By Staff
Last week's column on maintaining bird nest boxes prompted considerable response, mostly regarding bluebird houses. It seems as much effort has been expended on designing the perfect bluebird house as building the Space Station. About all that's come of all that is (1) – the realization there is no one perfect bluebird house and (2) – lots of confusion. The bottom line is bluebirds will nest in just about anything that resembles a cavity. The trick is making that cavity as safe as possible so they can successfully raise their brood.
One never ending debate is the best entrance hole size and shape. For decades a one-inch round hole was the standard. This is just big enough for the bluebirds to squeeze through but too small for starlings which readily take over the houses if allowed. Then a vertical, oval shaped hole measuring 1 3/8 inches by two-inches came into vogue. Bluebirds much prefer this configuration but, while it still deters most starlings, a determined starling or cowbird can gain access through this sized hole. Reducing the oval hole's size by even 1/8 inch causes most bluebirds to shun it so that's not an option. The trend now seems to be reverting back to the original one-inch round hole.
While a minimal hole size may deter most starlings, not so house sparrows and house wrens. Both wreak havoc on bluebirds and they easily fit through any hole a bluebird can enter. Wrens poke holes in bluebird eggs and kill the babies. House sparrows also kill the adult bluebirds if they can trap them inside the house. Some folks feel a slotted entrance hole across the whole front of the house deters sparrows by letting in more light. The slot should be 1 3/16 inch wide to block starlings. Sparrows may prefer a dark house but many will put up with the additional lighting so it's not entirely effective. If there's an advantage with the slot it's that the bluebird has more room to escape. With additional escape routes in mind, some folks swear by larger boxes with two entrance holes. The theory being while the raccoon, cat, sparrow or whatever is focused on one hole the bluebird can escape out the other. It may work to a degree but sparrows will gang up and cover both holes. As for the racoons and cats, a good predator guard on the mounting pole is a far better solution. Ants are deterred by keeping a band of grease around the mounting pole.
The most effective way of dealing with sparrows and wrens is choosing the right location for the bluebird house. House sparrows live around barns and buildings in conjunction with people so don't put up houses near these areas. Wrens live in brushy and wooded habitat so erect the bluebird house several hundred feet away from those areas. And obviously, don't put up wren houses if you want bluebirds.
House competition is also great between swallows and bluebirds. Some folks lessen this by putting up two houses about 10 feet apart. The swallows take one and the bluebirds the other. They seem to make quite compatible neighbors. I'm not sure how that works when there are more swallows than bluebirds. Bluebirds are territorial so they need at least 100 yards between their houses.
Another great debate is how high to mount the bluebird house. The norm has long been five feet but this is mostly a convenience for us in monitoring and maintaining the house. Many folks report bluebirds, when given a choice, choose houses eight feet up or more. Common in California is to hang bluebird houses by hooks on tree limbs as high as 30 feet. Cats can leap about six feet up which makes the predator guard on a five-foot high house useless. Mounting the house higher solves that problem.
As you can see, there are many ways to go about sheltering a bluebird. There is no single perfect bluebird house but there are many solutions to particular problems. Monitor your houses closely and adjust your houses to fit your situation and needs. Carpe diem.