HIGHLAND — HIGHLAND -- In the national crusade to preserve the gentle bluebird, Mark Wallace is a tireless soldier. Every day during the bluebird breeding season, from April to August, Mr. Wallace patrols Howard and Montgomery counties tending some 350 nesting boxes. He counts the blue-green eggs, puts identifying bands on baby bluebirds, and cleans the wooden boxes when the birds have flown. And, yes, he kills. All for the love of bluebirds. Mr. Wallace belongs to the Montgomery County-based North American Bluebird Society, a 4,500-member group that for 14 years has pursued the cause of the embattled bluebird. Across the United States and Canada, society members post nesting boxes by the thousands. Biologists say the organization has helped the bluebird rebound from severe decline. "We like to think so," said Lawrence Zeleny of University Park, who founded the society in 1978. What he would rather not discuss are methods bluebird fans have been known to use against the bluebird's chief nemesis, the house sparrow. Or, as Mr. Wallace calls them, "the little monsters." The other day, Mr. Wallace popped the hatch of his Dodge Ram to reveal a cage containing two house sparrows he captured that day. Like hundreds before them, they are doomed. "They're going to have an accident," said Mr. Wallace, 35, who works nights at a Columbia convenience store. He means he is going to kill them by bashing them against a sidewalk or the side of his truck. "I'd rather not be doing it," he said. But he sees no choice in coping with the aggressive house sparrow -- which invades bluebird boxes, kills the inhabitants and takes over the box. This year alone, Mr. Wallace says, he's probably killed about 200 house sparrows. "In a number of cases I have watched a sparrow and a bluebird fighting over a nesting box," said Mr. Zeleny, an 88-year-old retired biochemist who worked with the U.S. Department of Agriculture. "The sparrow comes out. The bluebird is lying there with blood running out of its head, the babies killed, eyes sometimes pecked out. When you see that sort of thing, you don't have much sympathy for the sparrow." Reluctantly, Mr. Zeleny acknowledges that in the course of tending to his nesting box trail in Beltsville, he has killed house sparrows. This human intervention in a battle of species vs. species ruffles the feathers of People for the Ethical Treatment of Animals, which learned of the society in a recent Wall Street Journal story. The story mentioned a 1980 pamphlet titled "How to Control House Sparrows," which described such sparrow-execution methods as immersing traps full of sparrows in water or gassing them with car exhaust. NTC "Our members were disgusted with the suggestions on how to control house sparrows," said Teresa Gibbs of Rockville, a PETA caseworker. "Killing one animal because you like another one better is weird." Bluebird Society Director Mary D. Janetatos, who runs the organization out of her home in Colesville, said it stopped distributing the pamphlet years ago. While the society does sell sparrow traps, it does not encourage its members to kill house sparrows. However, she said bluebirders have the law and the conservationist ethic on their side. Unlike the bluebird, the house or English sparrow is not protected by law because it is not a native species. Imported to this country deliberately in the mid-19th century, the house sparrow is more aggressive than the bluebird, has a more powerful beak and is more adaptable to cities and densely populated suburbs. While sparrows have thrived, the bluebird has declined as its rural habitat has been destroyed by development. Among people knowledgeable about birds, "sympathy for house sparrows is not very widespread," said Danny Bystrak, a biologist with the U.S. Fish and Wildlife Service in Laurel. "There are people who say sparrows are feathered rats." Mr. Wallace said he appreciates PETA's "live and let live" attitude but the crusade must go on. In this breeding season, Mr. Wallace says he's seen about 450 baby bluebirds fledge and leave the nest and expects another 75 or so before the season ends this month. Such efforts win the praise of National Audubon Society ornithologist Geoff LeBaron. He said he doesn't advocate cruelty to house sparrows, but he said the recovery of the bluebird is "a nice success story. It's one of the few success stories we have."