In some parts of the world，they're called muskegs or moors. In the United States，you probably know them as swamps or bogs. But no matter what they're called，wetlands（湿地；沼泽地）have one thing in common around the world—they're very important to plants，animals and the environment. Thousands of kinds of plants and animals，including many on the endangered animal list，depend on wetlands for a living. Some animals live in or near wetlands all year long. Others，like migrating birds（候鸟），stop by wetlands to feed and rest on their journey.

Wetlands are very important to people，too. They hold water that could cause floods（洪水）.During dry times，they slowly let water run back into rivers. Plants that grow in wetlands help cut down on pollution. That helps to keep our drinking water clean.

Many wetlands in the United States have been used to build shopping malls，roads and homes.

It's said that about 243 km² of wetlands are lost in the United States every year. Without wetlands，rivers dry up and streets become flooded. That also creates big problems for animals that can't live anywhere else.

May is American Wetlands Month. It's the perfect time to remind people to save wetlands.

The weather gets colder, days get shorter and the leaves change their colours and fall off the trees. Soon, winter is here. Snow covers the ground. People live in warm houses and wear heavy coats outside. Our food comes from the stores, but what happens to the birds?

Birds do many different, amazing things to get through the winter. Some of them migrate. This means they travel to other places, where the weather is warmer, or where they can find food.

Many birds migrate in autumn. Some travel in large groups, because the trip can be dangerous. For example, geese fly in noisy V-shaped groups. Other kinds of birds fly alone.

How do they know when it is time to leave for the winter? Scientist are still studying this. Many scientists think migration is part of a yearly cycle of changes that a bird goes through. The cycle is controlled by changes in the amount of daylight and the weather.

Birds can fly very long distances. For example, the Arctic tern（北极燕鸥）lives close to the North Pole in summer. In autumn, it flies south, all the way to the South Pole. Each spring it returns to the North Pole.

Most birds migrate shorter distances. Do you know how they find their way to the same place each year? Birds use the sun, moon and stars for direction.

Monarch butterflies from the Northern and Central United States and Southern Canada make the most amazing journey in the insect (昆虫) world Each year, they travel about 3,000 miles to their winter home in Florida and Mexico. How can they fly so far? And why do they make this long and dangerous trip?

For many years, people in Mexico wondered where the orange and black butterflies came from every winter. Then, in 1937, a scientist started to follow and study the butterflies. For the next 40 years, he discovered that one butterfly started its journey in Canada. Four months later, it arrived in Mexico.

The length of the butterflies’ trip is only part of the mystery (奥秘). Another amazing thing is that the butterflies always return to the same area in Florida and Mexico.

How do the butterflies find their way back to the same place? This is an interesting question because only every fourth generation (代) makes the tip south. In other words, the butterfly that travels to Mexico this year is the great-great-grandchild of the butterfly that traveled there last year.

Each year, four generations of a Monarch butterfly family are born. Each generation of the family has a very different life. The first generation is born in the south in late April. It slowly moves north, reproduces (繁衍), and then dies. On the trip north, two more generations are born, reproduce, and die. Each of these generations of butterflies lives for only two to five weeks. In the autumn, the fourth generation of butterflies is born. This generation has a much longer life. It lives for about eight months and makes the amazing journey back to the winter home of its circle of life.

Today, people are still studying Monarch butterflies. But they are not clear about everything.