Although this is not a hard and fast rule, for the most part, they will retire the middle bird in a Q. Most of the time, it also will be the longest mark. BUT with that being said, I have seen the middle mark be the shortest (and of course, not the go bird in order for it to be retired). I have also seen them retire the left or right bird, sometimes the longest, and sometimes not.
So, as you can see, there is really no good solution to say it is always one or the other.
Howard had a really good post above regarding some other concepts you will want to work on, in relation to retired guns.
No need to be nervous in the least. There are just a few basic differences between hunt test and Q. One being distance, but at the same time, the Q is given the worst grounds (between Open / AM / Q), so sometimes the distances will be exactly the same as you have seen in a Master test. The second being that you will run a land series of marks first, and then come back to do a land blind separately.
Some concepts for you to think about for the blind:
1. running the blind past a gunner from the first series, with gunner sitting out.
2. running blind past flyer crates, or through flyer scent from first series.
Good luck. HAVE FUN. It is only you going out and spending some good quality time with your best bud.