Chlidonias niger

Total length of adults 23-26.5 cm (9-10.5 inches). In alternate (breeding) plumage, head and body are black, fading to gray on the rump. Undertail coverts are white. Upper surface of wings and tail are dark gray, and wing linings are pale gray. The leading margin of the wing from the body to the first digit is white. Bill is black and feet are a dark reddish-purple (Goodwin 1960, Farrand 1983). Females are somewhat duller black than males, but this difference is often difficult to distinguish in the field (Goodwin 1960).

Prebasic (postbreeding) molt begins in late June when eggs begin to hatch. White feathers appear first around eyes and cheeks, then on forehead, neck, throat and breast, and finally on abdomen. Heavily molting adults take on a peculiar, piebald appearance. The prebasic molt is completed during fall migration (Goodwin 1960).

In basic (winter) plumage, underparts are pure white except for a small, dark patch on each side of the breast. The back becomes a shade of gray similar to the wings and tail. A blackish cap joins black ear coverts on the otherwise white head (Goodwin 1960, Farrand 1983). The juvenile plumage is similar to the basic plumage, but the feathers of the back are darker and the wing coverts and cap are barred and scalloped brown (Goodwin 1960, Farrand 1983).

VOCALIZATIONS: shrill, somewhat metallic alarm notes, described as "kik" or "keek", depending upon intensity and level of motivation, and a complex of contact calls described as "kyew", followed by one to four additional syllables, as "kyew-dik", "kyew-dik-ik", etc. (Goodwin 1960). The "kik" call commonly serves as a signal of impending danger in the nesting area. It may also be given during the ascent portion of the courtship flight. The "keek" call is similar to, but more shrill and forceful than the "kik" call, and is given during aggressive attacks on enemies in close proximity to the nest. The frequency of repetition increases as the terns become more aggressive. The "kyew" calls are given as parents approach and leave the nest, during foraging flights, by adults accompanied in flight by young, by parents calling to young at or near the nest, by parents at the nest during incubation, brooding and feeding, and during the courtship flights (Goodwin 1960).

EGGS: ovate with a tendency toward ovate pyriform (Bent 1921). Ground color varies from dark olive to light buff with markings of dark brown and gray. Markings vary from small dots and scrawls to very large blotches and are often particularly heavy around the larger end of the egg (Goodwin 1960). The average dimensions for 122 eggs in the U.S. National Museum were 34 x 24 mm (Bent 1921).

BREEDING: marshes, along sloughs, rivers, lakeshores, and impoundments, or in wet meadows, typically in sites with mixture of emergent vegetation and open water. Cattails, bulrushes, burreed, and/or phragmites commonly are present in nesting areas (Bent 1921, Cuthbert 1954, Goodwin 1960, Bailey 1977, Firstencel 1987, Novak 1990). Nested in greatest numbers where emergent vegetation and open water are in an approximately 50:50 ratio (Weller and Spatcher 1965). In Wisconsin, Tilghman (1979) found nests in areas where emergent marsh coverage was 51-75%. In British Columbia, nests occurred in areas with 33% open water, 42% matted vegetation, and 25% standing vegetation (Chapman-Mosher 1987).

Nests may be placed in a variety of vegetative situations, from dense stands of emergent vegetation to open water (Bergman et al. 1970, Novak 1990), but moderate or sparse vegetation appears to be preferred (Cuthbert 1954, Weller and Spatcher 1965, Dunn 1979). Nests are typically located in shallow water, close to open water or openings in stands of emergent vegetation. The range of water depths reported varies from a "few inches" (Bent 1921) to 1-2 m (Dunn 1979). Bailey (1977) found that nests were never more than 1-2 m from open water. Dunn (1979) and Cuthbert (1954) reported the average distance to open water as 4 m and 4.6-6.1 m, respectively. One site in New York where 9 nests averaged 25.3 m from open water (Novak 1990) appears to be the exception, although Firstencel (1987) also reported nests located "deep within the cattails".

Nests on heap of floating vegetation, on old muskrat house, old grebe or coot nest, or on floating wood (Cuthbert 1954, Bergman et al. 1970, Bailey 1977, Dunn 1979, Novak 1990). Floating mats of muck or algae, mud flats, and mud mounds and islands also have been used as nest substrates (Cuthbert 1954, Bailey 1977, Dunn 1979, Connell and Norman 1989, Novak 1990). The nest consists of a small gathering of aquatic vegetation with a simple, cup-like bowl (Weller and Spatcher 1965, Bailey 1977). Although the first egg may be laid before the nest takes shape, vegetation gathered at the nest site is added throughout the incubation period (Baggerman et al. 1956, Goodwin 1960, Bailey 1977). Will nest on artificial nesting platforms (Muller et al. 1992). The height of eggs above water has often been measured. Although the height may vary based upon substrate chosen for nesting, the eggs are rarely located more than a few centimeters above the water level. The range reported in the literature varies from an average of 2.3 cm for 23 nests on dead floating vegetation in Iowa (Bergman et al. 1970), to 20.0 cm for two nests located on mud islands in New York (Firstencel 1987). The latter is clearly the exception as 8.6 cm for seven nests on old muskrat houses is the next highest figure reported (Weller and Spatcher 1965).

Exposed perches, such as channel marker posts, floating logs, fallen trees, and old dock or fence posts are used as stations for feeding recently fledged young, resting, and copulation (Cuthbert 1954, Novak 1990).

NON-BREEDING: pelagic waters as well as seacoasts, bays, estuaries, lagoons, lakes, reservoirs, and rivers (Eisenmann 1951, Zaret and Paine 1973, van Halewijn 1973, Spaans 1978, AOU 1983, Williams 1983); prefers sheltered offshore waters and bays, comes to shore chiefly during migrations (Stiles and Skutch 1989).

Gregarious throughout the year. Has been described as a semi-colonial nesting species (Cuthbert 1954, Bergman et al. 1970). Nests may be clumped closely in favorable habitat or more widely scattered in other, perhaps less favorable areas. As is typical of colonial nesting gulls and terns, terns will join together to defend the nesting area from intruders (Cuthbert 1954).

Ectoparasites include feather mites and lice (Peters 1936, Perez and Atyeo 1984). A trematode, APORCHIS LARUS, was recorded in Russia (Mirzoeva 1980). The effects of these parasites have not been studied.

Black terns are susceptible to avian botulism. A few dead birds have been found in Nevada and Manitoba (Alcorn 1942, Manuwal 1967), but no major die-offs from this disease have been reported.

Commonly returns to previous nesting area but also commonly changes sites if conditions become unfavorable. Return rates may vary considerably among specific sites. Stern et al. (1985) found that 67% of recaptured terns nested within the same primary wetlands, while Bailey (1977) and Dunn (1979) reported return rates of 40% and 27% for sites in Wisconsin and Ontario, respectively. These return rates, which are low in comparison with other gulls and terns, may be the result of the relative instability of preferred habitat (McNicholl 1975).

Most terns in the northeastern United States and Canada return to breeding areas during the first two weeks of May, although birds may arrive in western New York as early as the last week of April (Laughlin and Kibbe 1985, Firstencel 1987, Gerson 1987). Conspicuous aerial courtship displays characterize the courtship period, which begins soon after arrival at the breeding site. In the "high-flight", a group of 2-20 terns ascend together to a great height then split into smaller groups of two or three and descend in rapid glides (Baggerman et al. 1956). During the "fish-flight", a male tern carries a small fish or large insect in its bill and is closely followed by a female as the two fly about the marsh. At the close of this aerial display the male follows the female to a perch and feeds her (Baggerman et al. 1956).

In the northeastern United States egg laying begins in late May, but may be initiated as late as the middle of July. Nests with eggs were observed at one site in western New York from 24 May to 12 July (Firstencel 1987). During a 1989 survey of colony sites throughout New York, nests with eggs were observed as early as 25 May and as late as 18 July (Novak 1990). Not known to be double brooded and late nests probably represent renesting attempts. At Rush Lake in Wisconsin, Bailey (1977) observed a trend toward three nesting peaks, one in late May, one in early to mid June, and one in late July. This pattern was attributed to two initial nesting periods characterized by a high degree of synchrony, followed by a period of renesting (Bailey 1977). Baggerman et al. (1956) also reported highly synchronous nesting activity.

One to five eggs may be laid, although the normal clutch is usually two or three (Bent 1921). Clutches with four eggs have been reported in only two recent studies (Bergman et al. 1970, Mossman 1980) and are apparently quite rare. Other than Bent (1921), there are no published reports of five egg clutches. Single egg clutches may often be replacement nests or nests where one or more eggs have already been lost (Bent 1921, Cuthbert 1954, Firstencel 1987). In a recent study in Wisconsin, the average clutch size for 41 closely monitored nests was 2.9 (Bailey 1977). Four nests had clutches of two, but no nests contained less than two or more than three eggs. Average clutch sizes reported in other recent studies where nests may not have been monitored as carefully, range from 2.25 to 2.75 (Cuthbert 1954, Goodwin 1960, Bergman et al. 1970, Mossman 1981, Firstencel 1987, Novak 1990). Incubation begins with the laying of the first egg, and eggs require 20-24 days to hatch (Goodwin 1960, Bergman et al. 1970, Bailey 1977). Both sexes incubate (Goodwin 1960).

Young are tended by both parents. Chicks are able to swim, walk and run by the time they are two days old (Goodwin 1960). The chicks grow rapidly, doubling their weight in less than three days and quadrupling their weight in less than six days (Bailey 1977). The rate of weight gain slows after the eighth day. In some cases, chicks may be relocated from the nest site to "auxiliary" nests within a few days after hatching (Cuthbert 1954, Firstencel 1987). If disturbance at the nest is minimal, young may remain at the original nest site for as long as 14-25 days, although they hide in the vegetation at the sign of danger and may be found swimming as far as 40 ft from the nest (Cuthbert 1954, Goodwin 1960). The age at fledging is difficult to determine. Bailey (1977) reported fledging at 18 and 19 days for two chicks of known age and suggested that the majority of chicks are flying at 21 days of age with a mean fledging age possibly less than 20 days. Baggerman et al. (1956) and Goodwin (1960) reported fledging at 21 days. Young are fully fledged at about four weeks.

Estimates of nest success (expressed as a percentage of nests where at least one egg was hatched successfully) from four nest studies are as follows: 27% (15 of 55 nests) in Ontario, 29% (56 of 192 nests) in Iowa, 34% (13 of 38 nests) in Wisconsin, and 50% (12 of 24 nests) in New York (Dunn 1979, Bergman et al. 1970, Bailey 1977, Firstencel 1987, respectively). There was no obvious correlation between nest success, height of eggs above water, and number of nests per substrate in the Iowa study (Bergman et al. 1970).

Survival of young to fledging is difficult to measure because of the mobility of chicks. Bailey (1977) attempted to measure chick survival by placing fencing around nests to prevent young from moving away from the nest site. Just three of 26 (12%) chicks monitored fledged successfully. Sixteen chicks were lost to predation and several chicks died in the pen netting. Fledging success at unfenced nests (perhaps a better representation of fledging success) was estimated at 15-20% (Bailey 1977). Recent surveys have presented estimates of reproductive success based on the number of fledglings produced per egg laid in the colony (Rabenold 1987, Novak 1990). Estimates for three small (less than 10 pairs) colonies in Indiana were 0%, 53%, and 67%, for an overall average of 30% (Rabenold 1987). Estimates also varied widely for 19 sites in New York, from 4% to 38%, with an overall average of 20% (Novak 1990). Mossman (1980) reported a 25% reproductive success based on the ratio of young: adults observed at one study area in Wisconsin.

The similarity between reproductive or fledging success rates and nest success (hatching success) supports the observation by Dunn (1979) that most losses occur during the egg stage. Wind and wave action, and storms were responsible for most nest losses in several studies (Bergman et al. 1970, Bailey 1977, Dunn 1979, Faber and Nosek 1985, Chapman-Mosher 1987). Nest losses have also been attributed to egg inviability, predation, muskrat activity, and intraspecific interactions (Bergman et al. 1970, Bailey 1977, Dunn 1979, Firstencel 1987).

Has been described as a semi-colonial nesting species (Cuthbert 1954, Bergman et al. 1970). Nests may be clumped closely in favorable habitat or more widely scattered in other, perhaps less favorable areas. As is typical of colonial nesting gulls and terns, terns will join together to defend the nesting area from intruders (Cuthbert 1954).