The Baltimore oriole, Maryland's official bird, has been caught in an avian sex scandal that has resulted in it being dropped from the blue book of ornithology.

Sen. Charles McC. Mathias (R-Md.) denounced yesterday the decision to relegate the Baltimore oriole to a subspecies in the upcoming revised version of Roger Tory Peterson's "A Field Guide to Birds." Along with its longtime consort, the Bullock oriole, the colorful songbird that is a Maryland symbol will share the new hybrid name of northern oriole.

Mathias expressed tongue-in-cheek outrage in the Congressional Record at the decision of taxonomists, who classify species, even though Mathias, an amateur birder, admits the problem is of the bird's own making.

"Alas, truth to tell," Mathias wrote, "the Baltimore oriole is its own worst enemy. Its curious breeding propensities have placed it in mortal danger, and indeed the peril may already be beyond Maryland's power to control."

Casting about for a sociological explanation, Mathias hinted that the Baltimore bird had falled prey to the lure of its western cousin, the Bullock oriole, and "lost all sense of decorum. It adopts the free and easy ways of the west and mingles with no thought of tomorrow."

Mathias, however, acknowledged that serious bird watchers have known about the liaison between Baltimore and Bullock for years.

The decision to downgrade the Baltimore oriole to a subspecies occurred in April 1973. "The Auk," official publication of the American Ornithological Union, reported that month that the AOU's checklist committee had ruled that "icterus bullickii is considered conspecific with icterus galbula."

The difficulty of deciphering the ruling may explain why it wasn't until recently that Mathias and other amateur bird watchers learned that the philandering eastern (Baltimore) and western (Bullock) orioles have rendered their respective offspring indistinguishable.

When Mathias discovered that Peterson's forthcoming revision of his popular bird guide had dropped the Baltimore oriole he protested the action to the Smithsonian Institution. There, the senator said, he was told "by a cultivated voice" that the ornithological union decision would be followed by the institution as well, assuring that "the offending oriole (is being) drummed out of civilized society."

Dr. George E. Watson, curator of birds at the Smithsonian, confirmed in an interview that the breakdown of the formerly blueblooded Baltimore bird occurred over a period of years during promiscuous meetings with its western cousins on countless fence-posts in the Midwest.

"It's old news," said Watson of Mathias' complaint.

Mathias, feigning anger at such a snooty attitude, shot back, "At the Smithsonian, at least, I had expected more sympathy." Mathias pointed out that yesterday was the 150th anniversary of the death of the institution's founder, and that, because his parents were never married, "James Smithson was a bastard."

Perhaps none of this sad story should surprise us," Mathias wrote to his Senate colleagues. "We are after all the only nation in the world which has landed its national bird on the endangered species list. To a nation capable of that enormity, defrocking a mere state bird is not trick at all."

But Mathias said Marylanders "do not despair. The Baltimore oriole, to us, is a thing apart, and always will be, no matter what the scandalmongers say." CAPTION: Illustration, no caption, By William T. Coulter for The Washington Post