To the casual observer, rugby is a bunch of mad dogs and Englishmen chasing an oblong ball around a football field. But to the Duck Brothers, rugby is a way of life.

The Duck Brothers successfully defended their Washington Seven-A-Side tournament title yesterday at Kenilworth Park, defeating Monmouth (N.J.), 24-0. After a scoreless first half, the heavily favored Ducks first scored when Steve Glahn raced 50 yards for what proved to be the winning goal.

In the women's division, the Virginia Women's Rugby Football Club won its first tournament in its five-year history by defeating Iris, 4-0. Thea Bertola ran 80 yards for the game's only score.

The Duck Brothers have waddled to national attention in amateur rugby circles. Formed in 1977 as an offshoot of the Northern Virginia Rugby Team, the team gained its name from the ducklike characteristics of its founders, brothers George, Charlie and Jimmy Wilinson.

"We were playing in a tournament in 1977 and my brothers were playing with me," said George, who is called "Duck Feet." "Anyway, after a while everybody on the sidelines started going, 'Quack, quack', and we just kept the name." Since then, the Duck Brothers have dominated East coast rugby.

The team won the national seven-a-side tournament in Hartford, Conn., last year and toured England in 1979. They have also won the last three Washington seven-a-side tournaments. Several of the players are of international caliber, including George Wilkinson, who is a member of the select Eastern Rugby Association all-star team.

Although different in rules and format from normal 15-a-side rugby, seven-a-side requires the same physial qualifications.

"You've got to be able to run nonstop for 80 minutes with no timeouts and no substitutions," said Bill Gardner, who plays hooker for the Duck Brothers. "Injuries are still just as much a part of the game, though. I tore my rib cartilage last year but you've got to keep playing. It's like football without pads."

Although the threat of rain (a rugger's worst enemey) caused cancellations by teams from California and New York, the tounament, turnout was still good, according to spokesman Bob Boyd.