Challengers Alice G. Bird and Betsey Brown toppled two progrowth incumbents in yesterday's Board of Supervisors elections in Loudoun County, while Ann B. Kavanagh retained her Dulles District seat by nine votes.

"I'm still in a little shock," Bird said last night after learning she had defeated her former husband, Andrew R. Bird III, with 54 percent of the vote in a startling upset in the Sterling District. "The key issue in this race was Andy Bird saying 'yes' to every developer that came to Loudoun County," she said. Alice Bird ran as an independent with Democratic Party endorsement; her former husband, a Republican, has served two terms as the eight-member board's most outspoken proponent of economic development and preserving property rights.

In the Catoctin District, Brown, a nonpracticing psychologist, defeated incumbent Republican Frank I. Lambert, one of Andrew Bird's leading allies on development issues. Brown, a Democrat who moved to Loudoun two years ago and quickly became a leader in slow-growth causes, won with 60 percent of the vote to Lambert's 40 percent.

Kavanagh, a first-term Democratic incumbent, got a spirited challenge from Republican Benjamin H. Hicks Jr. but managed to squeak by him with 45.8 percent of the vote to Hicks' 45.6 percent. Independent candidate Clements T. Berezoski got 8.6 percent.

Voters also returned incumbents Steve Stockman, James F. Brownell, Thomas S. Dodson and Charles A. Bos to the eight-member board. Board Chairman Betty W. Tatum ran unopposed.

Loudoun voters also approved a referendum to issue $15 million in bonds to purchase a 357-acre tract in the eastern part of the county and turn it into a public park. The bond referendum passed with 59 percent of the vote.

In the Blue Ridge District, Brownell staved off a two-way challenge to win a sixth four-year term. Brownell, who ran as an independent, got 42 percent of the vote to 41 percent for Ben F. Fordney, also an independent, and 17 percent for Larry W. Johnson, a Republican.

Stockman, a Republican who represents the Broad Run District, was reelected to a second term with 65 percent of the vote. Independent Gregory Rex Marquis got 35 percent.

In the Mercer District, two-term Democratic incumbent Dodson was reelected with 69 percent of the vote. Republican challenger G. Lawrence Moison received 31 percent.

In the Leesburg District, Democratic incumbent Charles A. Bos, who was appointed to the board last year, defeated Georgia White Bange, a Republican, 54 to 46 percent.

Turnout was described as heavy among the 33,290 registered voters in the largely rural county, which stretches from the suburban sprawl of Fairfax County westward to the Blue Ridge.

Seven of the eight seats on the Board of Supervisors were up for grabs. For the most part, challengers sought to portray the incumbents as open-armed to development without considering its impact, a characterization designed to appeal to Loudoun residents fearful that all of the 517-square-mile county eventually will be gobbled up by shopping malls and tract housing.

In the one uncontested supervisor race, Tatum, a Democrat representing the Guilford District, was elected to a second full term. Tatum, 46, was appointed to the board in 1981 to fill a vacancy, won a special election later that year and was reelected in 1983.

Other candidates who were unopposed were Commonwealth's Attorney William T. Burch, Sheriff John R. Isom, Treasurer George W. Titus and Commissioner of Revenue Catherine Ashby. All are Democrats.

The referendum on the 357-acre Claude Moore property, in Tatum's district, was the first time county residents have voted on whether to issue bonds to purchase parkland. Moore, a 95-year-old physician, donated the property in 1975 to the National Wildlife Federation, which then sold it to a McLean developer for $8.5 million. County officials, in voting to place the bond referendum on the ballot, said the action was taken because the Moore property is the last significant undeveloped tract in eastern Loudoun.

In the supervisor races: Dulles: Hicks, 41, who works for a Crystal City defense contractor, charged that Kavanagh, 45, a Fairfax County teacher, had neglected her district in favor of supporting complicated land-use schemes relevant primarily in the western part of the county. Kavanagh responded that she had tried to ensure that growth did not outpace the services, such as roads, needed to support it.

Berezoski, 47, who owns a snow removal company, had a campaign war chest of about $22,000 -- most of it from his own pocket -- which allowed him to outspend his rivals by a ratio of more than 2-to-1 and become a wild card in the race. Blue Ridge: This western district also had a three-way race, in which five-term incumbent Brownell, 69, a former Republican who ran this year as an independent, was challenged by Fordney, 56, an independent endorsed by local Democrats, and Johnson, 49, a Republican.

Although Brownell, a farmer, has long been considered one of the board's staunchest critics of growth, his opponents charged that he is too soft on developers. Fordney, a Voice of America employe and vice mayor of Round Hill, challenged Brownell four years ago and lost in a close contest. Johnson runs a national association of youth vocational clubs. Sterling:Andrew Bird, 42, an insurance company executive, ran on a record of improving the county's tax base by encouraging and shaping growth. Alice Bird, a 42-year-old nurse, charged that the incumbent has been inaccessible to constituents and too permissive with developers. Leesburg: Bos, 47, and Bange (rhymes with range), 57, waged a genteel contest in which both candidates agreed that controlling growth and improving transportation were primary goals. Bos is a veteran local politician; Bange is a counselor for a Leesburg funeral home. Catoctin: Brown, 57, a founding member of the Friends of Route 15, a group associated with keeping Loudoun rural, accused Lambert of being a staunch ally of developers, a charge he said was overstated. Lambert, 58, a businessman who was seeking a second term, had opposed efforts to extend the boundaries of the historic village of Waterford, arguing that nearby farmers have the right to sell their land for whatever the market will bear. Mercer: Dodson was challenged by Moison in this western district of large farms and estates. Moison, 58, a financial consultant and fiscal conservative, waged an uphill fight against the 38-year-old Dodson, a gas company executive and an ardent opponent of rapid growth, who was seeking a third term. Broad Run: Stockman, 38, was challenged by Marquis, 37, an employe of the Potomac Electric Power Co. Stockman, who owns a title and escrow business, is one of the board's most enthusiastic supporters of economic growth and commercial development.

Members of the new board, who will take office in January, will receive $18,000 annually.

Washington Post Staff writer Pierre Thomas contributed to this report.