Rats don't know, and probably don't care, that the seeds you put in your bird feeder are meant for your feathered friends. Besides being a nuisance, rats carry diseases, damage plants and your property and have been known to bite people. Getting rid of them preserves the food for the birds and protects your home against an infestation.
Rats like bird feeders for the same reason as birds, raccoons and squirrels: they're an easy source of food. While rats might not be able to get to the food in the feeders as easily as a bird or squirrel, they take advantage of the fact that birds are messy eaters and scatter seeds and shells all over the ground. The rats come in and clean up after the birds. These opportunists will also raid feeders directly if they get the chance, but rodent-proof feeders can usually keep them out.
Since rats move in because they've found a reliable source of food, a simple way to deter them is to stop filling the feeder for several months, or fill it with only as much food as the birds will eat in a single day. This disruption in their feeding habits will force them to look for other food sources and move away from the bird feeder. Rats won't always leave however, and sometimes they will find another food source on your property. Disruption works best in combination with other control methods.
Catch trays keep seeds from falling to the ground where they're easy for rats to collect. If cleaned every evening, they can deter rats. The difficulty with catch trays, however, is that they must be very large to catch the seeds flung by birds at the feeder. They can also harbor diseases dangerous to the birds if the bird droppings are not cleaned regularly. Like disruption, catch trays are best used in combination with other methods to get rid of rats.
It's nearly impossible to sweep up every seed and shell that falls from a bird feeder, but daily cleaning, especially in the evening, can reduce the amount of food available to rats. Remove as much fallen seed as possible and dispose of it in a rat-proof garbage container. This solution isn't likely to get rid of rats that have already made the ground under the feeder a regular meal place, but may reduce their numbers or prevent them from noticing the feeder in the first place.
Cats are a good natural deterrent for rats, but they are not for everybody, and a cat in the vicinity of a bird feeder may also scare away birds. If you decide to rely on a cat for pest control, make sure your bird feeder is well out of his reach. Poisons and baits work well for rat control, but can pose a danger to animals that might eat the dead carcasses. Traps are a non-chemical alternative, but should be put in place in the evening just before rats become most active and located where they won't harm other animals or children.