The time to watch birds is in the evening; as the sun sinks and the air cools, they come forth. Small gray-brown rock wrens hop among the boulders near the visitor center. Robins scurry through the leaves in the stream course below the Dinosaur Quarry. Here too, western flycatchers and Audubon’s warblers search among the cottonwoods for insects. A flash of red and white is seen as a red-shafted flicker darts from its nest in the hollow trunk of a tree. The sky is filled with wheeling, twittering rough-winged swallows and white-throated swifts that descend from their nests on the cliffs to feed upon the gnats and other flying insects. These are the birds that spend the spring and summer here. They raise their families and, young and old alike, depart in autumn when frost kills the insects upon which they feed. As they flee the cold of winter, they are joined by many other birds that make their summer homes at higher elevations or more northern latitudes. Ducks, geese, and swans join the hordes moving southward. So do the various shore birds, bluebirds, and hummingbirds. But the sagebrush flats and brushy ravines are not left vacant by this wholesale migration, for as the summer residents move out the winter residents move in. The Oregon and gray-headed juncos spend the entire winter here. Great flocks of mountain bluebirds descend from the mountains and piñon jays make the hills resound with their screams. Canada geese and golden-eye ducks live on the Green River and remain until it freezes. The harsh croak of the raven is seldom heard in summer but often in winter. Few birds live here in winter and summer—the golden and bald eagles, the red-tailed hawk, and the little sparrow hawk. Perhaps the most handsome year-round resident is the magpie with its long, iridescent tail, black and white body, and white patches on its wings. One other resident makes his presence known by his eerie cry on frosty, moonlit nights—the western horned owl. He hunts every night, summer and winter, but is seldom seen. Occasionally he is disturbed upon his daylight roost and as he skillfully dodges through the junipers, he makes a joke of the story that owls don’t see well in daylight. The mammals that live in the vicinity of the Dinosaur Quarry are almost never seen. There are several reasons for this—almost all of them are nocturnal, are very shy, and most of them are small. In spite of their retiring habits, they reveal their presence in a number of ways. Patches of bare earth under sagebrush and nearby sandy slopes are crisscrossed with tiny paths beaten into the dust by deer mice. Along the river bank, gnawed tree stumps, a few fresh chips, and perhaps a webbed footprint tell us beaver have been active during the night. The paired hind footprints of the kangaroo rat are common on the hillside. Freshly fallen snow records the preceding night’s activities in perfect detail. [Illustration: GOLDEN-MANTLED GROUND SQUIRREL]