Breeding

Pair Formation

Pairs form after arriving in nesting area; pair formation intimately associated with Fish Flight Display which culminates in copulation (see Behavior: sexual behavior, above). Courtship period is usually 2–3 wk in Apr and May, although second wave of courtship may occur in Jun and early Jul (Wolk 1974, Massey and Atwood 1981). Time frame varies geographically: early Apr on Gulf Coast, late Apr in California, early May on n. Atlantic Coast, late Apr–late May in interior. Arrival and courtship at interior sites generally is later because river water levels are generally high until summer (Hardy 1957).

First/Only Brood Per Season

See Figure 3 . Initiation of first nests seems socially facilitated and is fairly synchronous within a colony; nests started in Massachusetts after mean date of nest initiation are less synchronous (JLA). In coastal areas, first clutches are laid 2–3 wk after arrival on breeding grounds; in riverine areas, nesting often delayed until late Jun–early Jul, when water levels have receded and sandbar nesting habitat is exposed (Tennessee [Ganier 1930]; Iowa [Stiles 1938, 1939]; interior generally [Sidle et al. 1992]). First eggs appear about 20 Apr on Texas coast, 15 May in California, 20 May in Massachusetts, and 25 May in Nebraska; hatching begins about 20 d later (Massey 1974, Thompson 1982, Veit and Petersen 1993, Kirsch 1996).

Second/Later Brood(S) Per Season, If Any

No reliable record of >1 successful broods/season, although Bent (1921) inaccurately reported up to 3 broods/season. Renesting commonly follows nest or chick loss; up to 3 nests/season. Least Terns display second wave of nesting, involving birds that have lost clutches in the same colonies, renesting birds from other colonies, or younger birds (2–3 yr old) that are breeding for first time (Massey and Atwood 1981). These late-nesting birds usually insert themselves within existing colony, selecting sites used by earlier pairs or areas with little or no prior use (Massey and Atwood 1981).

Selection Process

Often makes several scrapes, only 1 of which becomes a nest. Both sexes appear to make scrapes; female selects scrape that will ultimately become nest.

Microhabitat

Nests may be closer to nests of other species (e.g., Piping Plover, Black Skimmer) than expected from random points (5–36 m versus 42 m) (Burger 1987). Least Terns nested within 2 m of a Snowy Plover nest but >6 m from Black Skimmer nest in coastal Mississippi (JAJ). Usually nests on ridges or other slightly elevated spots on sand (Burger and Gochfeld 1990a). Nesting substrate that has excessive silt or clay content can cause eggs to become stuck during wet weather, leading to embryo death when adult is unable to turn eggs properly (Thompson and Slack 1982, Palacios and Mellink 1996). Nest areas commonly have shell, gravel, or other fragmentary material in the substrate.

Site Characteristics

Generally needs open area largely free of vegetation, above high water levels, and safe from ground predators; thus islands commonly favored where available. Sand is the dominant substrate (Massey 1974, Thompson and Slack 1982, Gochfeld 1983, Burger and Gochfeld 1990a, MacLean et al. 1991). Sandy areas with sparse vegetation, mudflats, graveled rooftops and parking lots, and dredged-material deposits also are used. Preferred vegetation cover is 5–10% in New York (Gochfeld 1983), 7–18% in Nebraska (Faanes 1983), 0.2–5% in California (Minsky 1987). On Caribbean island of Bonaire, nests have been placed atop deserted flamingo (Phoenicopterus ruber) nests (Voous 1963).

Construction Process

Nest is shallow scrape in sand, soil, or pebbles, to which bits of shell, light-colored pebbles, or small bits of wood or grass stem are occasionally added after incubation has begun. Makes scrape by sitting on substrate and kicking feet backward while rotating body, using breast to form shallow depression made by feet.

Structure And Composition Matter

After incu-bation begins, incubating bird often picks up white or light-colored bits of shell, stones, or dried plant material and pulls them to nest in what has been referred to as sideways building (Wolk 1974), a possible displacement behavior. Least Terns make scrapes in some gravel, but in larger gravel (2–3 cm diameter), such as at some rooftop colonies, there are no scrapes or nests.

Dimensions

Shallow, round depression; inside depth about 2 cm; diameter 7–10 cm depending on fragment placement around edge.

Microclimate

Usually exposed, frequently near debris or vegetation; no specific orientation.

Maintenance Or Reuse Of Nests

Little evidence for reuse of nests, likely because of simple construction and changing substrate among years. Renesting birds rarely reuse same scrape after earlier failure.

Shape

Oval or subelliptical, with distinct smaller end; typically uniformly rounded on both ends.

Size

For 122 California eggs (range and mean ± 2 SE): length 27.5–35.7 mm (30.5 ± 0.2), width 20.7–25.3 mm (23.1 ± 0.2) (Massey 1974). For 202 Massachusetts eggs (range and mean ± SD): length 23.6–35.6 mm (30.9 ± 1.2), width 21.1–24.7 mm (23.0 ± 0.6) (JLA). For 59 eggs in Oklahoma: length 30.5 mm ± 1.0 SD, width 23.0 mm ± 0.5 SD (Hill 1985). Average of 63 eggs reported as 31 mm x 23.5 mm (Bent 1921).

Mass

5.5–10 g (8.1 ± 0.1 [2 SE], n = 122; Massey 1974).

Color

Ground color beige to light olive brown; spots or splotches medium brown to black. Ground and spot color, as well as number, size, and distribution of spots, vary greatly, even within a clutch.

Surface Texture

Smooth and dull.

Eggshell Thickness

Shells of 115 eggs collected pre-1947 were 0.152–0.156 mm thick; 127 eggs collected after 1974 were 0.149–0.160 mm thick (Koenen and Leslie 1996). Eggshell thickness in New Jersey did not change from early 1970s to early 1980s, but increased 21% from early 1980s to early 1990s; cause considered partly contaminant reduction and partly methodological (Burger at al. 1995). Eggshells from Oklahoma and Kansas during 1991–1994 were not thinner than pre-1947 eggshells (Koenen and Leslie 1996). Reported thickness varied nearly 2-fold among investigators; unable to compare between studies.

Clutch Size

Typically 2 or 3, sometimes 1; 3-egg clutch more prevalent at interior breeding areas than coastal areas (Thompson 1982, Kirsch 1996). For details, see Demography and Populations: measures of breeding activity, below.

Egg-Laying

Begins within 2 d of nest completion. Lays 1 egg/d, or on alternating days, usually in morning. Female may deposit egg elsewhere if disturbed from nest when ready to lay (Jackson 1973). Does not appear to replace single eggs lost, but readily renests if entire clutch destroyed. Renesting declines for nests lost later in season.

Onset Of Broodiness And Incubation In Relation To Laying

Incubation begins after first egg laid (Massey 1974), but nest may be inconsistently attended until clutch is complete.

Incubation Patch

None apparent on close examination during nesting.

Incubation Period

Generally 19–25 d (Hagar 1937, Hardy 1957, Davis 1968, Cornwell 1986); extremes of 28 d for a 2-egg nest that hatched in California (Massey 1974) and 35 d in Massachusetts (JLA). Average incubation period was 21 d for 16 nests in Mississippi (Hays 1980) and 22.8 d for 557 nests in Massachusetts (JLA). Nice (1954) detailed the history of an erroneous 14- to 16-d incubation period given by Bent (1921); subsequently reported in error by Sprunt and Chamberlain (1970).

Parental Behavior

Male and female share duties throughout nesting and chick-rearing, but not equally (Keane 1987). Female generally incubates about 80% of total incubation time (Hardy 1957, Davis 1968), but distribution of incubation between mates varies geographically (Cramp 1985). Female incubates more than male during first 5 d after clutch initiation and during early morning and late afternoon; female also attends and broods chicks more than male (Keane 1987). Eggs that roll or are kicked from nest may be either retrieved or incubated in the new position. Eggs of other Least Terns are readily accepted. Adults actively turn eggs (Wolk 1974). Adult may attempt to roll eggs to drier areas near a nest during rain (Swickard 1972). During hot weather, parent will stand over eggs or newly hatched young to shade them; if temperatures are excessive, parent soaks belly-feathers in water and drips onto eggs (Tomkins 1942) or chicks (Jackson 1994).

Hardiness Of Eggs Against Temperature Stress

Eggs are sometimes exposed to high temperatures when adults are disturbed from nests, but tolerance of eggs to excessive heating is unknown.

Preliminary Events And Vocalizations

Chicks cheep inside egg immediately before hatching.

Shell-Breaking And Emergence

Time from pipping to hatching averaged 2.1 d (range <1–4 d; Wolk 1974).

Parental Assistance And Disposal Of Eggshells

Shells from hatched eggs are removed by adult shortly after hatching and dropped elsewhere on or outside colony.

Condition At Hatching

At hatching, chick is wet, down-covered, and has prominent white egg tooth, open eyes, and limited thermoregulatory abilities. Color of down and flesh varies considerably within and among broods. Jackson (1976) distinguished 3 basic leg colors (flesh, yellow, orange) and found that 90% (109 of 121) 1- to 2-d-old young had flesh-colored legs and 10% had yellow legs. Some older chicks had orange legs. Down color was white or tan (closest to the buff of Smithe 1975) and usually spotted or streaked with black.

Growth And Development

At hatching in California and Nebraska, 77 chicks weighed 4.5–7.2 g; 148 chicks weighed 5–11 g <25 h after hatching (Massey 1974, Lingle 1988). Wings of newly hatched chicks are 10–11 mm long; wing grows slowly during first 3–4 d, then increases rapidly and steadily to 20 mm at 5 d, 50 mm at 10 d, 79 mm at 15 d, and 105 mm at 20 d (C. Collins unpubl. data). Increase in mass is rapid, with peaks of 36.4–41.7 g at 17–20 d; thereafter mass decreases slightly until fledging (C. Collins unpubl. data summarized in Schew 1990). Growth rates similar among years and colonies, but peak mass and mass at fledging differ among years and colonies depending on environmental conditions, especially food supply (C. Collins pers. comm.). Captive chicks still had egg tooth at 12 d (Wolk 1974). Chicks of all ages thermoregulate to some extent by gular-fluttering (Cornwell 1986); those old enough to leave nest (2 d) seek shade. Howell (1959) found that esophageal temperatures of 1- to 2-d-old chicks varied about 8°C, but never exceeded 42.5°C, despite higher ambient temperatures. His laboratory studies suggested great lability of chick temperatures (a range of 24°C) and an ability to thermoregulate effectively under severe heat for at least 38 min. Nonetheless, Abdulali (1940) reported death of nestling Little Terns after 15–20 min of exposure to hot sun.

In rooftop colonies, temperature can be severe problem. At a mall roof colony in Mississippi, black tar showing through the gravel became so hot that chicks’ feet were severely burned and blistered; soft tar stuck to chicks’ feet and down (Jackson 1994). With rooftop air temperatures reaching 45.6°C, the chicks gathered in large groups in shaded corners of the parapet or in the shade of air conditioners. Pools of water condensed by the air conditioners were used for bathing and drinking. Belly-soaking by incubating or brooding adults, described by Mabbett (1890), Tomkins (1942), and Hays (1980), provides relief for adults, their eggs, and small chicks (see Incubation, above).

Older chicks seek shade or shelter while adults feed, and often take refuge in vegetation or near debris. Chicks also use their feet and body to form a shallow scrape in which to rest. Fledged young from beach nests often gather at water’s edge to wait for adults returning with food. Later they follow adults to feeding areas, where they are fed; still later they begin to forage for themselves. Tomkins (1959) observed a young Least Tern repeatedly diving into a shallow pool in which there were no fish, apparently playing or practicing diving.

Brooding

Adults brood chicks during first 24–48 h after hatching, depending on site characteristics and ambient temperatures. After that time, chicks find shelter and thermoregulate.

Feeding

Both sexes feed young, bringing smaller fish to chicks than to mates (Moseley 1976, Atwood and Kelly 1984, Schweitzer 1994), and increasingly larger fish to chicks as chicks mature (Atwood and Kelly 1984, Hill 1985, K. Dugger pers. comm.). Occasionally, chicks peck at insects on the ground or in shallow water (Wilson et al. 1993). In California, adults brought fish 2.5 cm long to chicks 10 d old, and fish of varied sizes to older chicks and fledglings (Atwood and Kelly 1984). On Gulf Coast, adults fed chicks average of 2 fish/h (Brubeck et al. 1981). In interior, average 2 fish/h delivered to chicks on Platte River, NE (n = 6 nests) and on an Oklahoma salt flat (n = 17 nests; Wilson et al. 1993, Schweitzer 1994); 2–4 fish/h to chicks and 1–2 fish/chick/h for 26 nests at 3 sites on Mississippi River, MO (K. Dugger unpubl. data). Nebraska chicks ate 74% (n = 125) of fish offered and accepted fish 2.0–5.9 cm long (Wilson et al. 1993). In Oklahoma, 58% of fish brought to nests offered to chicks, and chicks consumed 60% of fish offered (Schweitzer 1994). On average, 204 fish delivered to chicks (11 nests during 129 h) at 37.9-min intervals in Nebraska and 24.5-min intervals for 38 fish delivered to 17 nests during 26 h observation in Oklahoma (Wilson et al. 1993, Schweitzer 1996). When feeding young, brings only 1 fish or shrimp/feeding trip. Parents continue to feed fledged young and lead them to fishing areas. In California, male fed chicks more frequently than female, but feeding rate didn’t differ with age of adult (Keane 1987).

Nest Sanitation

Eggshells removed by adults immediately after chicks free; dropped away from nest (Massey 1974). Feces accumulate at nest until chicks leave the nest (LAH).

Carrying Of Young

Not known; unlikely as chicks walk and swim soon after hatching.

Not reported.

Not reported.

Departure From Nest

Chicks leave nest at about age 2 d, well in advance of first flight at about 20 d.

Growth

Weigh about 40 g at fledging; growth after fledging unknown. Chicks often move away from nest soon after hatching, and parents brood and feed them, usually within 200 m of nest site. Chicks may travel up to 1 km from nesting area (Massey 1974), likely in response to continued disturbance.

Association With Parents Or Other Young

Adults and young from several to many nests intermingle for feeding, loafing, and roosting after young fledge (Massey 1974).

Most fledglings disperse from colony site within 3 wk of fledging (Thompson and Slack 1984, Atwood and Massey 1988). Distances traveled during initial dispersal may be 100 m (from a sand pit to an adjacent river sandbar; Lingle 1993b, EMK) to 5 km (Atwood and Massey 1988). On Texas Gulf Coast, juvenile Least Terns seen 90–200 km from their natal colonies 6 wk after fledging (Thompson and Slack 1984). Chicks are fed by parents for several weeks after fledging (Massey 1974); parents may feed chicks occasionally for up to 8 wk (BCT). Recently fledged chicks intermingle with adults and chicks from other colonies, feed inexpertly for several weeks, and ultimately depart colony area in preparation for migration within 4–8 wk of fledging.