If they are already there, it is illegal to harass them or destroy the nest. Try using another door for a couple of weeks. Also, you can try to remove all horizontal ledges the following winter, but remember that these birds are helpful insect eaters! Barn swallows are insectivores. Flies, grasshoppers, crickets,dragonflies, beetles, moths and other flying insects make up 99 % of their diet. They catch most of their prey while in flight, and are able to feed their young at the nest while flying.