Two perpetual mysteries surround the Baltimore Orioles.

Why they play well.

And why they play badly.

Few people understand either.

Nevertheless, the reason for both conditions is usually the same.

The state of the Birds and the state of the Bird bench is almost synonymous.

No other good team is so enormously dependent on its 10 worst players.

If the Birds are to perform like the team that won 100 games the last two years, then their bench must undergo a huge and sudden reawakening.

Until the arousal of the team's dormant "deep depth," the Birds aren't going anywhere the rest of this season.

Throughout 1981, the O's have been a rather mediocre team. Proof is simple.

In 1979, Baltimore outscored its foes by 175 runs. In 1980, by 165.

That's normal Bird form: an edge of more than a run per game.

This season, through 75 games, the O's have a paltry seven-run edge, 308-301.

In fact, it's remarkable that, with virtually no advantage in run scoring differential, the O's are 11 games over .500 for the season (43-32) and as recently as Sunday morning were in first place.

That's a testament to the team's ingrained self-confidence and resiliency, its belief that it is a winner and its knack in one-run games (15-6).

It's also plain dumb good luck.

What it definitely isn't is a good omen for September and October.

So, what's wrong? What must change for the Orioles to act Birdlike?

The answer is brief -- one word, in fact.

"Runs," says Pitching Coach Ray Miller. "We gotta get more of 'em."

The Birds' pace in run differential is down 70 runs over its average of the past two years. Of those 70 lost runs, 65 are a decrease in scoring since '80.

And runs, at least in the Orioles' case, means the bench.

You see, as always, Baltimore pitching remains a constant; the club ERA of 3.47 this season falls between last year's mark of 3.64 and the spectacular record of 3.26 in '79. Individual fortunes fluctuate -- Steve Stone and Jim Palmer may temporarily slump while Dennis Martinez and Scott McGregor blossom, or Sammy Stewart may pick up bullpen slack for an erratic Tim Stoddard. But, taking the nine-man staff as a whole, Bird pitching is par.

The team's defense (great) and speed (awful) are normal, too. Bird errors are worse than '80, but better than '79. Steals are down, but who cares?

Also, the six Oriole players who are good enough to be considered everyday starters have -- as a group -- produced almost exactly as expected. Ken Singleton (.322) and Doug DeCinces (43 RBI) are slightly better than expected while Al Bumbry (.256) and Rick Dempsey (decent .338 on-base percentage) are correspondingly a bit lower. Eddie Murray (14 homers, 46 RBI) and Rich Dauer (.273, 19 doubles) are normal.

So, if we want to find the root of the Orioles' current flatness, their mood that Singleton describes as "the irritating feeling that we're always about to break loose, but haven't yet," we have to look at Manager Earl Weaver's pride and joy -- the bench.

No alchemist of a manager makes journeymen look like stars as Weaver does. In '79 and '80, he gave chapter and verse on how to study stats, pick spots and juggle batting orders to get the most out of the 3 1/2 trouble positions in his lineup: left field, designated hitter, shortstop and backup catcher. This year, Weaver's spell seems to have snapped. His humble, hypnotized half-heroes are playing the way the teams that traded or released them always thought they would.

For instance, in the Series year of '79, the left field combo of Gary Roenicke and John Lowenstein had 36 homers and 98 RBI in 573 at bats -- almost MVP stats. Last season, that pair slipped to 14 homers and 55 RBI in 493 at bats. And this season the bad vibes grow as the two have only five homers and 33 RBI in 294 at bats. Increasingly, Roenicke looks like a big Punch-and-Judy with little knack in the clutch while Lowenstein is just the opposite, a more valuable player in a few crisis spots than in the regular lineup.

The DH is the Birds' other vacuum. In '79, Lee May and Pat Kelly -- the Big Bopper and the Preacher -- combined to hit 28 homers and drive in 94 runs in 609 at bats. Last season, that pair, plus Terry Crowley and Bennie Ayala, had these staggering DH stats: 27 homers, 114 RBI in 631 at bats.

Now, Jose Morales and (extremely disappointing) Jim Dwyer have replaced May and Kelly. The new pair, along with Crowley and Ayala, have only five homers and 39 RBI in 324 at bats. No wonder the Birds, whose homers slipped from 181 in '79 to 156 in '80, have now fallen another notch: only 58 homers in 75 games. Dr. Longball, it now proves, was a gentleman whom Weaver summoned from his bench.

Weaver has always accepted the fact that, in the middle infield and at catcher, defense had to come first. Nevertheless, he kept searching for nefarious ways to get bonus runs from those spots. In '79, Billy Smith had 33 RBI in 189 at bats at shortstop and second base. Then, last season, Weaver thought he had gone to heaven when Dan Graham went crazy and combined with Dempsey to hit 24 homers and drive in 94 runs in the platoon catching spot.

However, this year most of what the Genius of 33rd has touched has turned to dross. Utility men Lenn Sakata (.185) and even AAA phenom Cal Ripken Jr., who still looks like a future quality player despite a .118 average, have produced nothing. Graham, who has seen more curve balls and continually chases bad pitches, has been the biggest single disappointment (.185) as the O's catching stats (.219 with 20 RBI in 288 at bats) once more look distressingly familiar.

In short, the Orioles can tolerate miserable hitting at catcher and shortstop, so long as they get punch from both left field and DH. And, if they get lucky as they did in both '79 and '80 with unexpectedly high production from either short or catcher, they can get to that exalted 100-victory level.

However, when all of Weaver's 10 insurance policies lapse at once, when left field and DH become the province of Punch-and-Judies, when the shortstops can't hit a ball more than 150 feet in the air, when Dempsey must play too much and loses what little pop his bat has, the Oriole offense goes from dangerous to dormant.

Two explanations exist for the Oriole dilemma.

Either Weaver's particular brand of Oriole magic has run out and the era of the superstar journeyman is over in Baltimore.

Or else a loud, and overdue, explosion is about to take place when the Bird bench -- whose 10 denizens have hit .154 with three homers in 21 games in the second season -- come to their senses. History says that when one gets hot, others follow suit.