You may have design flaws in the housing you’ve provided that allows these aggressive, non-native birds to thrive and our native birds to decline.
Some ready-made bird houses may look cute but are not necessarily designed to keep the bullies of the bird world out.
Perches, for example, aren't needed by birds but attract house sparrows and starlings.
Design and construction of bird houses need to be species specific. The most important part is the entrance hole. If the hole is too small, the desired bird won’t be able to enter. If it’s too big (and this is more likely) undesirable wildlife – like non-native sparrows and starlings, and uninvited squirrels, can get in and harm, evict, or kill the desired bird.
As a rule, house sparrows can’t enter a nest box if the entrance hole is less than 1-1/8 inches in diameter. Starlings can’t enter if the hole is less than 1-1/2 inches in diameter.
Information in the Washington Department of Fish and Wildlife (WDFW) “Woodworking Projects for Backyard Wildlife,” available at http://wdfw.wa.gov/living/projects/, specifies entrance hole sizes to exclude these birds wherever possible. Even if you don’t want to make your own nest boxes, it’s a good resource for making better decisions when you purchase bird houses.
Bird houses often have to be maintained on a yearly basis to stick to these specs. You’ll need to patch or restore the entrance holes after squirrels or woodpeckers have tried to enlarge them. There are many ways to do this, from attaching wooden extensions or “donuts” over the holes to fortifying them with metal washers.
If you still have problems with aggressive non-native species even when you follow the standards, there are also design alternatives.
A diamond-shaped entrance hole that is no more than 7/8-inch deep and up to 3-1/2 inches wide, will exclude house sparrows and starlings. To accommodate the slightly bigger violet-green swallow, file down the area inside of the entry hole by just a quarter inch. You can see these specs at http://wdfw.wa.gov/living/projects/basic_songbird.html.
Anacortes bird enthusiast Gene Derig came up with that diamond design but recently told us about an even simpler way to accommodate and protect not only violet-green swallows, but also chickadees, nuthatches, and other native species.
“Try using a ¾-inch high slot configuration,” Gene said. “The slot is made by drilling three consecutive horizontal holes with a ¾-inch drill bit, and then just shaving out what’s left. It’s especially good for urban areas where there are more house sparrows.”
The purple martin is a species that could really use help with suitable nest sites within its range in western Washington, since its numbers seem to be declining. In fact, it’s a candidate for state protective listing, in part because of competition from sparrows and starlings.
WDFW biologist Chris Anderson notes that purple martins are colony nesters, but those attractive “multiple apartment complex” bird houses designed for them can be a problem in urban areas where starlings and house sparrows are numerous.
“A cluster of gourds drilled with holes seems to be much less preferred by starlings and house sparrows and the martins love them,” Anderson said.
Another way to help is to be more strategic with placement of bird nest boxes.
Martins, like all swallows, feed on insects, mostly on the wing, often over water where insects concentrate. Hanging a gourd collection over water makes it that much more enticing to martins and less inviting to sparrows and starlings.
Sparrows and starlings usually won't nest within ten feet of the ground. Placing nest boxes four to five feet off the ground and in brushy areas will discourage these birds, and will readily be used by many native species, from bluebirds to wrens. Nest boxes at this height, however, are vulnerable to predators such as cats.
If you want to get rid of house sparrows or European starlings nesting in a bird house, it is legal to remove their nests and destroy the eggs. Unlike most birds, these non-native, introduced species are not protected by state or federal law. Nests may have to be removed five to six times before sparrows or starlings finally abandon the house.
Sometimes the best thing to do if your area is plagued by starlings or house sparrows, and you can’t actively manage them, is to simply not use bird nest boxes at all.
There are many other resources to help you address bird house issues in addition to WDFW’s “Living With Wildlife” webpages. Some of the best information is available on the Sialis website, developed for people interested in helping bluebirds (which are in the genus Sialia) and other native cavity-nesting birds. For starling problems, see http://www.sialis.org/starlingbio.htm and for House sparrow problems see http://www.sialis.org/hosp.htm. Another good site is http://www.treeswallowprojects.com/index.html .