Crows are generally larger than most common feeder birds. Feeders, such as long seed tubes that are surrounded by a rubber-coated mesh, allow smaller birds to enter and feed and prevent access from larger birds.
Feeders that contain a weighted treadle also prevent larger birds from getting seeds. When a crow lands on the treadle, it drop down over the food blocking access. This method also works against squirrels. Some feeders are designed without perches that larger birds must land on to feed. Finches and smaller birds can cling to the side of the feeder and still stretch down to feed.
Metal caged feeders are also effective in preventing large birds such as crows from entering. There are combination caged feeders that allow small birds to feed on seeds and suet within the protected area.
Web sites such as BackyardWildBirds.com feature specially designed feeders such as the grackle proof feeder that feature a caged sunflower seed tube on the inside with four seed ports for smaller birds. The feeder has a 2-pound seed capacity. It has a metal cage constructed with a 1 ½ inch grid that smaller birds can penetrate but grackles and crows cannot. The cage also provides protection against predators as the birds feed.
Filling feeders with safflower seeds rather than sunflower naturally helps to deter certain species. Squirrels don’t like them and it is often the least favorite seed for some birds.