Caspian Tern (Hydroprogne caspia)

Photo by Scott FrankeCaspian Tern by Scott FrankeCaspian Tern distribution map


Population Information


Life History

Habitat Selection

The Caspian Tern nests on freshwater and coastal islands, beaches, and shorelines isolated from human disturbance (Cuthbert and Wires 1999, Strong et al 2004, Matteson 2006). Nest sites generally have little to no vegetation, sometimes resulting from thick layers of fecal deposits surrounding nests (Scharf 1963), and include a variety of flat nest substrates, such as cobble-gravel, crushed stone, sand, dredge spoil, (Scharf 1979, Cuthbert and Wires 1999, Matteson 2006) and occasionally artificial nesting platforms (Lampman et al. 1996). Nest sites are susceptible to flooding and thus nests often are located at the highest point of an island or shoreline, usually elevated 2-3 m above water level (Cuthbert and Wires 1999). The nest is a scrape, often lined with dried vegetation, sticks, small pebbles, bits of broken clam shells, feathers, or debris (Bent 1963).

The Caspian Tern generally nests in colonies ranging from several to hundreds of pairs, but a pair may nest singly near the nests of other tern or gull species (Cuthbert and Wires 1999). Erosion, drought, flooding, and vegetation encroachment can alter the suitability of colony sites from year to year and lessen site fidelity (Suryan et al. 2004). However, if conditions remain suitable, terns often return to colonies where they successfully raised young (Cuthbert 1988).

Habitat Availability

The Caspian Tern is a fairly common migrant in Wisconsin (Robbins 1991) but has never been a common breeder in Wisconsin or the Great Lakes region (Hyde 1996). Historically, the Caspian Tern nested irregularly in northeastern Wisconsin, primarily on isolated islands within Green Bay and Door County (Kumlien and Hollister 1903, Ward 1906). Recently, this species occupied a colony site near the Milwaukee U.S. Coast Guard Impoundment in southeastern Wisconsin (Matteson 2003) and continues to intermittently nest along the Lake Michigan coast in northeastern Wisconsin (Matteson 2006). Breeding habitat used by this species often is unstable and subject to flooding, erosion, vegetative encroachment, and other natural and anthropogenic disturbances. Furthermore, prey availability and the presence of nest predators also influence the establishment of colony sites (Suryan et al. 2004). Given these unpredictable conditions, the availability of suitable nesting habitat fluctuates from year to year. Fortunately, the Caspian Tern is able to respond quickly to habitat changes and rapidly colonize new areas (Wires and Cuthbert 2000).

Population Concerns

In North America, concerns for declining numbers or limited habitat led many states and provinces to designate Caspian Terns as a protected species (Wires and Cuthbert 2000). The Caspian Tern was officially listed as a Wisconsin endangered species in 1989 (Matteson 2003). Over the last three decades, populations have started to recover, particularly on Lake Ontario and along the Pacific coast where the largest colony in North America (>10,000 pairs) resides (Cuthbert and Wires 1999). Breeding Bird Survey (BBS) data indicate a non-significant population increase range-wide (Sauer et al. 2005), although BBS may not be the most reliable survey method due to this species’ semicolonial nesting habits and localized distribution. Since the late 1960s, the Great Lakes population has nearly tripled, mostly driven by dramatic annual increases on Lake Ontario (Wires and Cuthbert 2000). Matteson (1993) presented a recovery goal of at least 100 nesting pairs over a 10-year period and an average annual production of at least 100 young for at least 5 years of a 10-year period. The Caspian Tern is inadequately sampled by BBS in Wisconsin but Wisconsin Checklist Project data indicate a significant increase from 1983-2005 (Rolley 2005). It is no longer a rare nesting species here, although only two breeding colonies were documented during the Wisconsin Breeding Bird Atlas (1995-2000). In 2006, however, four colonies in southeastern, east-central, and northeastern Wisconsin contained a total of 1,338 nests (Matteson 2006). The primary factors limiting populations appears to be the availability of nest sites free from human and predator disturbance (Cuthbert and Wires 1999). 

Recommended Management

Partnerships between state and federal agencies and private organizations dedicated to the restoration, conservation, and management of Great Lakes coastal ecosystems will benefit the long-term management of both Caspian and Common Terns (WDNR 2005). The conservation and management of dredge spoil sites and other island sites as Caspian Tern nesting colonies also is recommended (WDNR 2005). Managers should control vegetative growth and minimize disturbance in areas designated for Caspian Tern conservation. Wherever applicable, managers should maintain water levels at a depth that will prevent land-bridging (i.e., allowing access to mammalian predators and human disturbance) and flooding during the breeding season (Strong et al. 2004). Also, regulations that eliminate or limit the presence of organochlorine contaminants are recommended (WDNR 2005).

Research Needs

Targeted surveys are needed to better monitor trends for populations of Caspian Terns in Wisconsin and across their breeding range. The impacts of interspecific competition, particularly with Ring-billed Gulls and Herring Gulls, needs more study at Great Lakes colony sites. Researchers should continue to monitor and assess the effects of chemical contamination on Caspian Tern reproduction and survival (Cuthbert and Wires 1999).

Information Sources


Contact Information

Wisconsin Department of Natural Resources. Madison, WI.

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