Chickens will eat pretty much anything plant or animal, so if unleashed in the garden, they’ll pick it clean.
Ducks, on the other hand, are more discriminating. I can leave them alone in the spinach patch and they’ll waddle through clearing out snails, grubs and insects and leave the plants intact and the soil nicely tilled.
For example, this grasshopper had emerged from the ground and was getting ready to fly away as soon as it got rid of its old shell. Fortunately for me, Judy the duck found it (and two snails) deftly picked it off with her bill, moved along leaving the spinach unharmed.
She even left a little “fertilizer” behind as a bonus.