Scientists call chicks that come out helpless and stay in the nest altricial. Songbird chicks come out with their eyes closed and almost no feathers, and they wait in the nest for their parents to bring them food. Pigeons, songbirds, and owls and eagles are altricial. Chicks that come out well developed and ready to move are called precocial. Most chickens, ducks and turkeys are precocial, and pigeons are the exception among farm birds. Birds born in open country hatch ready to run, and being able to move right away helps them avoid predators.

A precocial chick does a great deal in a short time. After it makes the first hole in the shell, a chick stops and rests for three to eight hours before it breaks out. A few hours after hatching, it is dry and fluffy and starts to walk around. A young bird that has left the nest but cannot yet fly well is called a fledgling, and a bald eagle chick leaves the nest, or fledges, at around 10 to 12 weeks old. A young bald eagle does not get its white head and tail feathers until it is about 5 years old.

Some mammals are ready fast, too. In its first two hours, a foal starts to breathe, lifts its head, stands, drinks its first milk, walks and runs, and takes its first nap. It stands within about 55 minutes and walks or runs within about 90 minutes. The mare's first milk, called colostrum, is full of antibodies that protect the foal from disease during its first few months. A giraffe mother gives birth standing up, so her newborn calf drops about six and a half feet to the ground, and the fall helps break the umbilical cord and starts the calf's first breath. In its first days it is often tucked into tall grass while its mother feeds.

Marsupials grow up in a different way. A marsupial mother's pouch is a safe place where her young finish developing. A wombat's tiny, underdeveloped baby grows in the pouch for 6 to 10 months. A joey spends its first six months relying on its mother's pouch for protection and milk, and then it comes out to learn how to forage and avoid predators. A koala joey stays in the pouch for about six months, then starts eating leaves and a soft food called pap that it gets from its mother.

Two kinds of mammals lay eggs: the platypus and the echidna. They are called monotremes. A baby echidna comes out of its egg after about 10 days, blind and hairless, and it then drinks milk in its mother's pouch for two or three months. When it grows prickly spines, its mother takes it out of her pouch and builds a burrow for it. A mother platypus keeps her eggs warm by curling around them and holding them against her belly with her tail. After the babies come out, they drink milk from patches on her belly.

A newborn Asian elephant weighs between 150 and 350 pounds, and a baby elephant, called a calf, may stand about three feet tall at birth. It drinks milk with its mouth, so it does not need a long trunk yet. Elephant calves spend their days practicing, and they are clumsy with their trunks at first, but they learn.

Sled dog puppies are born with their eyes and ears closed, and they cannot make their own body heat. They rely on their mother and littermates to stay warm, and for their first two weeks they sleep up to 90 percent of the time.

A tadpole's tail is not shed. The body slowly absorbs it and uses it as a source of energy. Tadpoles in ponds that are drying up can speed up their change into frogs, or they can delay it. Some frog species skip the swimming tadpole stage and develop into small frogs inside the egg. A single frog can lay up to 20,000 eggs at once, but only a small percentage of the eggs survive to adulthood. Bullfrog tadpoles may take several years to turn into frogs.

A monarch egg is about the size of a pinhead or the tip of a pencil. A monarch caterpillar is less than 1 centimeter long when it hatches, and it grows to about 5 centimeters. It grows to almost 2,000 times the mass it had when it hatched. The other way insects develop, incomplete metamorphosis, skips the pupa stage and has only three stages: egg, nymph and adult. In a monarch, the caterpillar's chewing mouth changes into the straw-like tongue of a butterfly, used for sipping nectar, while it is inside the chrysalis.

Words to know

• altricial: helpless at birth
• precocial: ready at birth
• colostrum: first milk
• marsupial: pouch mammal
• monotremes: egg-laying mammals