Population Status Trends
Caspian Tern populations are stable or increasing across most of North America; recently their breeding range has expanded to southern Alaska. Breeding Bird Survey (BBS) data shows increases in some populations. However, Caspian Terns are decreasing in Eurasia and Africa, and are listed as vulnerable, rare, or even extinct in parts of their former range, due in part to the scattered nature of their breeding colonies.
Protection of Caspian Terns and their traditional nesting sites (bare sand substrate on islands free of mammalian predators) have succeeded in stabilizing populations in most parts of their North American range. The terns have also benefited from their ability to make use of human-created dikes and dredge spoil islands for breeding. In most regions of North America, however, the number and size of Caspian Tern colonies appears to be limited by suitable available nesting habitat.
Erosion of nesting islands, changing water levels, gull predation on eggs and young, and harassment by predators and humans all reduce nesting success. Breeding colonies that are not situated on islands are especially vulnerable to disturbance and predation; Caspian Terns readily desert their colonies if disturbed by mammalian predators, including humans, early in the breeding season.
In the Columbia River estuary, near the Washington-Oregon border, the world's largest colony of Caspian Terns was feeding upon juvenile salmon and was causing conflicts with efforts to restore threatened salmon runs. During 1999-2001, the entire colony was successfully relocated to an island 16 miles closer to the ocean, using a combination of vegetation management, decoys, and recorded tern vocalizations. In this same area, Bald Eagles that flush nesting terns, and hybrid Western/Glaucous-winged Gulls that eat tern eggs and chicks, pose additional challenges.
In North America, studies including nesting, feeding, breeding, parental behavior, courtship, and nest site selection of Caspian Terns have been conducted along the Great Lakes and the Pacific and Gulf coasts. Band recovery data have revealed information on the migration patterns and over-wintering sites of these populations.