Demography and Populations

Age At First Breeding; Intervals Between Breeding

Most Least Terns begin breeding in their third year (Massey and Atwood 1981). Two-year-old birds have nested in California, Massachusetts, and interior U.S. (Massey and Atwood 1981, Boyd 1993, G. R. Lingle unpubl. data, EMK, JLA). On central Platte River, NE, one-third of terns banded as chicks that return to breed in that area return when they are 2 yr old (G. R. Lingle unpubl. data). Adults typically attempt to breed every year once they have started nesting, as shown by color-banded California terns nesting in 8 consecutive years (B. Massey pers. comm.). In years of high water, terns breeding on rivers may not nest until flows recede (e.g., Yellowstone River, MT, in 1995).

Clutch

Typically 2–3 eggs; usually 2 for individuals nesting along marine coasts (Massey 1974, Roche 1979, Massey and Atwood 1981, Corbet 1990) and 3 in interior freshwater portion of range (Kirsch 1996, G. R. Lingle pers. comm.). One-egg clutches are common, especially later in season, and may be renesting attempts or nests of first-time breeders (Massey and Atwood 1981). In Netherlands Antilles and Missouri, clutches of up to 4 eggs reported (Voous 1963, R. Renkin pers. comm.). Clutch size related to variation in food supply (Massey et al. 1992).

Readily renests at same site or up to 70 km from original nesting site if eggs or chicks are lost early in breeding season (Massey and Atwood 1981, Massey and Fancher 1989, Boyd 1993, Lingle 1993b). Can renest at least 3 times during season; interval between egg loss and re-laying is 4–16 d in California (Massey and Fancher 1989) and 5–30 d on central Platte River, NE (Lingle 1993b).

Annual And Lifetime Reproductive Success

Only 1 successful brood/season. Annual reproductive success can vary greatly among colonies and among years (Burger 1984, Corbet 1990, Gore and Kinnison 1991, Kirsch 1996). Annual production estimated as 0.59 fledgling/breeding adult on Gulf Coast during 1979–1981 (Thompson 1982), 0.48 fledgling/pair in New Jersey during 1975–1982 (Burger 1984), and 0.47 fledgling/pair on lower Platte River during 1987–1990 (Kirsch 1996), comparable to regional production of interior U.S. during 1982–1992 (LAH). Proportion of females that rear 1 brood/season has not been estimated. In most breeding areas, probably insignificant numbers of nonbreeders present and number of fledglings/pair is the same as number of fledglings/breeding female. Limited estimation of lifetime reproductive success. In California, lifetime productivity estimated as 1.49 fledglings/adult on basis of calculated breeding life of 9.6 yr (Massey et al. 1992). Substantial variation in estimated productivity relates to annual variation in colony outcomes, difficulty in locating young that fledge, and uncertainty about contribution of renesting.

Record longevity is 24 yr 1 mo for a Least Tern banded in Massachusetts and recovered in New Jersey (Klimkiewicz and Futcher 1989). Least Terns banded in New Jersey as chicks recovered at 20 and 22 yr (Burger 1989). A 20-yr-old Least Tern nested successfully (JLA). Five California Least Terns recovered were up to 15 yr old (Massey 1973).

Reliable estimates of annual adult survival obtained only from band recoveries for California population (0.88; Massey et al. 1992) and Least Terns that nest along Mississippi River adjacent to Missouri (0.85; Renken and Smith 1995a). Annual adult survival estimated from leg band resightings at 1 California colony over 6 yr was 0.84–0.94 (Collins et al. 1996). Survival of terns from fledging to 2–3 yr estimated in California as 0.80–0.82, progressively higher for 4- to 9-yr-old terns with 0.87–0.93 survival, respectively (Massey et al. 1992).

Autopsies of Least Tern adults killed during hailstorm in Kansas revealed no parasites or indications of illness (Schweitzer 1994). Mallophagan ectoparasites have been collected (C. Collins unpubl. data). Jackson and Jackson (1985) found dead and dying chicks in Mississippi, on which autopsies revealed necrotizing hepatitis, poxlike lesions, and infection with a paramyxovirus. Mallophaga and salmonella infection have been reported in similar Little Tern (Nadler 1976).

Exposure

Eggs and chicks frequently are killed by storm tide flooding in coastal areas, flooding of river sandbars, and sheet flooding of salt flats or mudflats. On sloping dredge piles, eggs have been swept out of nests during heavy rains (BCT, JAJ, JB, LAH, EMK, and JLA). Eggs can die when exposed to cold or heat when adults are kept away from nests by human intruders, predators, or human recreation near nests. Eggs sometimes buried by drifting sand from strong winds. Chicks, eggs, and some adults have been killed by hailstorms in the Midwest (Schulenberg and Ptacek 1984, Boyd 1992, Schweitzer 1994, EMK). Chicks also succumb during prolonged very hot (Overstreet and Rehak 1982, Hill 1985) or cool, wet weather (Kirsch 1996). In coastal Mississippi, Jackson (1976) found 21 chicks with mortal wounds from sandspurs (Cenchrus sp.). A massive die-off of chicks in Mississippi in part of 1 colony appeared to have resulted from heat stress (Overstreet and Rehak 1982), but brain cholinesterase levels suggested organophosphate poisoning (Jackson and Jackson 1985); the colony area with high mortality was directly across the street from a golf course that used pesticides. Dead chicks have been found covered with ants in e. U.S. and California, and there is some concern that fire ants (Solenopsis spp.) may kill chicks (Hays 1980), but ants may be a secondary factor after a disturbance has left chicks unattended.

Predation

Susceptible to wide variety of avian and terrestrial predators reported to kill eggs, chicks, and adults (see Behavior: predation, above). Mammalian predation may be higher at sites closer to vegetation (Koenen et al. 1996b).

Competition With Other Species

Interspecific competition, especially as it affects survival, has not been investigated in detail. In Massachusetts, Common Terns regularly steal fish from Least Terns flying to colonies (JLA).

Initial Dispersal From Natal Site

Young of year move from natal colony within few weeks of fledging.

Fidelity To Breeding Site And Winter Home Range

Degree and spatial scale of breeding-site fidelity differs for terns from different populations and breeding areas. Return rate of terns banded as adults to site where banded was 59% of 438 adults (36–86%, range for annual and intercolony fidelity) in 3 s. California colonies (Atwood and Massey 1988), 42% of 78 adults along Mississippi River (Renken and Smith 1995b), 28% of 109 adults on central Platte River, NE (Lingle 1993b), and 81% of 146 adults at 8 colonies at Quivira National Wildlife Refuge in Kansas and on the Cimarron River in nw. Oklahoma (Boyd 1993). Renken and Smith (1995b) observed that 97% of 78 terns they banded as adults and subsequently resighted returned within 1.5–80 km (median 9.1 km) of colony where banded. For 64 birds that changed colony sites, 77% moved <15 km, and 15% moved to nearest available colony site (Atwood and Massey 1988). Lingle (1993b) reported 1 tern banded as an adult on central Platte River in 1988 was recaptured breeding on the lower Platte in 1989, and after successfully hatching its eggs was seen about 160 km away on central Platte later in 1989.

Fidelity to natal site is difficult to estimate because resightings or recaptures of terns banded as chicks are limited (0.5–1%: Atwood and Massey 1988, Boyd 1993, Lingle 1993b, Renken and Smith 1995b). For banded chicks that return to breed in a study area, estimates of natal-site fidelity vary: 5% (n = 19) on Mississippi River (Renken and Smith 1995b), 16% (estimated from 1,940 chicks banded) in California (Massey et al. 1992), 26% (n = 222) on central Platte River, NE (Lingle 1993b), and 82% (n = 11) in Kansas and nw. Oklahoma (Boyd 1993). Terns banded as chicks returned to natal colonies more frequently than predicted if they settled at colonies randomly (Atwood and Massey 1988). Renken and Smith (1995b) reported that all but 1 of the 19 terns banded as chicks and resighted as adults were found 4.1–72.7 km from their natal colony. Least Terns have been recaptured at or near their natal colony as much as 21 yr later (Massey 1976, Clapp et al. 1982).

Some terns banded as chicks were resighted breeding for the first time far from their natal area. One chick banded on Gulf Coast of Texas later nested in Kansas (Boyd and Thompson 1985). Chicks banded on central Platte River in Nebraska nested in central Kansas and vice versa (Boyd 1993, Lingle 1993b). A chick banded on Missouri River in S. Dakota was recaptured as an adult on lower Platte River in Nebraska. An adult banded in Missouri later nested in Indiana (Johnson and Castrale 1993).

Dispersal From Breeding Site Or Colony

Wanders widely after breeding season, especially in interior U.S., before migrating southward. An adult and 2 fledglings of unknown origin were reported near Minneapolis, MN, in late summer 1992 (Bardon 1992), at least 400 km from nearest known breeding area (Missouri River in S. Dakota and Nebraska), and about 1,400 river-km upstream from closest Mississippi River colony that year. On coast, generally does not wander northward of the most northerly colonies.

Home Range

Limited radiotelemetry and observation studies of foraging birds suggest that they travel no farther than 3–12 km from colony sites (Atwood and Minsky 1983, Lingle 1993b, K. Dugger pers. comm.). Adults with older chicks (>7 d) have been observed leaving colony site at dusk and returning at dawn (EMK). Usually forage close to nesting area, but opportunistic. Foraging concentrated within 100–300 m of riverine nesting sites (Hill 1993b, Wilson et al. 1993), but may forage up to 4.5 km away (Hill 1993b). In response to localized abundance of suitable fish, terns foraged up to 3 km from coastal colonies and 12 km from inland salt flat colonies, but 75% of foraging in s. California occurred within 1.2 km of nesting area (Atwood and Minsky 1983, Schweitzer 1994). Nesting Least Terns foraged up to 4.9 km away when carrying food to a colony in Georgia (Tomkins 1959) and 4.5 km from rooftop nest sites in Mississippi (Jackson 1994). How far from the colony site these birds roosted overnight is unknown. Daily or seasonal range at winter areas is unknown.

Numbers

Species difficult to count accurately. Shifts colony sites between and sometimes within years. Timing of nesting varies locally because of weather, habitat availability (e.g., seasonal duration and timing of flooding of sandbar habitats or tidal flooding on marine coasts), and latitude. Often nests in small colonies that are difficult to detect because of bird’s small size and tendency to space nests far apart.

Although highly variable in procedure and extent, counts during 1980s and 1990s indicate at least 55,000 Least Terns at U.S. breeding locations (Table 1). From Maine through Virginia, numbers have remained relatively stable, with yearly fluctuations (Engstrom et al. 1990). A comprehensive survey of colonial waterbirds conducted in 1982 (Spendelow and Patton 1988) revealed about 42,500 Least Terns nesting from Maine to Texas. No information on numbers at wintering locations.

Trends

Current distribution probably reflects historic distribution. Species most abundant along Gulf and Atlantic Coasts, but nevertheless classified Threatened, Endangered, or species of concern for most states because of loss of nesting habitat. Historic data are sparse and probably do not represent actual population sizes. Concern for habitat conditions and reduced populations led to Endangered status for California populations in 1970 and interior populations of U.S. (>50 km from coast) in 1985. Subsequent evaluation indicated that probable underestimates of interior populations had resulted from incomplete surveys (Downing 1980, USFWS 1985, Sidle et al. 1988).

In California, increased from approximately 600 pairs in 1973 to 2,750 pairs in 1994 (California Fish and Game annual resources reports). Number of colonies in California, however, has remained relatively stable at 25–30 sites from 1978 to 1994. Classified Endangered in Mexico; population there difficult to estimate, but at least 400 pairs in the Gulf of California in 1992–1994 (Mellink and Palacios 1993, Palacios and Mellink 1993, 1996, Carmona et al. 1994).

Interior population surveyed 1984 to present; federally listed as Endangered in 1985. Survey effort and coverage increased during 1984–1986, yet because colonies are ephemeral, riverine nesting habitat is remote, and salt flat nesting habitat is vast, consistent timing and coverage of surveys have not been logistically possible. Colonies in interior are typically small: 5–20 nests, rarely >50 nests. Best information available suggests that interior population has increased during 1986–1991 from approximately 4,125 to 6,830 (EMK and J. Sidle unpubl. data). Tern numbers increased 100% along lower Mississippi River (Cape Girardeau, MO, to Vicksburg, MS) between 1989 and 1990 (from 2,503 to 5,038 birds), which cannot be accounted for by increased survey effort or change in survey method. This area of the lower Missis-sippi supports 52–65% of all known nestinginterior Least Terns.

Species increasingly nesting on rooftops (especially from N. Carolina southward), and dredged-material islands (Fisk 1978, Parnell et al. 1986, Hovis and Robson 1989, Gore 1991, M. Harris pers. comm., M. Dodd pers. comm.), but productivity is variable relative to natural sites (Robertson and Woolfenden 1992).

Few quantitative data; needs study. Numerous factors combine to alter age structure in a breeding colony, as well as colony size and its future population trajectory (Massey et al. 1992). The importance of age-specific survival rates and the effect of renesting on demographic models was discussed by Thompson (1982).

Colony size and repeated use of nesting areas appear limited in part by predators or human disturbance at natural nesting sites (Jackson and Jackson 1985). Where these factors are eliminated or controlled, nesting pairs sometimes increase. Where vegetation is controlled or naturally reduced by winter storms, same site may be used many years. Natural flooding limits populations by destroying eggs and chicks, but creates suitable nesting habitat by limiting vegetation and depositing new substrate (Sidle et al. 1992).

In California, periodic El Niño–Southern Oscillation (ENSO) events have caused major nesting failure, especially the 1982–1983 ENSO event (Massey et al. 1992). Collectively, effects of these factors (plus human impacts; see Conservation and management, below) on populations are unknown and must be balanced with subtle features of breeding-site interference caused by major landscape changes such as water diversion.