What's the toughest bird to kill? Some would say sea ducks, particularly eiders and scoters, given their thick feathers. Others may point to late-season pheasants, which I've seen absorb shot, shake it off, and fly off no worse for wear. Then there's Canada geese, scaup, and canvasbacks—all tend to act as if immune to being shot.
However, if you ask me, the toughest bird to kill has nothing to do with species. Any bird presenting a going-away shot—as illustrated in the accompanying photo--presents certain challenges.
Namely there is a tendency to hit the bird in the rear-end and, as a buddy is fond of saying, you have to hit the birds "where they eat, not in their feet." Beyond 25-30 yards, it can be difficult to achieve sufficient pellet-penetration to reach the vitals. You might drop the bird, but your dog will likely have a tracking job ahead of it.
The key to clean kills is putting approximately three or more pellets in the front half--or ideally the front third—of the bird. Fortunately, going-away birds often present a better angle to the vitals than may be apparent. Generally the bird will either rise gently during its escape or "tower" into the air before making its getaway. These birds are quite killable. The shot pattern should be positioned just above the rising bird, while a high, going-away shot may be achieved by leading below and forward of the bird (some people shoot the bird at the peak of the tower, but I prefer to let it sort out a direction—just a personal preference). The biggest mistake people make is putting their bead directly on the bird, which causes them to hit the rising bird "where it feets" and to shoot over the high bird.
Occasionally, however, you will encounter a going-away bird that truly offers no angle to the vitals. Ethics and a knowledge of your pattern and loads come into play here. I'll shoot crossing pheasants and tall, passing geese all day long at 40 yards, but I'm much more conservative when it comes to pulling the trigger on going-away birds. I can't stomach wounding them, and the risk is too great.