A bird is an endothermic vertebrate that has feathers and a fourchambered heart. A bird also lays eggs. The bodies of birds are adapted for ﬂight. For example, many bones in the bird's body are hollow, which makes the skeleton light. The bones of a bird's front limbs form wings. Flying birds have large chest muscles. And all birds have feathers.
Contour feathers are large feathers that give shape to a bird's body. The contour feathers that extend beyond the body on the wings and tail help it balance and steer during ﬂight. Birds also have short, ﬂuffy down feathers that are found against the skin and keep the bird warm.
Birds have no teeth. Instead, they have bills that are shaped according to the type of food they eat. For example, a hawk has a pointy, curved bill that acts like a meat hook.
Many birds have an internal storage tank, or crop, that allows them to store food inside the body after swallowing it. Birds also have a stomach where food is bathed in chemicals that help to break it down. This food then moves into a thick-walled, muscular part of the stomach, the gizzard,which squeezes and grinds the food.
Flying takes a lot of energy. Birds have a system of air sacs inside their bodies that connect to their lungs. Air sacs enable birds to get more oxygen from each breath than other animals can.
A bird's heart has four chambers. There is no mixing of oxygen-rich and oxygen-poor blood in a four-chambered heart.
Birds need a lot of energy to maintain their body temperature. Each day, an average bird eats the amount of food equal to about a quarter of its body weight. Feathers also help birds maintain their body temperature.
Birds have internal fertilization and lay eggs. Bird eggs will develop only at a temperature close to the body temperature of the parent. Most parent birds care for their young at least until they are able to ﬂy.
Birds are the most diverse land-dwelling vertebrates. Birds are adapted for living in diverse environments. You can see some of these adaptations in the shapes of their legs, claws, and bills.
Birds play important roles in the environment. They act as pollinators, seed dispersers, and predators.
The faster air moves, the less pressure it exerts. Faster-moving air above a bird's wing exerts less pressure than the slower-moving air below it. The difference in pressure above and below the wings as a bird moves through the air produces an upward force that causes the bird to rise. That upward force is called lift.
Three types of bird ﬂight are ﬂapping, soaring and gliding, and diving. However, wing shape alone does not enable birds to ﬂy. They must get off the ground. To get off the ground, birds push off with their legs as they pull their wings down.
Some birds, like sparrows, ﬂap nearly constantly. To ﬂap, a bird sharply pulls down its wings as it did when it pushed off the ground. Other birds will soar or glide without ﬂapping after they get in the air. When birds soar, they rise up into the sky on currents of warm air. When birds glide, they coast downward through the air.
Diving does not involve lift. When a bird dives, it tucks in its wings from their open position. Pulling in the wings changes the pelican's body shape into one that produces no lift, and the bird falls from the sky. A bird may achieve great speed during a dive.
Mammals are a diverse group of vertebrates that share many characteristics. All mammals are endothermic vertebrates that have a four-chambered heart and skin covered with fur or hair. Most mammals are born alive, and every young mammal is fed with milk produced by organs in its mother 's body. These organs are called mammary glands.In addition, mammals have teeth of different shapes that are adapted to the type of food they eat. Most mammals have teeth of four different shapes to tear and chew their food. Incisors are ﬂat-edged teeth used to bite off and cut parts of food. Canines are sharply pointed teeth that stab food and tear into it. Premolars and molars grind and shred food into tiny bits.
Mammals breathe with lungs. Mammals breathe in and out because of the combined action of rib muscles and a large muscle called the diaphragm located at the bottom of the ribs. The lungs have a huge, moist surface area where oxygen can move into the blood. Mammals have a four-chambered heart and a two-loop circulatory system.
All mammals have fur or hair at some point in their lives. Mammals that live in cold climates usually have thicker fur than mammals that live in warm climates. A layer of fat beneath the skin also helps keep mammals warm.
Mammals have adaptations that allow them to move in more ways than other vertebrates. Also, the brains of mammals enable them to sense their environment and behave in complex ways. The senses of mammals are highly developed and adapted for the ways that individual species live.
There are three main groups of mammals-monotremes, marsupials, and placental mammals. The groups differ in how their young develop. Monotremes are mammals that lay eggs. There are three species of monotremes-two species of spiny anteaters plus the duck-billed platypus. Marsupials are mammals whose young are born at a very early stage of development. They usually continue to develop in a pouch on their mother 's body. Koalas, kangaroos, and opossums are some of the better-known marsupials. Marsupials have a very short gestation period, which is the length of time between fertilization and birth.
Most mammals, including humans, are placental mammals. A placental mammal develops inside its mother's body until its body systems can function independently. The placenta is an organ in pregnant female mammals that passes materials such as food and oxygen from the mother to the developing embryo, and carries the embryo's wastes away.
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