Found here:

Q: I thoroughly enjoyed last nights “Dvorak & Rachmaninoff” program – kudos to all involved! I wondered why there were so many personnel changes in the woodwind section from one piece to another – it seemed like there was a revolving door for the flutes and clarinets. Thanks again for a memorable evening.

Well, first of all, thanks for coming, Gail! It’s definitely a fun program to play, and the audiences have been great all week. The reason for the wind and brass changes is twofold: first of all, the instrumentation is not exactly the same for all three pieces on the program. (For instance, there’s a saxophone in the Rachmaninoff, and there are differing numbers of winds involved in the two pieces on the first half.)

Secondly, it is traditional in most orchestras for principal wind players to divide up the duties on long programs, simply because there is a physical limit to the strength of the human lip, and if a principal flute, for instance, tried to play every single piece on every single program, s/he would likely do serious physical damage by the end of the season. Sometimes, a principal will play the entire program one week (as our principal oboist, Basil Reeve, is doing this week) and then not play at all the next week. Other times (as you saw with our flutes and clarinets,) the principal will sit out one of the pieces on the program, and be replaced by the assistant principal. The non-principal wind players may shift around as well, but since their parts tend not to be as physically taxing, they are more likely to play everything on the program.

This is why I’m playing Assistant Principal this week; Beethoven’s ninth is a killer for first oboe. (It’s actually a killer for second oboe as well. In the past we’ve doubled all the winds, but not now.)

But four concerts of B’s 9th in a row took its toll. (Update: Oops! Three. Today was our fourth. See what happened by reading my current blog entry.) Our principal oboist is now home, and sounds quite ill. So we are going to play musical chairs today. I’ll play principal in the Brahms. It’s a work I love but have never played before. I’ll probably remain on AP for the Beethoven, and our second oboist for this set will move to principal. Some might think I should switch over, but the second oboist has played principal at several performances in the past, and I’ve never done it. So we just thought that might be a better way to go. Someone new will come in (we hope!) to play second. (If we can’t find anyone I’ll move over, but we really, really need an AP for that piece. As I said, it’s a killer.)

Sometimes—many times, actually—I wish for the “days of yore” (the “olden days” … you know … “back in the day”) when there were three of us: principal oboe, second oboe, and (me!) English horn. I would play assistant at times, and move to other positions when necessary. It gave us a bit more “wiggle room”. I miss that. I’m glad, though, that we were allowed the extra player for AP this time, so there were three of us (and I do hope there will be three today!). I just think three on the roster would be much better at all times.