Conservation and Management

Shooting, Egging, And Trapping

Collection of eggs for food widespread during eighteenth and nineteenth centuries; probably important factor limiting numbers and distribution along Atlantic Coast and at some sites inland (Drury 1973–1974, Courtney and Blokpoel 1983). Shooting for millinery trade was main cause of population crash in 1870s–1880s (Drury 1973–1974, Nisbet 1973b). Shooting and egging now infrequent in North America, but continue sporadically in some areas (Burger and Gochfeld 1991). Egging still prevalent in Prairie Provinces at least into 1970s (Weseloh 1973, Stelfox and Brewster 1979); no recent information from this area.

Wintering birds trapped in numbers on coast of Guyana, mostly using throw nets at night roosts on mudflats, and sold for food in markets (Blokpoel et al. 1982, Trull 1984). In 1960s–1970s, one specialist trapper caught large numbers (up to 200 in 1 throw; Trull 1984), resulting in large concentration of band recoveries in that area, mainly in Oct–Dec (Hays et al. 1997). Birds also trapped on shrimp boats off Guyana, but said to be released alive (Trull 1984). Some caught by hand at beach seines in Trinidad (Blokpoel et al. 1982, Morris et al. 1982). No information on extent of trapping in other wintering areas.

Pesticides And Other Contaminants/Toxins

DDE-induced reproductive failure probably contributed to major population declines during late 1950s and 1960s, especially at freshwater sites.

Organochlorines. Organochlorines measured primarily in eggs, sometimes in muscle or liver. PCBs and DDE the predominant residues in Common Terns since first measurements were made in late 1960s; PCB levels much higher than DDE levels in all areas except Prairie Provinces (see Table 1). Concentrations of both contaminants were already decreasing rapidly when first reported in period 1968–1973 (Switzer et al. 1973, Nisbet and Reynolds 1984, Bishop et al. 1992), and have continued to decrease in all areas for which data are available (Table 1). Highest reported levels of DDE were in Prairie Provinces and Lower Great Lakes, but high levels also reported in eggs and chicks from Long I., NY, in 1966–1970 (Woodwell et al. 1967, Hays and Risebrough 1972). Levels of PCBs were more variable than those of DDE, with locally high concentrations in industrial areas, including Green Bay (Lake Michigan), Saginaw Bay (Lake Huron), the Detroit River, Port Colborne (Lake Erie), Hamilton and Toronto Harbors (Lake Ontario), Boston and New Bedford Harbors (MA), Providence Harbor (RI), and Long Island Sound (Table 1).

Congener-specific PCB data available only from Green Bay, Lake Michigan (Smith et al. 1990, Ankley et al. 1993, Jones et al. 1993), Bird I. and Nauset, MA (Aquatec, Inc., 1990, Nisbet et al. 1996, Hart 1998). Using toxic equivalency factor (TEF) method, PCB congeners 126, 77, and 105 were principal contributors to dioxin-like toxicity of PCB mixtures found in eggs from Green Bay; calculated equivalent concentrations of TCDD (TEQs) in range of 200–1,300 pg/g in 1988 (Jones et al. 1993), decreasing to <270 pg/g in 1996 (K. Stromborg). Congeners 126 and 77 not measured in eggs from Massachusetts, but based on an in vitro method (induction of ethoxyresorufin- O -deethylase [EROD] in tern or rat hepatocytes in culture), biological activity of residues in eggs from Massachusetts much less than that from Green Bay (means 35 pg TEQ/g at Nauset and 114 pg/g at Bird I. in 1995, vs. 104–187 pg/g at Green Bay in 1986 and 315–440 pg/g there in 1988; Tillitt et al. 1991, Jones et al. 1993, Hart 1998).

Many other organochlorines reported in eggs or tissues of Common Terns in North America (sources in Table 1). Contaminants reported at potentially toxic levels include dieldrin (1.1 ppm in eggs from Hamilton Harbour, Lake Ontario, in 1970) and mirex (0.71 ppm in eggs from Toronto, Lake Ontario, in 1981; Weseloh et al. 1989). Levels of all these contaminants decreased in parallel with those of DDE and PCBs; none were detected in eggs from Massachusetts or Maryland in 1997 (French et al. 2001).

Common Tern is one of the most sensitive bird species to embryotoxic effects of DDE. Low hatching success and productivity in Alberta and Great Lakes in 1968–1973 associated with defective eggshells and DDE contamination (see Breeding: eggs and hatching, above). Although DDE caused some eggshell thinning, main effects were disruption of eggshell structure and composition, reduced gas exchange, and egg breakage (Fox 1974, 1976). Severe effects observed with colony mean levels of DDE in eggs as low as 4.0–4.4 ppm (Fox 1976); some hatching failures at levels as low as 1.9 ppm (Nisbet and Reynolds 1984). These levels of contamination reported at many interior sites in 1970s and at some coastal sites as late as 1966–1972 (Table 1); levels already decreasing when Common Terns first studied in 1966–1973. Levels of DDE high enough to reduce hatching success persisted at some Great Lakes sites into 1980s (Weseloh et al. 1989, Hoffman et al. 1993), at Hamilton Harbour as late as 1992 (Weseloh et al. 1995).

Levels of PCBs in terns at same period were also sufficient to impair reproduction in many areas. Common Tern is the most sensitive wild species yet tested to dioxin-like toxic effects of PCBs (Bosveld and Van den Berg 1995, Lorenzen et al. 1997). Calculated and measured biological activity of extracts from eggs from Green Bay, WI, in 1988 (200–1,300 pg TEQ/g and 315–440 pg/g, using TEF and EROD methods, respectively; Jones et al. 1993) were well into embryotoxic range determined from experimental studies with PCB126 in Common Tern eggs (Hoffmann et al. 1998); calculated activity was still in embryotoxic range in 1996. As with DDE, exposure to PCBs was much higher and more widespread in 1960s–1970s. Best evidence for independent effect of PCBs was from Saginaw Bay, Lake Michigan, in 1984: hatching success lower and more abnormal embryos at sites with 10–11 ppm total PCBs in eggs than at control sites with 4–7 ppm PCBs but similar levels of DDE (Hoffman et al. 1993). Similar findings from Germany (Becker et al. 1993b). Congenital deformities more frequent in early 1970s, associated with PCB contamination (Hays and Risebrough 1972, Gochfeld 1975, Gilbertson et al. 1976). Field studies in 1990s (after decline in contamination) showed only minor effects or no effects associated with PCBs (Nisbet et al. 1996, Hart 1998, French et al. 2001). PCBs cause biochemical changes in Common Tern embryos at current field levels of exposure (Hoffman et al. 1993, Jones et al. 1993, Hart 1998).

In Europe, major population decline of Common Terns in The Netherlands attributed to lethal poisoning by cyclodienes (telodrin and dieldrin; Rooth and Jonkers 1972). No similar reports of poisoning in North America, although other fish-eating birds were killed by dieldrin in several areas until 1970s (Ohlendorf et al. 1981).

Metals and Selenium. Concentrations of mercury (Hg), lead (Pb), cadmium (Cd), chromium (Cr), other metals, and selenium (Se) in Common Tern feathers, livers, or eggs reported extensively. Available data compiled and reviewed by Gochfeld and Burger (1987), Burger (1993).

Elevated levels of Hg reported during 1970s and 1980s at many sites, including Lake Winnipeg in 1970s (Vermeer 1971), the Great Lakes (Vermeer and Peakall 1977, Heinz et al. 1985), and several parts of the Atlantic Coast (Fimreite et al. 1971, Burger and Gochfeld 1988b, Burger 1993). Extremely high levels reported in eggs and tissues from locations downstream of industrial sources in Ontario and Michigan in 1969–1971 were thought to be associated with reproductive failure (Fimreite et al. 1971, Dustman et al. 1972, Fimreite 1974), but this not supported by data from another site (Vermeer et al. 1973). Elevated levels of Hg in chicks suffering from premature feather loss in New York in 1970–1971 (Gochfeld 1980b). Highest level reported in feathers was 12 ppm at Bird I., MA, in 1991, much higher in feathers grown locally than in feathers grown in winter quarters (Burger et al. 1992).

Elevated levels of Pb, Cr, and/or Cd reported at several sites in New Jersey, New York, Rhode Island, and Massachusetts (Custer et al. 1986, Burger and Gochfeld 1988b, Burger et al. 1992). No clear evidence of adverse effects, but levels of Pb in feathers of adults from Cedar Beach, NY, in 1975–1982 and 1989–1992 were within range associated with adverse effects on behavior and growth in laboratory studies (Burger et al. 1994a). Biochemical changes associated with levels of Cr in chicks at a contaminated site at Providence, RI, but no detectable effect on survival (Custer et al. 1986). Levels of Cr and Se in feathers of adults increased with age, but those of Hg, Cd, and Pb did not (Burger et al. 1994b), probably because these metals are excreted in feathers at each molt (Braune 1987). Thus, molt appears to be important detoxification mechanism.

Oil. Oiling of plumage prevalent among wintering birds in Trinidad in 1980s; thought to have caused some mortality (Blokpoel et al. 1984), but lightly oiled birds showed no physiological effects (Erwin et al. 1986). Oiled birds infrequent on Long I., NY, in 1970s (Duffy 1977, Gochfeld 1979c).

Ingestion Of Plastics, Lead, Etc

Plastic particles found in gizzards of 2 of 46 birds collected at sea off N. Carolina (Moser and Lee 1992). Particles eliminated in pellets (Hays and Cormons 1973). No evidence of adverse effects. Adults and chicks occasionally killed by entanglement in monofilament line, kite strings, etc. (Nickell 1964, Gochfeld 1973).

Collisions with Stationary/Moving Structures Or Objects

Many fledglings killed by collisions with automobiles on roads adjoining 2 colonies on Long I., NY (Gochfeld 1976, 1978). No other reports from North America.

Fishing Nets

No information from North America. For association with human fishing in winter quarters, see Habitat and Food habits, above.

Displacement By Gulls

Between 1930s and 1980s, many colony sites on Atlantic Coast occupied by Herring and Great Black-backed Gulls (Drury 1973–1974, Kress et al. 1983); many colony sites in Great Lakes occupied by Ring-billed and Herring Gulls (Courtney and Blokpoel 1983). Common Terns were displaced to less suitable sites on or near mainland, hence subjected to higher levels of predation and other adverse factors (Kress et al. 1983). Although direct predation by gulls was probably minor factor in many areas (see Behavior: predation, above), this displacement was probably important contributor to population declines at this period. For recent attempts to restore former tern breeding sites by removing or displacing gulls, see Management, below.

Degradation Of Habitat

Human development, building, and recreational activity on barrier beaches and islands have made some formerly-used sites unavailable for breeding (Erwin 1980, Kress et al. 1983). However, recreational beaches and islands can be managed for multiple uses (e.g., Gochfeld 1978). A large colony at the Toronto Island Airport, Lake Ontario, was deliberately destroyed for safety reasons in the 1960s (Courtney and Blokpoel 1983). No evidence that marine habitats used for feeding have been degraded significantly, but no systematic studies reported.

Disturbance At Nest And Roost Sites

Human disturbance can prevent occupation of sites, promote desertion, or cause egg and chick losses due to chilling or overheating. Vandals occasionally destroy nests and eggs, or throw stones at mobbing birds. Despite many anecdotal accounts of such incidents, however, little scientific evidence that human disturbance causes substantial adverse effects on breeding birds in North America (Nisbet 2000). Staging and wintering birds roost at remote sites and may be susceptible to disturbance there (Hays et al. 1999, Trull et al. 1999).

Direct Human/Research Impacts

Readily habituates to predictable human activity and can become extremely tolerant of research activities, including close approach, trapping, blood sampling, and other intrusive procedures. With modest precautions, can be studied intensively without adverse effects (Nisbet 2000).

Conservation Status

Listed as Endangered, Threatened, or Special Concern in most U.S. states around Great Lakes and in several coastal states (Cuthbert and Timmerman 2001, S. Hall), but not in any Canadian provinces except Nova Scotia, where listed as Yellow (= sensitive; A. Boyne).

Measures Proposed And Taken

Many colony sites along U.S. Atlantic Coast and on Great Lakes are actively managed; some major sites in Atlantic provinces and Ontario are managed, but few elsewhere in Canada. Early measures included protection of remnant colonies from plume-hunters in 1890s (Dutcher 1901), translocation of chicks in 1920s (Jackson and Allan 1931), habitat management in 1930s (Austin 1934, Floyd 1937), and site restoration in 1960s (Heilbrun 1970). Management plans exist for this and other terns in Atlantic Canada (Can. Wildl. Serv. 1990) and in several U.S. states.

Protection of Existing Colony Sites. Many sites are protected by government agencies (federal, state, provincial, and local) and by private conservation organizations, landowners, and individuals (Kress et al. 1983, Cuthbert and Timmerman 2001). Degree of protection ranges from simple posting of advisory signs to complete exclusion of visitors, enforced by daily presence of wardens (e.g., at wildlife refuges or sites of intensive research). A few sites are managed for multiple use, with public areas separated from nesting areas by buffer zones and/or fences. At a few sites (e.g., Bird I., MA), visitors are permitted or encouraged to approach or pass through the nesting areas on fenced paths, or are guided by wardens. Some of these measures are augmented with public education programs, such as informative signs, notices at docks or marinas, leaflets, newspaper articles, etc. Blanchard (1994) pointed out importance of education programs in areas where conservation is not part of cultural attitudes.

Habitat Management. Vegetation is managed at many sites, usually to prevent overgrowth and maintain desirable mix of open substrate with scattered cover. Measures used at various sites include mechanical clearance, hand thinning, burning, tilling (plowing or harrowing), herbicides, rock salt, or periodic deposition of gravel or dredge spoil. At Great Gull I., NY, meadow voles (Microtus pennsylvanicus) were introduced to control vegetation (Hays 1984b). At other sites where vegetation is too sparse, grass may be planted, or wooden or rock shelters set out to provide cover. Chick shelters also used to reduce exposure of chicks to predators (Burness and Morris 1992). At a few sites, erosion is controlled using walls or breakwaters constructed from boulders (rip-rap). Fences or barriers are placed around some elevated artificial sites such as piers, barges, and navigation cells, to prevent chicks from falling into water.

Site Restoration, Gull Control. About 15 island sites between Nova Scotia and New York have recently been restored by removing or displacing Herring and Great Black-backed Gulls which had occupied the sites and displaced terns during preceding decades (e.g., Kress 1983, 1997; Harlow 1995). At several sites, nesting gulls were initially removed by poisoning hundreds or thousands of birds, followed up with shooting or egg-breaking in subsequent years (e.g., Kress 1983, Blodget and Henze 1992, Anderson and Devlin 1996). At other sites, attempts were made to displace gulls using harassment, nest destruction, and egg-breaking (Cuthbert and Timmerman 2001). Recently, trained dogs have been used to displace gulls from 3 sites (Seavey I., NH, Penikese and Muskeget Is., MA; D. De Luca, ICTN). In Great Lakes, Ring-billed Gulls are excluded from some sites using overhead grids of monofilament line, spaced to admit terns but not gulls (Blokpoel et al. 1997). Restored sites usually require annual follow-up and management action to prevent gulls from recolonizing (Kress 1997).

Predator Control. Predators have been controlled at several sites by shooting or trapping; these actions usually targeted at individual specialist predators observed taking eggs, chicks, fledglings, or adults. Black-crowned Night-Herons shot at sites in Maine, Massachusetts, New York, and Connecticut (e.g., Nisbet and Welton 1984). Herring and Great Black-backed Gulls shot at several sites (B. Blodget, S. Koch, ICTN). Great Horned and Short-eared Owls trapped and killed or translocated from sites in Maine and Massachusetts (Holt 1994, S. Kress, ICTN). A migrant Peregrine Falcon was trapped and translocated from Bird I., MA (Nisbet 1992). Striped skunks (Mephitis mephitis) and red foxes sometimes trapped and killed or translocated. Coyotes (Canis latrans) shot each year at Monomoy I., MA (S. Koch). Norway rats poisoned at several sites (e.g., Austin 1948). Ants controlled using baits at Lake Champlain, VT (LaBarr 1996). American Crows and Common Ravens (Corvus corax) displaced by nest destruction at Country I., Nova Scotia (A. Boyne). Mink removed from a site in Lake Superior (Cuthbert and Timmerman 2001). Electric or wire mesh fences used to exclude foxes, dogs, snakes, and other potential predators (Cuthbert and Timmerman 2001, S. Hecker).

Creation of Artificial Sites. Many artificial sites such as dredge spoil islands, confined disposal facilities, or derelict piers were created for other purposes, but became important sites for nesting terns and other birds in areas where natural sites are scarce or absent (Shugart and Scharf 1983, Parnell et al. 1986, Cuthbert and Timmerman 2001). Dredge spoil and other artificial islands can be designed, modified, or managed to make them more suitable for ground-nesting birds. Floating rafts designed for tern nesting have been deployed at several sites in the Great Lakes (Dunlop et al. 1991), and are in wide use on non-tidal waters in Europe (Becker and Sudmann 1998). A fixed platform was built in Boston Harbor, MA, to replace a derelict pier designated for removal (Hatch 2001). Platforms also placed in N. Carolina salt marshes to protect nests against predation and flooding (Dennis 1996). At new and restored sites, terns are often attracted using decoys or sound recordings to speed up recolonization (e.g., Kress 1983).

Other Management Actions. Regulatory actions to eliminate uses of DDT, PCBs, and other organochlorines, and to limit discharges of Hg, in North America were taken primarily for other reasons (although protection of fish-eating birds was one reason for ban on DDT); these resulted in major declines in exposure of Common Terns to these contaminants since early 1970s (Table 1). Management actions to phase out municipal landfills in the U.S., to regulate fisheries, and to reduce discards of fisheries wastes and by-catch, were similarly taken for other reasons, but have had important effects on populations of Herring and Great Black-backed Gulls.

Effectiveness Of Measures

All measures mentioned above have had some success in attracting Common Terns to suitable sites and/or in allowing them to breed successfully (e.g., Dutcher 1901; Austin 1934; Heilbrun 1970; Kress 1983, 1997; Hays 1984b; Dunlop et al. 1991; Harlow 1995; Blokpoel et al. 1997). Attempts to control gulls by non-lethal methods (harassment and nest destruction) were usually unsuccessful while gull populations were increasing in the 1960s and 1970s, but have had some success in 1990s, especially with use of dogs. By 2000, about 60% of the Common Terns nesting on U.S. Atlantic Coast were on managed sites, about 25% on recently restored sites (ICTN); about 25% of the birds nesting on Great Lakes and St. Lawrence River were on managed sites, about 80% on artificial sites (F. J. Cuthbert, L. Harper, C. Pekarik). Together, these management programs appear to have been successful in 1970s and 1980s, permitting steady increase in numbers on Atlantic Coast and at least halting decline in Great Lakes (see Demography and populations: population status, above). Main limitation of the programs is that managed and restored sites often are subject to predation, hence are not favorable places for terns without efficient predator control. These problems have recently become worse, as some predators have increased or spread into coastal regions (e.g., coyotes, Great Horned Owls) or have been promoted by human activity (e.g., rats, feral cats, striped skunks). Control of specialist predators is difficult, labor-intensive, and frequently controversial; not often carried out successfully. About half the restoration projects on the Atlantic Coast have failed for this reason, and the regional increase may have ceased during the 1990s. Management programs at many sites must be continued indefinitely if present numbers are even to be maintained.