Caspian terns are highly migratory and are nearly cosmopolitan in distribution. In North America , nesting has been reported on the west coast from Baja, California to the Bering Sea, in the interior from the Gulf Coast of Texas to Lake Athabasca, Saskatchewan, and on the east coast from the Florida panhandle to Labrador. Outside of North America, nesting has been reported in Australia, New Zealand, South Africa, Asia, and Europe. Caspian terns winter primarily on the Pacific Coast from southern California south through west Mexico and Central America.

Early estimates of the Pacific Northwest population were as many as 500 pairs nesting with gulls and cormorants as far north as Klamath Lakes in Oregon. Nesting colonies were first discovered in Washington near Moses Lake and Pasco in the 1930s, but coastal colonies were not recorded until the late 1950s, when one was found in Grays Harbor. Since the early 1960s, the population has shifted from small colonies in interior California and southern Oregon to large colonies nesting on human-created habitats along the coast. The population in the Columbia River basin is part of a dramatic northward and coastward expansion in range, and an overall increase in Caspian tern numbers in western North America.

Caspian terns arrive in the Columbia River estuary in April and begin nesting at the end of the month. Terns construct their nests on islands to avoid predators, and show a preference for barren sand. They eat fish, requiring about 220 grams (roughly one-third of their body weight) of food per day during the nesting season. The timing of courtship, nesting and chick rearing corresponds with the out-migration of many of the salmonid stocks in the basin.