The internal structure and reproduction of birds

Birds usually breed when warm days come, because food must be plentiful when the chicks appear. A male and female form a pair: in small birds the pair lasts one season, in storks and birds of prey many years. Males invite the female by singing or dance-like movements. Most birds build a nest: birds of prey and storks make a bowl nest of twigs, woodpeckers and tits nest in tree holes, ducks and cranes on the ground, swallows in mud under beams. The cuckoo builds no nest but leaves its egg in another bird’s nest. In the egg a central yolk is surrounded by white, held to the shell by cords, with the embryo on the yolk; the bird turns the egg so it warms evenly. Chicks of hens, ducks, geese, pheasants and quails hatch covered in down with open eyes and soon follow the mother – these are precocial birds. Chicks of doves, swallows, sparrows, storks and birds of prey hatch with closed eyes and naked, and are fed by the parents for a long time – these are altricial birds. Birds incubate eggs, warm, feed and defend chicks; some distract the enemy’s attention. Birds descended from ancient reptiles. Archaeopteryx (about 150 million years ago) resembles reptiles in its teeth and long tail and birds in its feathers and wings, but it had no keel. Scientists today think birds came from small feathered predatory dinosaurs (theropods). Protoavis, mentioned in the textbook, is disputed: its relation to birds is unclear.