It appears we have Mother Nature to thank for Avner the Eccentric -- the 37-year-old mime, clown, magician, acrobat and generally blithe spirit who currently is tickling funny bones with his one-man show at Arena Stage's Kreeger Theater.

As an undergraduate at Tulane University, Avner (last name: Eisenberg) was on the road to becoming a herpetologist, which is, of course, a specialist in snakes. But as he headed back to his apartment one afternoon, a sudden rainstorm came up and he took refuge in the theater department. Auditions for a production of "The Mandrake" just happened to be in progress, so he tried out and got a bit part. Goodbye, snakes. Avner was hooked.

Part of the wave of young vaudevillians to emerge from the 1970s (which, you may recall, liked street performers a lot), Avner has taken his show all over the country, toured 10 foreign countries (he doesn't talk in the act, so there's no language barrier) and last season held forth for eight months off-Broadway.

"A lot of people hate clowns. But what they really hate is the clowns they've got -- the Bozos and the Ronald McDonalds," he says. "What happened with American circus clowning is that as the arenas became bigger and bigger, the clowns grew more and more grotesque. Now their eyes are made up to here. The props are oversized. The humor is violent and often sexist.

"For me, the clown is a mirror of the people in the audience, and my show is a compendium of accidents that happen to everyone. The clown has an acceptance of failure, almost as a condition of life. The point is to find humor through frustration in dealing with everyday props and situations. Every prop I use is something that everyone in the audience has at home -- 2-by-4s, a clothesline, napkins, a red tablecloth stolen from a Chinese restaurant. And the show has a moral: If you can't succeed every time, learn to fail magnificently."

Emmett Kelly and Buster Keaton, he says, were his inspirations. But so were Estragon and Vladimir, the baggy-pants tramps who while away the days with jokes and routines in Samuel Beckett's "Waiting for Godot." (Avner played Estragon at the Pittsburgh Public Theater.) "Actually, my act is a little like a Beckett play," he says. "I mean, the feeling is the same. You've got this character -- me -- who is, in a sense, trapped on stage and doesn't know how to get off. I was once described as a guy with a 10-minute slack-rope walking act, who'll do anything to avoid doing it."

Up to now, Avner's show business identity has come mostly from playing the Eccentric. But that may change shortly. He's got a featured role in "The Jewel of the Nile," the sequel to the blockbuster adventure film "Romancing the Stone." He'll be seen as an Arab holy man (one of the good guys), alongside Kathleen Turner and Michael Douglas in the Christmas release.

Still, he remains committed to his one-man show, which allows him to juggle, balance on the slack rope, mime, do acrobatics and perform sleight of hand. "Those were always my hobbies and I get paid for doing them," he says. "The trouble is, now I don't have any hobbies for my time off."

What was going to be the American National Theater's first attraction in the Eisenhower Theater next month, a star-studded production of Robert Sherwood's "Idiot's Delight," has been postponed until early next year. The undisclosed stars whom director Peter Sellars wants for the project won't be available until then. "This is why I never announce anything. When it's ready to be served, it will be served," says Sellars, who also has put the Mae West comedy "Embassy Row" on hold until the proper director is free. Meanwhile, Sellars is juggling possibilities for the October slot.

The National Theatre, which has been sitting prettier and prettier since the Shubert Organization took over its management, has more than the American premiere of the musical "Chess" in December 1986 to look forward to. Bernard B. Jacobs, president of the Shubert Organization, also has indicated that two strong possibilities for the 1986-87 season are "Biloxi Blues" and "Broadway Bound," the second and third installments in Neil Simon's autobiographical trilogy that began with "Brighton Beach Memoirs." "Biloxi" already is a smash in New York; "Broadway" presumably would premiere here.

Larry Shue's "The Foreigner," which was such a hit this summer that Olney Theatre is bringing it back Oct. 1 for a second engagement, is going to be turning up lots of other places, too. Shue's agent says there are 68 productions of the zany comedy scheduled across the country this season, as well as productions in five foreign countries. . . . The Capitol Steps, a group specializing in political satire, has taken up residence at the Bread Oven at 1201 Pennsylvania Ave. NW. The eight singer/dancers perform every other Saturday.