The main feature associated with the fact that a bird needs to have a minimum mass for flight is that birds have a “through” digestive system, i.e. The digestive system of a bird is a hollow tube from the beak to the cloaca. The system works on the trail. principle: it takes feed, produces juice with enzymes, absorbs substances and removes undigested residues through the cloaca. If it is a bird of prey, then bones, claws and hard objects, they vomit.
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