Thursday was the 51st anniversary of the first Earth Day, and strides have been made since 1970 to clean up many of our rivers and lakes, recycle, protect natural ecosystems, and wake up to the threat of hazardous materials.

Still, we have a long way to go to live in a sustainable way in harmony with nature.

There are many things each of us can do to become good stewards of the earth. The best way might be taking pleasure in the true beauty of the earth's ecosystems and its creatures, and to take time to learn about some of the plants and animals that share the earth with us. It's just about impossible to destroy something you understand and love.

The air is filled with spring bird music in April. We hear the loud rattling sounds of belted kingfishers, the "what-cheer, cheer, cheer" song of northern cardinals, woodpeckers drumming, the chattering of the tree swallows and barn swallows, cooing of mourning doves, and honking of Canada geese.

Added to this is the "cheer-up" song of the American robin, and remarkable melodic accomplishments of the song sparrow. The song sparrow song is three or four repeated notes followed by a rich warble. Put into the English language it might come out as "Marge-Marge-Marge, put-on-your-TEA-kettle, ettle-ettle."

So, why do birds sing, call, or make mechanical sounds? It's their means of communicating with other birds and the world around them. The chief function of song in most species is to proclaim territory. It warns males of the same species to stay away, and attracts females. The songs also help maintain and strengthen the bond between the mated pair.

There are social songs such as a canary-like one used by American goldfinches in flock formation. At times a bird sings because it appears to bubble over with the pure joy of living. Call notes are used in a variety of situations. There are calls of alarm, anger, scolding, and location. As an example, the cheeps of warblers help these nocturnal migrants stay together in flight.

Even though birds do not call and sing for our benefit, we can still enjoy their efforts. They keep us in tune with the changes that are coming our way, especially in spring.

Jim Gilbert's observations have been part of the Minnesota Weatherguide Environment Calendars since 1977, and he is the author of five books on nature in Minnesota. He taught and worked as a naturalist for 50 years.