I vividly remember the arrival of the house sparrows. They descended one autumn morning in a flock of 20 or so and stayed all winter, hogging my feeding station, tearing up my yard, even leaving their mark on the hood of my car. I silently cursed their arrival but endured their feathered reign of terror, hoping they would disappear. So I was naturally quite shocked to find, in the spring of the following year, that these ignominious little ruffians had a lesson to teach me about bird behavior.
It was late March, and several house sparrows and other birds were hopping about my lawn looking for seeds dropped during the season´s final snowmelt. As the day wore on, some birds shifted their attention to a pile of sand that had accumulated at the end of my driveway.
Frozen in an ochre mound for most of winter, the sand had been left behind by snowplows and had turned to dust during a week without precipitation. The bold house sparrows discovered it first and other birds soon joined them in this seemingly bizarre frivolity. Eventually, three species--house sparrows, dark-eyed juncos and white-throated sparrows--were shaking their bodies like wet dogs, sending sand flying in every direction.
Captivated by this odd behavior, I watched intently as one house sparrow spread its wings, fanned its primary and tail feathers, and then plopped belly-first on the sand. Moments later, in a flurry of flicks and twitches, the bird engulfed its body in a nimbus of dust. I later learned that the exact benefits of dustbathing, as the behavior is commonly known, are still a mystery to scientists.
"We´re not sure precisely why birds do this," admits Ron Rohrbaugh, assistant director of education at the Cornell Laboratory of Ornithology in Ithaca, New York. "But the primary purpose of dustbathing is thought to be to clean the feathers and get rid of ectoparasites."
Rohrbaugh notes that birds have a gland at the base of their tails called the uropygial gland. When preening, the creatures collect oil on their bills from this gland and distribute it over their feathers. The oil protects the feathers and makes them more flexible.
However, like motor oil in a car engine, a bird´s lubricants must be changed regularly. When their preening oils get dirty, many birds dip their tails and wings in dust in an activity that seems like a contradiction. But, says Rohrbaugh, "the oil is sucked up by the dust, making it easier to remove from the feathers. The bird can get rid of the oil by simply getting rid of the dust."
A secondary reason why birds bathe in dust may involve ectoparasites, a group of infinitesimal creatures (including mites, lice and fleas) that dwell on or near the skin. Ectoparasites come in two forms: blood eaters that live on the skin itself, and skin eaters that reside in the tiny spaces between feathers. Researchers believe that the populations of these microscopic animals dwindle after a vigorous dusting session. Rohrbaugh notes that ectoparasites breath through their exoskeletons and the dust probably clogs pores in those exoskeletons, killing the parasites.
In his book Bird Garden, Maine naturalist Stephen Kress dedicates a section to explaining how to use dustbaths to attract birds to a garden. Kress recommends a mixture of soil, sifted ash and loam for a fine light consistency that the birds enjoy. For sifted ash, he advises, you can turn to your fireplace and remove the larger chunks of coal through hardware cloth. If that´s not possible, Kress notes that common sand is sufficient for most dustbathing birds.
Don´t expect songbirds to flock to your dustbath, however. Most of the species that come to dustbaths are ground birds. "It´s fascinating to watch this behavior up close," says Kress.
There´s a downside, though, to inviting birds to dustbathe in your yard. "Most of the visitors to a backyard dustbath, at least in the eastern half of the country, are going to be house sparrows," notes NWF Chief Naturalist Craig Tufts. "And the presence of these exotic, aggressive birds may well discourage native birds, particularly cavity nesters, from nesting successfully on your property."
Kress agrees that house sparrows can intimidate some native birds. The sparrows often discover a source of dust first and, he says, "advertise its location to others." That´s certainly what happened in my yard not long ago.
Now, when the house sparrows arrive en masse in early spring, I still wince and worry that they will drive away cherished native species. But I must admit, I´m also thankful for the lesson these unwanted ruffians taught me about a little-studied facet of bird behavior.
Evan Mallet lives in New Hampshire.