The U.S. Air Force is having a hard time getting Master Sergeants to volunteer to become First Sergeants. Both enlisted ranks are the same pay grade (E-8, on a scale of E-1 to E-9). But First Sergeants in the air force must be volunteers (and can actually be either E-7, E-8 or E-9), and over the last few years there have been fewer volunteers. Currently there is a shortage of over a hundred qualified volunteers.
First sergeants are the senior NCO (Non-Commissioned Officer) in the basic unit of the air force; the squadron. In addition to the familiar aircraft squadrons, there are also squadrons that do all sorts of other jobs (maintenance, administration, security and so on.) In effect, the air force squadron is roughly equivalent to the company in the army and marines. The first sergeant takes care of all administrative details and, in general, sees to it that things run smoothly. The officers may command the squadron, and do things like fly aircraft, but the first sergeant is the guy (or gal) who makes sure things happen when they are supposed to. The first sergeant also works with the other sergeants in the squadron to prevent any of the junior enlisted personnel from getting into trouble, and also keeps an eye out for exceptional performers. When the squadron commander has to decide which enlisted personnel to promote, it is done after consultation with the first sergeant. To enlisted personnel, their first sergeant is something of a god. The first sergeant can intercede with officers if an airman gets in trouble, and will often crack down on wayward airmen before misbehavior catches the attention of an officer. To officers, the first sergeant is more than just a senior NCO. The first sergeant knows more about what is going on in the squadron than any of the squadron officers. Junior officers are told bluntly, before they report to their first squadron, that to cross the first sergeant is a very bad career move.
Currently, the air force is short about ten percent of their first sergeants, and part of the problem is that the job has gotten a lot harder in the last decade. Unlike in the army and marines, air force first sergeants have to supervise a lot more people. The "company first sergeant" in the army and marines rarely has more than 300 personnel to look after. Air Force squadrons often have over 700 people in them. This is why some air force first sergeants are E-9 (a rank used in the army and marines for Sergeants Major, the senior NCO in battalions and larger units.) Since the end of the Cold War, more women have been joining the air force, and more junior enlisted personnel are getting married, and having families. In the Cold War days, most of the airmen were, well, men, and they lived in a barracks. The enlisted accommodations were actually more like a college dorm, with as few as two airmen to a room. But they were still all in one place and the first sergeant could go through the barracks building to keep an eye on his charges. Most of the NCOs were married and lived houses or apartments (both on the base or in civilian neighborhoods.) But the NCOs were older, usually in for a twenty year career and willing and able to help the first sergeant run the squadron. But now you often have up to twenty percent female airmen (there's no such thing as an "airwoman") and over half the enlisted personnel living outside the barracks. Not so easy to keep an eye on all those junior troops any more. The air force also discovered that supervising female airmen required some different management skills than those long used to supervise males.
And the first sergeant is the first responder when it comes to domestic problems among his troops. And all those young airmen (male and female) with spouses, and often young children, have lots more problems than the old, "single young males, living in the barracks" days. So added to the usual problems of getting the squadron to do its job efficiently, the first sergeant often has a dozen or more domestic crises to deal with (divorces, debt, sick children and so on.) While the air force has added many "family support" services, the first sergeant is still getting hit first with these problems. And no matter what any support effort outside the squadron does, the first sergeant is still dealing with side effects (poor performance, absence, bad attitude) inside the squadron. The job is basically a lot harder, and there just aren't as many volunteers as their used to be. While being a first sergeant gains one a lot of prestige and respect, the price to be paid is seen as too high. And right now the air force is short over a hundred first shirts.
Air Force first sergeants are usually referred to as the "first shirt," or just "the shirt." NCOs who are thinking of becoming first sergeants will often first serve as an unofficial assistant to the first sergeant, and fill in for the first shirt when he (or she) is away (on leave or for training.) This apprentice first sergeant is often called the "under shirt."