When reading discussions on fly-offs and birds' anatomy, well... — it seems like
we all kill a lot more quarry than we can recover bodies.
Here's a quote from one of those discussions (by Bob Sterne, GTA):
"I do a lot of Grouse hunting, mostly head shots, and occasionally have a fly-away, despite a
solid hit, red mist, and if the bird is recovered often half the skull missing....
My son, who is
a Nurse and has studied a lot of anatomy gave me THE ANSWER.... Birds have their central nervous system spread out along the upper part of the spinal cord, the brain-stem if you prefer....
It is that part of their nervous system that controls the flight muscles.... Even with a solid hit to the brain, providing an instant kill,
the message that the bird is dead just doesn't reach the flight control system....
The same thing can happen with a hit to the heart and/or lungs....
The "flight response" takes over on impact, the bird launches, and can fly for some distance, often under what looks like control,
before running out of blood and folding.... It doesn't always happen, of course, and is more common with a chest hit than an impact to the fusebox…. but it happens often enough to be really frustrating when you're hunting for food...
Bob"

Different shooters, different ways of counting....

Here are some examples I've seen:
●
Some classify their hits like this: ▪
DRT = dead right there
▪
F&D = fly and die (feathered quarry)
▪
R&D = run and die (furred quarry)
▪
H&D = hide and die (hide in burrows and holes)
▪
HBS = hit but survive :-(
▪
Miss ●
Others define the kill probabilities: ▪
Confirmed kill = 100% (death visually confirmed)
▪
Probable kill = 70% (for example: "audible thud, and at least 10 feathers flying", or "red mist", "dropped straight down and out of sight", etc.)
▪
Possible kill = 40% (for example: "at least 5 feathers flying, but no audible thud", or "5 feathers, but flies away without visible impairment", etc.)
▪
Miss
It seems if we limited ourselves to only counting DRT hits we are more strict than most powder burner hunters.... I explain:

The phenomenon that quarry after getting hit hurries off for long ways before dropping dead seems to be very common with
powder burners.... For example,
deer, after getting a perfect hearth/lung shot, can run off for hundreds of yards(!), before dropping dead....
Why would
killing be any different with airguns?

However, an important difference between powder burners and airguns is not in the
killing, but in the
recovery: Airgun quarry is
very small — so
finding it after a non-DRT hit is much more difficult than finding a deer, with blood trail and all. Airgun quarry
can hide and die in burrows and holes,
typical powder burner quarry can't....
So, it seems that only counting DRT's as kills is more strict than how power burner hunters count their kills....
Just my 2c.
Matthias