Nesting Birds

Their brains are small but their will is strong

April 1, 2005

The arrival of spring incites passion in all creatures that fly — human as well as feathered — and those passions seem to urge both to the airport. Human fliers want to get airborne to knock the wintertime rust off their piloting skills and feathered fliers are searching for a place to build a nest and propagate their species. Those entirely different agendas are a conflict waiting to happen.

In May 2001, the pilot of a Cessna 182 boarded two passengers at Jack Northrop Field/Hawthorne Municipal Airport in California, for a flight to a private airstrip. Shortly after takeoff, the pilot noticed a strong odor similar to sage, but continued the flight. About an hour later, according to the NTSB accident report, the original odor abruptly changed to a smell of something burning. The pilot did not detect any visible smoke, but the smell worsened — so much so that he elected to attempt an off-airport landing on a highway. An unsuspecting driver forced the pilot to veer to the highway shoulder and the airplane struck a signpost, nosed over, and came to rest inverted.

The accident investigation revealed remnants of a bird nest in the engine compartment lodged on top of the number-one cylinder near the cabin-heat air intake system. According to the NTSB report, the nest would not have been visible during a normal preflight inspection. The nest remained in place throughout the one-hour flight and accident sequence.

Similar accident reports are in the NTSB database, but most bird nests built in airplanes don't result in accidents. Usually they create only a messy nuisance with an occasional ethical conundrum thrown in when a nest is discovered to contain eggs or young birds.

But undiscovered nests can result in serious damage by blocking cooling airflow to cylinders or the oil cooler, or nesting materials can trap water and debris in the wing or fuselage structures, which may promote corrosion. Starlings are known as particularly untidy nesters and will use all sorts of debris to build their nests.

Tom Rouch of Top Gun Aviation in Stockton, California, says that birds building nests in engine cowls usually go as far back into the cowl as possible. "On many engines it puts the nest right at the oil cooler...and the nest cuts the airflow through the fins, causing high oil temperatures," he says. Rouch also reports that he has seen nests removed from Mooney tail cones that contained so much material that they would fill a grocery bag.

"While we frequently remove eggs and even slightly cooked chicks," says Rouch, "I think the most bizarre [bird incident] was when my mechanics were pulling an airplane in that had come from the Bay Area, and we all could hear a chirping sound. We found four live chicks in the tail cone." So much for that pilot's preflight inspection.

Don Maxwell of Don Maxwell Aviation Services in Gladewater, Texas, has seen damage to nav light wiring and strobe light power supplies when birds nest in the wing. "Of course, the bird poop is a little tough on the paint, too," he adds.

Nesting instincts

In temperate areas of the United States, the longer and warmer days of spring trigger birds' instinct to mate and raise a brood. This mating period, roughly from March through July depending on where you live, is the time of maximum nest-building activity. The primary threats are from the European starling and the house sparrow. These cavity-nesting birds find the nooks and crannies of an airplane irresistible because they offer what nesting birds seek — shelter from the elements and protection from predators.

Birds see an airplane as just another structure, according to biologist Brainard Palmer-Ball. "Before man came along, cavity-nesting birds built nests in cliff faces and tree hollows. Now the majority of nests are built in artificial structures," he says. Once birds select a site, they can build a nest in a couple of days if nesting materials are nearby.

Starlings and sparrows are very persistent once they choose a nesting site. If they happen to select your airplane as a place to call home, expect that the nest builders will return even if you completely remove all nesting materials while the nest is being built. "Birds don't associate nest removal with predation," says Palmer-Ball. "Nesting materials will naturally drop so they're used to seeing some disappear." However, removing a nest that contains eggs or young birds may be a deterrent because the birds associate that with predation, according to Palmer-Ball.

After the birds build the nest and lay a clutch of eggs, they incubate them for 11 to 14 days. The young sparrows leave the nest after about two weeks and starlings after about three weeks. Unfortunately for pilots, both starlings and sparrows produce more than one brood per season. And if sparrows select your airplane as a nesting site, expect a season of trouble. A male sparrow's bond with a nest site is stronger than that with his mate. He may lose a mate, but he won't give up his nesting site.

Preflight clues

When nesting time arrives in your part of the world, birds will give you clues about what they're up to. Seeing birds around your airport with small pieces of straw, dried grass, and other debris in their beaks is a sure-fire sign that nesting season has arrived. Simply watching your airplane for a few minutes is sometimes all that is necessary to tell if your aircraft has been selected as a nesting site. If birds are building a nest, they will make numerous trips to the site with bits of nesting materials.

Other indications that birds have selected your airplane as a nesting site are the droppings they leave behind as they enter or exit the cavity site. If one side of the cooling air intake on your airplane's cowling has one or more bird droppings, there almost certainly is a nest on that side of the engine compartment. "A flashlight looking down the intake will almost always show some sticks or grass," says mechanic Rouch.

Sometimes the telltale bird droppings are on the ramp under the airplane. A starling nest was discovered in the horizontal stabilizer of a Cessna 172 after multiple droppings were noticed on the asphalt underneath. Incredibly, the birds gained access to the interior of the horizontal stabilizer through the tiny gap between the horizontal stabilizer and elevator. The birds entered the horizontal stabilizer through lightening holes in the structure.

Other signs on the ramp are loose clusters of grass or debris under or near the airplane when there usually isn't such material around. Birds may have dropped it there during their nest-building activities.

If you do discover a bird nest during preflight, the location determines what needs to be done. A nest in the engine compartment needs to be completely removed. This can be a time-consuming chore because some or all of the cowling usually needs to be removed to gain sufficient access for thorough removal. Nests found in airframe or landing-gear wells can be removed and any leftover debris cleaned up at the next scheduled inspection.

Keeping them out

Palmer-Ball says the only completely effective way to keep birds from nesting in your airplane is to close up or block access to cavities that birds might find attractive. Many companies that make canopy covers for airplanes also make custom-fit plugs that block access to the engine compartment and certain other intakes.

But pilots of some airplanes, such as Mooneys and Piper Cherokees, have to custom make their own plugs for openings in the tail cone that are nest-prone areas. Rags or foam rubber stuffed in those cavities keep birds out; however, pilots must remember to remove these nonstandard items before flight. A note attached to the yoke or instrument panel may help.

Even if access to likely nesting areas has been closed, pilots must still carefully preflight during nesting season. As discovered by the Cessna 172 owner mentioned previously, birds can utilize very small openings for nesting activities. If they find the easy openings to be blocked, then they will be forced to select others. And they will.

Some pilots use predator decoys such as owls and snakes around their airplanes to frighten birds away, but Palmer-Ball says that these fakes provide only temporary deterrence, if any at all. "If a decoy owl had a cavity in it, a starling would use it to nest," he says.

Keeping your airplane in a hangar provides some protection, especially if it is located away from areas that have a ready supply of nesting materials. But don't let a hangar lull you into a false sense of security: Hangars harbor lots of nest-building resources such as pieces of safety wire, bits of paper, and paper towels to go along with grass and twigs from outside the hangar.

So this spring when the south breezes usher in pleasant temperatures and the days turn long enough for an after-work flight, be sure to check your airplane carefully for the work of our feathered friends in flight.


Bill Kight, AOPA 658477, of Goshen, Kentucky, is a commercial airline pilot.