Birds range in size from thumb-sized hummingbirds to ostriches larger than a big man. Most birds, of course, fall somewhere between these two extremes. One common bird, the American Robin is about 10 inches in length, approximately the handspan of a large man. The smallest american birds are some species of hummingbird that could nest and raise a family in the cupped palm of a child. The nearly extinct California Condor has a wing span of almost 10 feet, and the Bald Eagle spans 7 feet.
Birds walk, run, hop, swim, perch, cling, fly and even dig.
They lay their eggs and raise their young in holes in the ground, in nests of varying complexity in vegetation or on the ground, in holes in trees, in human-constructed nest boxes, and in or on various parts of buildings. According to the best evidence paleontologists have found, birds evolved from dinosaurs or from a near relative of the dinosaurs during the Mesozoic Era. Birds evolved feathers and light hollow bones which allow them to fly, though some species later returned to life on solid ground. Flying birds have powerful muscles anchored to special bones in the breast. Many birds are migratory, nesting and raising their young in temperate or higher latitudes during the warm months and spending the winter in tropical or sub-tropical areas.