The internal structure and reproduction of birds

Birds have no teeth, so food is torn off with the beak and swallowed whole. In grain-eaters such as the dove the widened part of the gullet, the crop, stores spare food. The stomach has two parts: in the glandular part food is softened by gastric juice, and in the muscular part (gizzard) it is ground with the help of small stones that the bird swallows. Flight and constant body temperature need much energy, so birds eat often and food is digested quickly in the intestine. The intestine, the urinary tract and the reproductive ducts open into the cloaca, and urine leaves together with the droppings. The lung is a complex organ made of tiny air chambers; air sacs connected to the lungs lie between the internal organs. In flight the sacs expand and contract with the up-and-down wing beat, and air passes through the lungs twice: on breathing in and on breathing out – “double breathing”. The faster the wings beat, the faster the breathing; a dove breathes about 26 times a minute at rest and 400 in flight. The air sacs also cool the body. The heart has four chambers: two atria and two ventricles, so venous and arterial blood do not mix and organs receive oxygen-rich blood. A dove’s heart beats up to 165 times a minute at rest and 550 in flight. Body temperature is about 42 °C. The eye is very sharp and sees colour, hearing is good, smell is weak. The brain is large, with strongly developed forebrain hemispheres and a folded cerebellum. Many behaviours (nest building, incubating, feeding chicks) are inborn instincts, but conditioned reflexes also form: a chick first pecks at everything, then learns edible from inedible and recognises the owner’s voice. Birds communicate by sounds; the parrot, mynah and crow can repeat some words.