Birdfeeders discouraging blackbirds?

Last year was my first summer of birdfeeding......in the fall a huge number of blackbirds descended upon the feeder a couple of times but pretty much moved on quickly as they were migrating south, when huge numbers showed up a month or so ago thought they would move on as well as don't remember any blackbirds at my feeder during summer months. A core group of 15 or so blackbirds seem to be hanging out near my feeder. I wait a few days to refill in the hopes they have moved on but they swarm as soon as I refill....so my question is are these blackbirds likely here to stay for the summer or are they passing through migrating somewhere else -- my only objection is they are discouraging the other birds --especially my cardinals...appreciate any advice on how to discourage them.

I used to feed the birds several years ago and found the blackbirds/grackles (whatever they are) stole all the food all spring/summer long. Between that and all the mice that suddenly appeared, I stopped feeding the birds and now I only have feral cats in my yard.

They are probably grackles or starlings. I don't know if I've ever seen an actual blackbird in Maplewood. Here's a link to attracting small birds and discouraging larger ones: http://www.wildbirdhabitatstore.com/index.php?option=com_content&view=article&id=58:out-witting-squirrels-and-blackbirds&catid=9&Itemid=170