Between Birds and Pigs.

I would like to challenge the phrase "eats like a bird", and the reason it denotes a modest and light eater.
True, most smaller birds feed on little more than berries, seeds, and insects, and their larger, "of prey" counterparts probably don't make a habit of gorging themselves multiple times per day.
But still, birds feed themselves enough to induce a (seemingly) constant state of defecation, and their young greedily gobble-down the regurgitated contents of their parents' stomachs with vigor, which, when applied to the human world, sounds more like a serious health problem than a dainty dietary preference.