Questions 38-42 Birds, like mammals, have a four-chambered heart that separates oxygenated and deoxygenated blood. The blood temperature of birds is from 2˚ to 14˚ higher than that of other mammals. Though the use of the phrase “bird-brain” as an insult would suggest otherwise, birds in fact have a relatively large brain. They also possess keen sight and acute hearing, though they have little sense of smell. Birds are highly adapted for flight; their body structures combine lightness and strength. Body weight is reduced by the presence of a horny bill instead of heavy jaws and teeth and by air sacs in their hollow bones as well as in other parts of the body. Compactness and firmness are achieved by the fusion of bones in the pelvic region and in other parts of the skeleton. The heavier parts of bird bodies – the gizzard, intestines, flight muscles, and thigh muscles – are all strategically located for maintaining balance in flight. Feathers, despite their lightness, are highly protective against cold and wet. Feathers used for flight, especially, have great strength. Some birds, such as the ostrich, the penguin, and the chicken, lack the power of flight and have a flat sternum or breastbone, without the prominent keel to which the well-developed flight muscles of other birds are attached. In the majority of species of bird there are differences between males and females in plumage coloring. Male birds are usually the more brilliant or the more distinctly marked and are the aggressors in courtship. Most birds build a nest in which to lay their eggs. Some birds, such as the oriole, weave an intricate structure, while others lay their eggs directly on the ground among a few seemingly carelessly assembled twigs. Eggs vary in size, number, color, and shape. In spring and fall many birds migrate. Not all of the factors motivating this behavior are fully understood. These trips often involve flights of hundreds and even thousands of miles over mountains and oceans.