Its its latest effort to defend two proposed garbage landfill sites in Montgomery County, the county government this week released a detailed $22,000 report on the migratory patterns, birth rates, mating habits and diets of 13 separate species of birds.

The report was designed to blunt a major criticism of the proposed sites in Laytonsville and Potomac: that they would attract scavenger birds who, to get to the landfills, would fly across airplane flight lanes that serve Dulles and National airports.

The Federal Aviation Administration has shown that birds can cause fatal airplane crashes it they fly near enough to a plane to be sucked into its engines.

So the problem of bird and airplane hazards, a serious one by FAA standards, has become another weapon for Potomac and Latonsville residents to use in their battle against county proposals for landfill sites in their communities.

"We take this issue very seriously," said Andrea Weirich, the county planner who oversees landfill site selection in Montgomery. "In fact, we are so serious that we went into some depth to evaluate this problem."

In the course of their investigation, suggested last winter by both the FAA and county residents, Dames & Moore, the Washington consultants hired by the county, studied birth rates, wing span, bird size, mating habits, mortality rates and diets of 13 particularly on four "known to aggregate at Washington, D.C., area landfills.

"They are: the herring gull, Larus argentatus: the ring-billed gull. Larus delawarensis, the common crow, Corvus brachyrhynchos; and the starling, Sturnus vulgaris," according to the report.

In addition, the consultants counted birds at the Rockville landfill for 32 consecutive days and hired a helicopter to follow the birds on several occasions as they flew along the Potomac River. The consultants also sought the advice of a leading Illinois ornithologist, one of the few seagull experts in the world, according to one county official.

Results showed that two landfills in Potomac and Laytonsville and one solely for construction debris near the Montgomery County Airpark would pose "no increased risk" to airplanes. However, one Laytonsville landfill would pose increased risk, but the risk would be avoided if nonfood wastes were dumped there.

All this is chronicled in a report brimming with intricate detail about every particular of the lives of the area's scavenger birds.

In geography textbook fashion, they chronicled the birds' lives - for the common crow, there are winters in Mexico (although some are all-year residents of central United States), nonbreeding seasons while they feed at landfills and a courtship in mid-March.

For the herring gull, the winter is spent in the South, courtship and nesting are on sand bars and then there is a southward migration of the "immature individuals."

Their movements were watched closely: "A gull 'spiral' was observed at approximately 6 p.m. over the Potomac on April 1. About 100 individuals were soaring and spiraling over the river . . . as an airplane approached, the gulls in the spiral dispersed, moving completely out of the approach path," the report stated.

Their landfill trips were also clocked: "They begin to leave the landfill in the afternoon around 4 p.m. The return course to the river appears less organized than the morning flight and individuals and flocks appear more scatttered than in morning hours."

The results show that the activity along the Potomac River gull flyway will remain about the same if a new landfill is opened in Potomac or Lavtensville.

Weirich denies that the report is a rebuttal to an FAA report released last month, which condemned the prospective landfill site in Potomac, saying it would increase the hazard of "bird strikes" on aircraft.

"It was disappointing that some of this material wasn't pulled together by the FAA," said Weirich, alluding to the fact that the FAA report is a brief five pages long compared to the county's detailed 60 pages. "But they didn't, we did."