Okay, so they're not actually zombies, they're parasitic worms that snails have consumed. But these parasites are so powerful that they can take over the snail's tentacles and use the snail's body for its own devilish intentions: survival.
Once the parasites have entered the snail, it uses the snail as a go between in order to reach it's ultimate goal: a bird. The snail's tentacles, when engorged from the parasite, resemble maggots, which are the bird's favorite prey. Soon, the snail is inside the bird's stomach, just where the parasite wanted.
In the bird's stomach, the parasitic worm reproduces, thus giving its offspring a push in the right direction to continue the cycle. For when the bird poops out its waste, the parasite offspring are disposed of as well, and right into a perfect position for snails to eat (who knew snails like to munch on bird poop?).