I count any rooster that FALLS from the sky as a result of my shot, but doesn't end up in my vest, as a lost bird. Sometimes a somewhat wounded rooster might make a controlled landing at some distance, & we might be able to go see if we can find him again, to either be recovered on the ground or flushed & shot again. We most commonly never see those birds again (I believe most will heal & live mostly normal lives), but either way, if he doesn't end up in the vest, that kind does NOT count as a lost bird. He simply won.
Obviously, sometimes I lose a pheasant. But in almost all cases, the REASON I lose one is because I screwed up somehow. Therefore, it's a direct reflection of my shooting, decision making, or something, & it's never acceptable. We go to great lengths to try to recover every wounded/dead rooster, & I absolutely HATE losing one. It makes me feel miserable.
I've read in the past that the "average experienced pheasant dog" loses 12%-17% of shot birds. I have a hard time calling that acceptable or reasonable. But I can see it for the "average", because that person really doesn't hunt a lot, he consequently doesn't GET as many high percentage shots, ISN'T a great pheasant shooter, & his "average experienced pheasant dog" really isn't terribly experienced. I've studied how to find & hunt pheasants my whole life. I usually hunt 30-35 times a year, in areas where good numbers of pheasants live, & I can put my puppies on them from day 1. As a result, I consider myself very experienced, & my dogs become very experienced quicker than most. That experience level only increases with time. We don't lose 12%-17%. Most years it's probably in the 2%-5% range.
One thing I've noticed with my current dog Ace is that, while he makes amazing recoveries occasionally, he's not quite as accomplished at finding/recovering runners as my other 2 springers were. I attribute it to this (which may sound a bit pompous). I've become a considerably better shot in the last 20 years, particularly in the past 10 or so. A fairly high percentage of roosters I shoot hit the ground & stay pretty much where they fell. I believe this has made Ace more prone to try to SEE a "dead" bird, particularly right when he reaches the drop zone, rather than immediately turn on his nose & use it. This gives a runner a head start that can result in a lost bird. My first 2 dogs hunted with me while I was a less accomplished shot. They were having to find runners WAY more often than Ace does, & I think that experience made them better at it. That said, Ace is still only in his 4th season, does very, very well, & I suspect will only improve in the next few seasons. He's not perfect, but at this point, without him, I don't hunt pheasants.