Good news for eagle enthusiasts: Not only is the $3 million federal budget appropriation to buy the Mason Neck Wildlife Refuge and eagle preserve still alive, but there also may be a baby eagle in a refuge nest for the first time in four years.

The House Interior Appropriations subcommittee this week put $3 million for Mason Neck in the proposed supplemental 1983 appropriation.

The proposed funding still faces formidable hurdles, from both the Senate and President Reagan, who could decide to veto the entire budget supplement, but the Virginia congressional delegation expressed elation at the subcommittee's action.

The Mason Neck peninsula on the Potomac River, 10 miles south of the Capitol, contains the nation's first wildlife refuge established to protect the bald eagle, the national symbol. The federal appropriation has been sought for two years to buy the last 355 acres of privately owned land in the 1,131-acre refuge.

A pair of eagles has been nesting at the refuge regularly for a decade, and more than a dozen migrating eagles have wintered there each of the last three years.

Wildlife refuge manager Domenick Ciccone said yesterday, "There's a fairly good chance we have an eaglet in the nest now, although we can't be sure since the nest is so big it's hard to see inside from the ground."

On April 13 the annual aerial eagle survey by the National Wildlife Federation detected "something in the nest and both eagles were standing by," said Ciccone. The surveyors "thought it was a young eaglet, and the fact that the adults are hanging around is a good sign."

The nesting pair produced one eaglet in 1979, "which fledged and flew away," said Ciccone. But 1980 was a barren year and high winds blew down the aerie in 1981 and 1982.

The past two years were equally barren for eagles on Capitol Hill, although "we came oh-so-close last fall," said a spokesman for Rep. Stan Parris (R-Va.). "But we're delighted, we're elated that the subcommittee has put it in the supplemental budget."

"This is the best thing that's happened to us so far," said Elizabeth Hartwell, who helped lead a decade-long fight to create federal, state and regional parks on Mason Neck. "But the Senate could still block it again as it did last year and then the President could veto the entire supplemental budget, so we're tempering our enthusiasm," she said.