Swallows and Yuanyang belong to birds and have the same characteristics as birds. Birds are all vertebrates, egg-laying, feathered, with winged forelimbs and four claws on hind limbs, with high and constant body temperature and horny beaks.
Most birds have the ability to fly, and use flight to feed or migrate to adapt to the changing environment. The flying ability of birds is closely related to many unique structures of their bodies. Birds suitable for flying have developed breasts, and the contraction and relaxation of pectoral muscles drive the wings to flap up and down.
There are many kinds of birds, which are distributed all over the world and have diverse ecology. Birds can be divided into three superfamilies, flat-chested superfamily, including a bird that is good at walking but can't fly, such as ostrich. Penguins include a class of birds that are good at swimming and diving but can't fly, such as penguins. The order Convex Chest includes birds with developed wings that can fly, and most birds belong to this order.
Birds eat all kinds of food, including nectar, seeds, insects, fish, carrion or other birds. Most birds move during the day, and some birds move at night or dusk. Many birds migrate long distances to find the best habitat, and some birds spend most of their time at sea.