Demography and Populations

Age At First Breeding; Intervals Between Breeding

No precise information on distribution of ages at first breeding based on intensive studies in small colonies or mark-recapture methodology. Based on age-distribution of birds trapped on nests in Cape Cod colonies, MA, a few (<10%) breed at age 2 yr; most breed at age 3; a few additional birds (<20%) first breed at age 4 (Austin and Austin 1956). Not known if any postpone breeding until after age 4 yr; records of breeding at age 1 yr (Austin and Austin 1956) require verification. At Wilhelmshaven, Germany, 76% of survivors first returned to natal colony at age 2 yr, 21% at age 3, 2% at age 4, and 4% at age 5, based on automated detection of implanted transponders (n = 129; Wendeln and Becker 1998). Birds aged 2 yr nest only at end of season (>30 d after peak); birds aged 3–4 yr nest progressively earlier; birds aged 5–6 yr nest a few days earlier than colony mean; laying dates continue to advance slightly until end of life span (Hays 1978; Nisbet et al. 1984, in press).

Most birds breed annually; no clear evidence that any skip breeding, at least after age 3. Proportion of birds at Bird I., MA, that skipped breeding in one year estimated as 0–7% (Nisbet and Cam 2002). Sex ratio of breeders thought to be close to 50:50, even amongst oldest age-classes (Nisbet and Cam 2002).

Clutch

Usually 2–3 eggs (Appendix 2). Colony/yr means vary from <2.1 to >2.9 (see Breeding: eggs, above).

Annual And Lifetime Reproductive Success

Colony/yr mean values for productivity vary from 0 (resulting from heavy predation or flooding) to >2.5 fledged chicks/pair (see Appendix 2 and Breeding: young birds, above). Productivity consistently higher in larger colonies, in part because birds tend to change sites after failures, accumulating at sites where predation and flooding are infrequent. On Atlantic Coast, reported mean productivities usually in range of 1.0–2.0 fledged chicks/pair at island sites from Québec to New York, but 0.5–1.0 fledged chicks/pair at beach and saltmarsh sites from New York to Virginia. At interior sites, reported values of productivity almost all below 1.0 fledged chicks/pair (Appendix 2). Especially low values (0.03–0.65 fledged chicks/pair) reported at several freshwater sites in 1969–1973 (Switzer et al. 1971, 1973; Fox 1976; Morris and Hunter 1976; Morris et al. 1976), associated with DDE contamination and low hatching success.

No information on lifetime reproductive success of individual birds. At Bird I., MA, median breeding lifetime about 7 yr (Nisbet and Cam 2002). Several measures of reproductive performance improve with age, rapidly from ages 3 to 5 yr, less rapidly thereafter until ages 10–15 yr (Hays 1978, Nisbet and Welton 1984, Nisbet et al. 1984, Galbraith et al. 1999). Productivity continues to increase until end of life span at ages 18–23 yr (Nisbet et al. in press).

Number Of Broods Normally Raised Per Season

One (see Breeding: phenology, above).

Proportion Of Females That Rear At Least One Young

No precise data, but usually very high in absence of predation or flooding.

Median age of breeders about 9–10 yr. At Bird I., MA, 95th percentile 18 yr; oldest recorded were 2 that bred successfully at age 26 (Nisbet and Cam 2002, ICTN). Only 1 estimate of survival based on rigorous mark-recapture methodology: annual adult survival rate at Bird and Ram Is., MA, 0.88 ± 0.04 SE or 0.91 ± 0.05 SE, depending on selection of sample (n = 2,367 or 1,599, respectively; Nisbet and Cam 2002). Other estimates of annual adult survival rate: 0.89–0.925 for Cape Cod region, MA, in 1940s–1950s (Nisbet 1978b, recalculated from data of Austin and Austin 1956); 0.92 at Great Gull I., NY, in 1970s (DiCostanzo 1980); 0.89 at Wilhelmshaven, Germany, in 1990s (Wendeln and Becker 1998). Last 2 estimates were of local survival (survival and return to same site) only. Adult survival rates in Cape Cod region thought to have been lower in early 1970s, associated with rapid population decline (Nisbet 1978b).

Estimates of survival to first breeding vary widely in scope and methodology. At Oneida Lake, NY, estimates of survival from hatching to age 3 ranged from 0.102 to 0.123 over 3 yr in 1980s; estimates of survival from hatching to age 4 ranged from 0.090 to 0.103 (Yuan 1993). These estimates derived using mark-recapture methodology, but based on small number of recaptures (n = 11–28/yr) and did not distinguish between pre-fledging and post-fledging survival. At colonies around Cape Cod, MA, survival from fledging to age 4 estimated as 0.06–0.10 in 1940s and 1950s (recalculated from multi-site retrapping data of Austin and Austin [1956]) and 0.07–0.13 in 1970s (calculated from multi-site retrapping, n = 39; Nisbet 1978b). At Great Gull I., NY, local survival from banding to age 4 in 1970s estimated as 0.143, based on one-site retrapping (n = 1,029; DiCostanzo 1980); this estimate also did not distinguish between pre-fledging and post-fledging survival. At Wilhelmshaven, Germany, local survival from fledging in 1990s estimated as 0.46 to age 2, 0.42 to age 3, 0.23 to age 4, 0.22 to age 5 (n = 26–81 returns/age-class; Wendeln and Becker 1998). No evidence that survival from fledging to first breeding depends on hatch order, hatching date, or parental age (Nisbet 1996).

Very little information from North America.

Diseases

Mortality of breeding birds at Eastern Egg Rock, ME, in Jun 1988 caused by avian cholera; only 33 birds found dead on island, but all birds deserted colony and only 37% recolonized later in season (Anon. 1984, Kress 1997). Antibodies to ornithosis virus detected in Common Terns from Texas (Pollard 1947). Clancey (1977) contracted ornithosis after handling dying birds in South Africa. Large die-off in South Africa in Apr–May 1961 traced to virulent strain of influenza (Becker 1967). Influenza also isolated from Common Terns in Russia, but not in North America (Stallknecht and Shane 1988, P. Luttrell).

In early 1970s, chicks with developmental defects of bill, eyes, legs, or wings noted at several sites (Hays and Risebrough 1972, Gochfeld 1975, Gilbertson et al. 1976). Some chicks also displayed premature feather loss—shedding of primaries shortly before fledging — at same period (Gochfeld 1971). Both conditions now rare (M. Gochfeld, G. A. Fox, ICTN); possibly associated with toxic chemicals (see Conservation and management, below). A few chicks develop subcutaneous emphysema, probably from infections following injury (Gochfeld 1974).

Biological Toxins

Paralytic shellfish poisoning (toxin of dinoflagellate Gonyaulax excavata) killed numbers of breeders in United Kingdom in 1968 and 1975, and at Monomoy I., MA, in Jun 1978. Last incident affected primarily egg-laying females (n = 70); other birds vomited and survived (Nisbet 1983c). Of 6 birds found dead on islets off Aruba, Netherlands Lesser Antilles, in May–Jun 1984, 2 tested positive for botulism (toxin of Clostridium botulinum Type C; van Halewyn 1985).

Body Parasites

Other than Mallophaga, both ecto- and endoparasites infrequent in adults and nests, at least in coastal colonies (Floyd 1928, C. Fiorello, ICTN). Saemundssonia sternae (Mallophaga) characteristic of Common Tern (Ward 1955). Schistocephalus solidus (Cestoda) frequent in birds in Finland, associated with feeding on sticklebacks (Lemmetyinen and Raitis 1972).

At breeding sites, main cause of adult mortality is predation by Peregrine Falcons, Great Horned Owls, or mink (see Behavior: predation, above). Reported incidents have been sporadic and only 3 (Floyd 1925, Austin 1940) involved >1% of total number of breeders. Main causes of chick mortality are predation, flooding, and starvation; occasionally bad weather (see Breeding: young birds, above). During premigratory staging, juveniles occasionally taken by Great Horned Owls or gulls (Nisbet 1976, Trull et al. 1999).

Although most mortality apparently occurs on migration or in winter quarters, no information on causes or manner of deaths, except for locally important trapping for human food (see Conservation and management: effects of human activity, below). For possible effects of oiling in Trinidad, see Conservation and management, below.

Initial Dispersal From Natal Site

Fidelity to natal site variable, but usually high. Most birds banded as chicks return to natal site at age 2 yr and many breed there at age 3 (Wendeln and Becker 1998). In Great Lakes, 75% of 61 breeding-season recoveries of birds banded as chicks were outside the 10’ × 10’ block containing the natal site, and 17% were in a different lake; mean recovery distance 61 km (Haymes and Blokpoel 1978). At Tern and Bird Is., MA, however, 82% and 76%, respectively, of birds banded as chicks were first found breeding at natal sites, although some of these later moved to other sites (Austin 1949). Natal fidelity lower at other sites around Cape Cod during same period, mainly because these sites became less suitable (Austin 1949). No other statistical data published on natal fidelity among Atlantic coast birds, despite many anecdotal accounts. At Bird I., MA, 7% of breeding birds in 1997 had been banded as chicks at other sites, ranging from New York (250 km WSW) to Nova Scotia (950 km ENE; Nisbet and Cam 2002).

Fidelity To Breeding Site

On occasions when colony sites are abandoned because of predation or other causes of breeding failure, breeding birds move en masse to other sites, sometimes settling synchronously in contiguous groups (group adherence; Austin 1951, Gochfeld 1979a). Otherwise, breeding site fidelity usually very high (e.g., DiCostanzo 1980, Wendeln and Becker 1998, Nisbet and Cam 2002). In Great Lakes, 16% of 61 breeding-season recoveries of birds banded as adults were outside the 10’ × 10’ block containing the breeding site; mean recovery distance 37 km (Haymes and Blokpoel 1978). At Tern I., MA, 98% of birds banded as adults and retrapped in succeeding years returned to same site, although some of these moved to other sites in subsequent years (Austin 1949). During colonization of Ram I., MA, 8% of birds breeding at Bird I., 10 km away, in 1995 moved to Ram I. in 1996; 4–8% of those breeding at Ram I. in 1995 moved back to Bird I. in 1996 (Nisbet and Cam 2002). No other statistical information on breeding site fidelity of Atlantic Coast birds.

For movements between Great Lakes and Atlantic Coast colonies, see Systematics: geographic variation, above. No information on fidelity to wintering sites.

Home Range

Breeding birds range commonly up to 10 km from colony, less frequently to ≥30 km (see Food habits: feeding, above). Individuals from colonies in Lakes Erie and Ontario tended to forage predictably in same directions from colony site, often at same locations (Moore 1993, Burness et al. 1994). Coastal birds often hold feeding territories (see Behavior: spacing, above). Otherwise, no evidence that individual birds return regularly to same feeding areas or locations. Following breeding season, birds travel widely through breeding area and probably range widely each day before returning to communal roosts (see Migration: timing and routes, and Behavior: self-maintenance, above). No information on home range in winter quarters.

Numbers

Total numbers breeding in North America probably on the order of 150,000 pairs. Most North American breeding areas censused during 1990s (Appendix 1). These censuses conducted in different ways and not necessarily comparable; most figures were peak-of-season nest counts, which typically include 80–90% of total number of pairs that nest during entire season (footnote 1 to Appendix 1). Summing these peak-of-season counts yields totals of about 90,000 pairs along Atlantic Coast (including about 25,000 pairs in Gulf of St. Lawrence), about 9,000 pairs in Great Lakes, about 30,000 pairs on Lake Winnipeg, about 6,000 pairs on other large lakes and rivers, a few dozen in Bermuda and n. Gulf Coast, and a few hundred in s. Caribbean. Little information available on numbers breeding on small lakes and rivers across the continent, but at least 650 such sites mapped in Breeding Bird Atlases and other state and provincial books, including 195 in Saskatchewan (Smith 1996), 117 in Alberta (Semenchuk 1992), 111 in Québec (Razurel 1996), 56 in Ontario (Blokpoel 1987), 38 in Maine (Adamus 1987); also widespread inland in Newfoundland, Labrador, and Manitoba, where no atlases available (Nero 1967, Koonz and Rakowski 1985, W. Koonz, P. Thomas). Most of these inland sites support small colonies of 1–20 pairs, but several colonies of hundreds reported in recent years (Guertin and Pfannmuller 1985, Hjertaas 1990).

Trends

Historical data summarized for Atlantic Coast by Nisbet (1973b), Drury (1973–1974), Kress et al. (1983), Buckley and Buckley (1984), Clapp and Buckley (1984); for Great Lakes by Courtney and Blokpoel (1983), Cuthbert and Timmerman (2001). No quantitative information prior to 1870. On Atlantic Coast, a credible record of hundreds of thousands nesting at Muskeget I., MA, in 1870 (Brewster 1879) and large but unspecified numbers at other sites. Hundreds of thousands were killed for the millinery trade in 1870s and 1880s; numbers breeding in the U.S. were reduced to a few thousands at <10 documented sites by early 1890s (Nisbet 1973b). Numbers increased under protection to peak in 1930s, when about 45,000 pairs in New England and 7,000 pairs in New York (Drury 1973–1974, Nisbet 1973b). Numbers in Canada poorly documented at that period, but a credible report of hundreds of thousands on Sable I., Nova Scotia, in 1901 and still large numbers in 1921 (A. R. Lock in Kress et al. 1983). Numbers then declined again to low point in mid-1970s, when only about 9,000 pairs in New England and 11,000 pairs in New York; numbers on Sable I. had dropped to about 1,300 pairs by 1971 and 340 pairs by 1982 (Kress et al. 1983). From mid-1970s until early 1990s, numbers in most areas of Atlantic Coast increased rapidly, reaching levels shown in Appendix 1 . During 1990s, numbers have continued to increase in Massachusetts and Maine, but have leveled off or declined in other areas (sources in Appendix 1).

In Great Lakes, numerical information for nineteenth century virtually non-existent. Numbers in Lower Great Lakes increased between about 1900 and 1960, peaked in 1960s, then declined to about 5,000 pairs by 1980 (Courtney and Blokpoel 1983). Courtney and Blokpoel estimated peak numbers in early 1960s as around 16,000 pairs, but overlooked records of up to 5,000 pairs in Ohio at that period (Peterjohn and Rice 1991). Numbers in Upper Great Lakes less well documented, but apparently a similar decrease, at least in Lake Michigan (Cuthbert and Timmerman 2001). Since about 1977, total numbers in Great Lakes have remained approximately stable at 9,000–10,000 pairs (Appendix 1; Cuthbert and Timmerman 2001).

Increased recently in St. Lawrence River and Lake Champlain, decreased slightly in Oneida Lake, and may have decreased in Lakes Winnipeg and Winnipegosis (sources in Appendix 1). Little or no information on trends in the hundreds of colonies on smaller lakes, but some sites that once contained hundreds or thousands of pairs now much smaller or deserted (Roberts 1936, Stewart 1975, Guertin and Pfannmuller 1985, Hjertaas 1990). Breeding Bird Survey (BBS) indicates a highly significant increase between 1966 and 1996 at about 2.5%/yr on survey routes in w. Canada, but a highly significant decrease at about 6.5%/yr in the Maritime Provinces. At least in w. Canada, these changes primarily reflect changes in numbers seen on inland routes, but BBS was not designed to sample colonial waterbirds with patchy distributions, so validity of BBS data for inferring regional population trends is uncertain.

At Bermuda, almost extirpated in late nineteenth century, but increased to about 30 pairs between 1950 and 1980; now approximately stable at about 20 pairs (D. B. Wingate). No information on current trends on Gulf Coast (Appendix 1), but former large colonies in Texas have disappeared and range may be contracting in other states (see Distribution: historical changes, above). In s. Caribbean, no evidence for major changes in numbers since 1960s (van Halewyn and Norton 1984, A. del Nevo).

For information on numbers and trends in Europe, see Glutz von Blotzheim and Bauer 1982, Cramp 1985, Lloyd et al. 1991 . For Russian Far East, see Kondratyev et al. 2000 .

No evidence that density-dependent factors regulate populations under present-day conditions. Numbers at most sites have changed greatly over short and long time-scales; populations in most regions are well below historical highs (see Population status, above). One local exception is Bird I., MA, where numbers reached plateau and breeding success declined in 1980s after rapid increase during 1970s (Appendix 2). Population models (e.g., Nisbet 1978b, DiCostanzo 1980, Wendeln and Becker 1998) remain speculative until better information is obtained on metapopulation dynamics.