Sometimes, you can switch to only safflower seed to get rid of unwanted birds but I don't know if that will work with cowbirds. I think I've seen it suggested to discourage starlings or grackles. I can't remember which. Do the cowbirds seem to like both the swinging feeders and the ones that are stationary?
I put out birdseed in December right after Christmas and it lasted a long time - I assume because of the warmer winter. The birds weren't as hungry this year. So I decided to wait until it snowed to put out more and it didn't really snow so I have lots of birdseed leftover. I don't usually feed birds in the spring and summer.

Here's a link to a thread about Cowbirds in the Birdwatching forum.

Here is a link that might be useful: Cowbirds

I have thought about feeding birds in my vegetable garden, but so far I am too cheap to buy seed. I wouldn't mind any kind of bird if it would eat caterpillars and bugs.

If I remember right, Cowbirds are the birds that raid other birds' nests and leave their own eggs ??? I wouldn't encourage them to stay, nashonii.
Sunny