Conservation and Management

Population declined rapidly in late 1800s and early 1900s because of exploitation for millinery trade and egg collection. Rebounded following passage of the Migratory Bird Treaty Act of 1916 and with changing attitudes toward conservation. However, breeding distribution of interior populations has fragmented further in this century (mostly since the 1940s) as nesting habitats have been destroyed by flooding behind dams, channelization, and untimely release of water from dams. Populations declined in many breeding areas during 1950s–1970s, possibly because of use of organochlorine pesticides and nesting-habitat disturbance by humans, then rebounded again since 1980 as result of increased conservation efforts.

Because of Endangered status of California and interior Least Terns, federal and state agencies have closely monitored these populations for 10–30 yr. Data from these studies are rarely published in technical literature, but are available as unpublished agency reports and are often cited in regional studies (e.g., Blacklock et al. 1978, Schulenberg and Ptacek 1984, Sidle et al. 1988). Early estimates were often low because of inadequate survey of available habitat, thus increasing numbers do not necessarily indicate growing populations. Furthermore, because Least Terns are relatively long-lived, numbers show delayed response to reproductive problems in the population. Thus it is imperative to monitor not just numbers, but also reproductive success. Recent unpublished USFWS Breeding Bird Survey (BBS) route data indicate for 10 regional summaries with 10 or more routes reporting Least Terns, that 1 region showed a significant downward trend and 4 regions showed a significant reduction in detection on routes (data examined by BCT). However, BBS routes have little application to detection of Least Terns because routes are rarely placed to contact tern nesting areas.

Shooting And Trapping

Destroyed on New Jersey beach-nesting areas between 1881 and 1886 for New York millinery trade (Stone 1937). In Mississippi Delta, Least Terns and other fish-eating birds have been shot at some commercial fish ponds to reduce perceived fish depredation. On wintering grounds, may be killed by humans for food or sport (Whitman 1988, Boyd 1993).

Pesticides And Other Contaminants/Toxics

Because diet consists primarily of fish, bioaccumulation of pesticides, heavy metals, and PCBs has potential for negative effect on Least Tern populations. Impacts not well understood or quantified, however. Boardman (1988) found DDT or its metabolites in all of 16 abandoned eggs (4.5–23.64 ppm [parts per million] DDT wet weight) and livers of 13 adults (2.35–49.13 ppm) found dead on s. California beaches. Oil pollution is a threat in coastal and inland habitats, on breeding and wintering areas, and along migration routes.

Ingestion Of Plastics, Lead, Etc

Not known to be a problem.

Degradation Of Habitat

Beach resort development in late nineteenth century hurt New Jersey populations (Schick 1890), and nesting in Massachusetts in mid-1930s was precarious. Dredged material with a high clay content can make eggs stick to nest when clay gets wet and dries, resulting in broken eggs when adults attempt to turn them (Thompson and Slack 1982, Jackson and Jackson 1985). Increased jet ski activity near nesting areas can result in constant churning of water, which increases turbidity and presumably decreases tern foraging success (JAJ; Mississippi). In riverine ecosystems, creation of reservoirs, untimely release of water from dams, and channelization to remove sandbars and islands that might be used for nesting continue to have deleterious effect on interior Least Terns (Faanes 1983, Schwalbach 1988, Sidle et al. 1988, 1992, Dirks 1990). In arid plains areas, diversion of river water and withdrawal of water from aquifers for irrigation has lowered water tables such that some former rivers that supported Least Terns are now dry (Schulenberg and Ptacek 1984).

Disturbance At Nest And Roost Sites

Destruction and disappearance of Least Terns from some islands of the Dry Tortugas and Florida Keys were caused by egg-collecting and by introduced rats early in twentieth century (Robertson 1964). Eggs described as “delicious eating” (Thompson 1903) and are still eaten in the Caribbean (Haynes 1987). Birds nesting in areas that are frequented by humans often suffer from disturbance. Humans kill eggs and chicks by stepping on them inadvertently, by deliberately collecting eggs for food or fun, or by off-road-vehicle traffic and earthmoving equipment (Goodrich 1982, Burger 1989, Cowgill 1989, Lingle 1993b, Smith and Renken 1993, Kirsch 1996). On Gulf and Atlantic Coasts, chicks and eggs are lost each year in beach-nesting colonies to beach maintenance equipment and during annual Fourth of July fireworks and associated nighttime human presence on nesting areas (JLA, JB, JAJ, and BCT). Airboats, off-road vehicles, and human recreationists have destroyed eggs and chicks at interior colonies (LAH and EMK).

Human/Research Impacts

Creation of dredged-material islands, construction of flat graveled rooftops, and clearing of vegetation sometimes provides new nesting habitat for Least Terns, but productivity of nests at such sites may be much less than for natural sites. In coastal Mississippi, a concrete seawall prevents chicks from straying away from beach-nesting areas, and a busy major highway limits access of mammalian predators to the colonies (Jackson and Jackson 1985). Species may respond favorably (increased pairs, improved productivity) to management and protection of nesting areas (Toups 1976, Britton 1982, Jackson and Davis 1996).

Conservation Status

Rangewide conservation status is difficult to assess because Least Terns are highly mobile, recognized subspecies are almost indistinguishable and some mix extensively, standardized surveys are difficult, and leg-banding produces few recoveries and resightings. Undoubtedly the interior population requires continued attention to maintain water flows that promote natural nesting substrates wherever possible. Production of young appears restricted in many areas, and rate of postfledging survival is unknown. Ultimately, comprehensive management plans are needed that integrate breeding, migration, and wintering habitat in context with potential mixing of the various recognized populations.

Measures Proposed And Taken

Efforts to protect and manage Least Terns are almost always directed at nesting areas and include signs and symbolic fencing (Burroughs 1966, Buckley and Buckley 1976, Hays 1980, Burger 1989), steel cables to exclude vehicles (Swickard 1972), barriers to exclude predators, electric fences to deter large mammalian predators, artificial nesting sites (Swickard 1974, Koenen et al. 1996a, 1996b), construction and clearing of islands in rivers (USFWS 1990, Hill 1993b, Latka et al. 1993, Sidle and Kirsch 1993), decoys to attract terns to abandoned or unoccupied sites (Kotliar and Burger 1984), shelters for chicks (Jenks-Jay 1980), wire barriers to confine chicks of roof-nesting terns to safe areas (Cornwell 1986), hired guards at times of anticipated excessive human activity in the area (Jackson and Jackson 1985, Burger 1989), elevated nesting ridges (Koenen et al. 1996a), and raked beaches to eliminate vegetation (Jackson and Jackson 1985). Least Terns have been attracted to abandoned colony sites with use of decoys, if there is sufficient space for pioneering pairs (Kotliar and Burger 1984). Breeding habitat vulnerable to human activities such as recreational use, direct disturbance, and habitat modification (Downing 1980, Boyd 1981, Burger 1984, Carreker 1985, Kruse et al. 1993). Poison baits containing brodifacoum have been used to control Norway rats at colonies, and a strobe light system reduced nocturnal predation by owls and night herons in Indiana (Johnson and Castrale 1993). Kotliar and Burger (1986) recommend using salt water or Ureabore (a highly concentrated salt compound) to control vegetation at colony sites.

With a threat of flooding, it is possible to successfully move Least Tern nests a few centimeters at a time, in the manner described by Prellwitz et al. (1995) for Piping Plovers, to higher ground or to artificially build up nests a few centimeters, but moving nests is not recommended as an alternative to managing water at proper levels for nesting birds. During 1995–1996, Least Tern eggs and chicks were rescued from rising flood waters on Missouri River by U.S. Army Corps of Engineers, raised in captivity until capable of foraging, and released at various locations in Missouri River drainage (C. Kruse pers. comm.). Swickard (1974) covered salt flat nesting areas with a layer of sand or rows of small mounds of sand to prevent flooding of nests. Both sites were used, although the larger area of sand was preferred over the small mounds.

Effectiveness Of Measures

Despite many measures attempted, success in increasing nesting and production is mixed (Koenen et al. 1996a, 1996b). Generally, conservation measures have been directed at altering vegetation succession, reducing predation, or controlling human activities. Controlling or redirecting human activities can be effective, but is very labor intensive. Attempts to alter vegetation succession and predation require serious evaluation in context with biology of Least Terns, which are adapted to shift nest areas quickly and which have small, cryptic-colored young that make mortality estimation difficult. In riverine habitat, attempts to restore or mimic natural flooding should take precedence over manual control of vegetation. Predator control barriers are sometimes effective locally where nesting is concentrated, but such barriers are labor intensive and should only be applied where losses to predators are adequately substantiated. Effectiveness of captive-rearing and release of tern chicks on Missouri River is currently under evaluation (C. Kruse pers. comm.).