_Cats_
[Illustration: Cat]
Nesting birds are extremely vulnerable to cats, as are fledglings and
birds roosting for the night. Bell collars on cats offer birds little
protection. Nailing a sheet metal guard or cone to a tree trunk is
unsightly, but may deter less agile felines. Houses mounted on metal
poles are the most difficult for predators to reach, especially if you
smear the poles with a petroleum jelly and hot pepper mixture. The
best deterrent is for owners to keep their cats inside whenever
possible.
_Dogs_
Pet dogs are a hazard to nestlings in the spring and summer. Don’t let
your dog run loose during nesting time.
_Squirrels_
Red squirrels, and sometimes gray squirrels, can become a serious
menace to bird houses and the birds themselves. If you find your nest
hole enlarged, chances are a red squirrel is the culprit. Once inside
the box, squirrels make a meal of the eggs and young. Adding a
predator guard made of sheet metal to the entrance hole is usually
enough to keep squirrels out.
_Raccoons and Opossums_
Raccoons and opossums will stick their arms inside nest boxes and try
to pull out the adult, young, and eggs. Adding a ¾-inch thick predator
guard to the bird house or an inverted cone to its pole support is a
simple solution.
_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.