with the same result.  I think this bird may have been blinded in one eye and that’s why it was flying in the spiraling pattern… it was the only way it could see where it was going.

It’s just a theory but I suspect it’s a good one.

The House finch tumor disease is one of the reasons that you are admonished to clean your feeders regularly.  While I'm certainly not an opponent of regular feeder cleaning, I've observed that the finches with the tumors are more dependent on the feeders than other birds.  They're often too sick or blind to obtain food from any other sources.  If you try to chase them away from a feeder they often won't fly far since they're already sick and/or partially blind.  So that sick finch can and probably will be quickly back to a feeder you have just cleaned. 

If the bird is allowed to hang around it will only die a slow and painful death and the longer it is in the area the greater the chances that it will transmit the disease to the other finches.