I'd be hesitant to use shears. It's not that they would not cut through the neck, but because it would be pretty slow compared to an axe/hatchet or even a quick head snap. I have no problem dispatching an animal but if I have a choice they will never know what is happening. I hunt, it doesn't always go as planed. Bothers me every time it doesn't. Think about it this way, chop a small twig/branch on a chopping block and the cut the same branch with shears. The axe is instantaneous, you can actually hear/feel the fibers giving way with the shears. I know I can feel pain faster than that. If I HAD to cut off a finger I'd want the axe. Maybe a chicken reactions aren't as fast as a humans, but I doubt there could be too much of a difference. Just my opinion, I've never dispatched anything with shears before, so I can't really really say yes or no. BTW I don't really like the idea of the commercial dispatchers (the neck breakers) either. If you could spring load them or shears, maybe.
Swamp