Most of the time, Jim Mills makes a living training bird dogs in Texas. But last week he was training the bomb sights of his A37 jet attack plane on the target range at this Air National Guard base in south-central Wisconsin.

Mills, 33, is a captain in the Air Force Reserve, which for the past two weeks has been holding the largest nationwide exercise in its 32-year history.

The exercise is not only to test the reserve's ability to mobilize but also to shed some light on what reserve chief Maj. Gen. Richard Bodycombe calls "the best-kept secret in the Air Force."

The General is not talking about some new supersecret weapon.

Rather, he is talking about the strength and readiness of the two reserve components of the U.S. Air Force -- the Air Force Reserve and the Air National Guard -- a force of 152,000 paid reservists, many with more experience and training then active-duty airmen, plus 1,900 aircraft in 144 flying squadrons, an armada larger than all but three or four of the world's regular air forces.

As overseas crises have suddenly refocused attention on defense, the sorry state of the nation's military reserves have become more apparent. These troops would have to be called up fast to fill the gaps and expand the fighting forces in an emergency.

The Army reserves are estimated to be as many as 500,000 troops short of what they would need in the early stages of a battle, a situation brought on by the phasing out of the draft six years ago. The Navy and Marine Corp face similar problems though not as severe.

For a variety of reasons, the Air Force is the happy exception.

Judging from interviews with dozens of enlisted and officer reservists here, there are waiting lists to get into many Air Force Reserve and guard squadrons, maintenance of the airplanes is first-rate and most of these units could be ready within 24 to 72 hours of a call-up notice.

And though many of the planes are old, there is still a widespread feeling here that even old planes in the hands of experienced mechanics and pilots can play an important part in the overall threat the U.S. presents to any enemy.

For example, though it usually doesn't show up in the U.S.-U.S.S.R. balance of power calcualations, there are 20 squadrons of 18 to 24 aircraft each of F4 Phantom jet fighter-bombers in the Air National Guard along, and the equivalent of another 10 squadrons of F105 fighter-bombers between the two Reserve organizations.

While many of these planes are 15 to 20 years old, they all can carry atomic bombs far and fast.

In a battle against the Soviets, says Brig. Gen. John B. Conway, 45, the deputy chief of the Air Guard, it's not just sophistication that counts.

"Numbers count too," he says. "You just can't afford to boneyard these old planes. The enemy has got to pay lots of attention to them because if you don't they'll blow the hell out of you."

There are also 14 guard squadrons of relatively new A7 jet attack planes used for conventional bombing and four guard squadrons of the A10 attack planes.

Last week, Bodycombe went to the Fairchild Industries plant at Hagerston, Md., to accept the first A10 for the Air Reserves right off the production line -- the first time a frontline warplane has been turned over to reservists right out of the factory.

It also is not generally known that reservists provide close to 50 percent of the flight crews and 40 percent of the maintenance for the regular Air Force's fleet of C5 and C141 oceanspanning jet transports. About 36 percent of all the tactical troop and cargo airlift -- involving smaller propeller-drive planes such as the C130, C123 and C7 used closer to the battlefield -- is provided by the reserves.

"There is just no way that the Rapid Deployment Force can move without us," says Bodycombe about President Carter's plan to have a special 110,000-man force of Army and Marine troops that can quickly be dispatched to global troublespots.

The Air Force Reserves, numbering almost 58,000 persons and federally controlled, and the Air National Guard -- which has 94,000 people and is state-controlled but available for national call-up -- are not trouble-free. i

There is still a shortage in some guard units, says Conaway, of skilled photo reconnaissance and electronic technicians and munitions handlers.

Sgt. Billie Humphries, from a reserve C130 squadron in Alabama complains there is much too great a disparity in pay and travel allowances between skilled reserve technicians and reserve officers, a complaint echoed by some other enlisted men on this exercise.

There are other gripes too, but there are far more positive comments, suggesting the air reserves are not only in better shape than the reserves of other services but also are probably in better shape then ever.

Part of it is experience.

T.W. Johnson, a reserve major who runs a computer business in Minneapolis and is a navigator in a C130 squadron, says half his squardon saw service in Vietnam. An F105 pilot here says 75 to 80 percent of the pilots in his squadron in Texas are combat experienced, yet still young enough to to active in the reserves.

The real "secret weapon" of both the reserves and guard, however, lies in the enlisted technicians who maintain the planes.

While many of these are the traditional "weekend warriors" who come to train one weekend a month and two weeks in the summer, some 28,000 actually work full-time for the guard and reserves in civil service jobs. But they also have reserve commitments, which means if their unit gets called up or ordered on a mission, they go too.

The average enlisted-man weekend reservist gets from $1,200 to $1,600 a year. But tech Sgt. Robert Dawes, with a transport squadron in Columbus, Ohio, gets about $400 at week as a civilian reserve technician. He has been fixing airplanes for 16 years, which is typical of the experience in this force.

"The maintenance is unbelievable," says reserve Maj. Sam Roane, who flies jet airliners for Northwest Orient in civilian life and C130s in the reserves.

"It's 10 times better than active duty," adds Johnson. "They've got guys as mechanics who've been doing it since World War II. I think all of us would just love to compete with the active duty force. We'd really wax'em," he says.

A colonel in the regular Air Force in Texas says watching some of the reserve and guard squadrons "makes my eyes misty, they're so good." Mills says his A37 squadron hasn't had a major accident since 1973.

The Air Force Reserves -- like the air reserve portions of the Navy and the Marine Corps -- have always benefited from the natural inclination of pilots to keep flying, and that explains a lot of the appeal. "It sure beats having 200 passengers strapped to your back and trying not to spill their drinks," says one reserve pilot who says he enjoys flying his Air Force jet more than a DC9 airliner.

The camaraderie also is a big part of it, even among enlisted people who like being around flying units and moving with them. A group of sergeants here from Minneapolis talks of trips to Panama, of lifting refugees out of Nicaragua, of relief work after the Peoples' Temple massacre in Guyana and of deployments with fighter and transport squadrons to Germany and Turkey.

But money is also a big part. In a sense, the reserves are benefiting from people who are being driven out of the active force by low pay.

"When a guy with eight years' experience leaves the Air Force, it usually isn't because he doesn't really like it, but because he can get more for his skills on the outside, or maybe he doesn't want to travel so much," says one reserve officer.

"It wasn't really like getting out," said Mills of joining the reserves after nine years on active duty. "It was sort of like switching places and doing the same thing."

But Mills put in 135 days last year flying his A37 for the reserves and made an extra $12,000 to $15,000, more than many career soldiers make who are leaving the Army. That is well above normal, he says, but many pilots earn several thousand dollars a year from the reserves because they are encouraged to fly -- seven or eight times a month in the case of fighter pilots -- to keep up their skills.

Reservists get two days' regular pay for one day's reserve work. The reserve technicians, making about $400 weekly, earn more than their counter-parts on active duty. The sergeants who come in one weekend a month plus their two-week stint make about $1,200 to $1,600 a year extra now, while the time served adds to a partial retirement check when they reach age 60.

To keep the units full, however, both the guard and reserves have a reasonably sophisticated system of contacting enlisted and officer personnel while they are still on active duty if they are leaving the service.

"Our job is to capture these people, not steal them from the Air Force," says guard deputy commander Conaway.

Having two different reserve organizations also seems to give the Air Force an edge, with the guard having some home state indentification advantages plus some protection against too much time away from home. The reserves also offer a nationwide setup more closely linked with the federal role of military forces.