Snakes
Snakes play an important part in the balance of nature. If you find one in your bird house, don’t kill it. Snake-proof your house by putting it on a metal pole lathered with petroleum jelly or red cayenne pepper.
House Sparrows and Starlings
If you don’t discourage them, these two nuisance species introduced from Europe will harass or kill cavity-nesting birds. Since house sparrows and starlings are not protected by law, you may destroy their nests. But remember, other birds are protected by law.
House Wrens
House wrens sometimes interfere with the nesting success of other birds by puncturing their eggs. But, unlike the house sparrow and starling, these birds are native to North America and are protected by law. Don’t be tempted to intervene when wrens appear at your backyard birdhouse.
Insects
Many insects lay their eggs and pupate in bird houses. Inspect your bird houses for signs of gypsy moths, blow flies, wasps, ants, gnats and bees. Keep bees and wasps from attaching their nests by coating the inside of the roof with bar soap. In areas where gypsy moths abound, avoid placing boxes in oak trees, which the gypsy moths favor.
Pyrethrin and rotenone insecticides are recommended for killing fly larvae, bird lice and mites after birds have finished nesting for the season.