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Common Tern
Sterna hirundo
Order
CHARADRIIFORMES
– Family
LARIDAE
Authors: Nisbet, Ian C.

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Breeding

Common Tern chicks, 2 days old.
Common Tern, adult incubating; Duluth, MN; June

Phenology

Pair Formation

Not known whether established pairs arrive together, but some nesting and feeding territories reoccupied by pairs within a few days of first arrival (ICTN). Divorce likely if one member of pair arrives >5 d after the other (González-Solís et al. 1999c). Aerial courtship displays start immediately after arrival and continue throughout season.

First/Only Brood Per Season

See Appendix 2 . At established breeding sites, birds arrive 15–25 d before laying, start to occupy nesting areas after 2–5 d, gradually reoccupy nest territories over ensuing 10–20 d. Earliest eggs laid in early or mid-May on Atlantic coast south of Cape Cod, MA (Smith et al. 1981, Safina et al. 1989, Burger and Gochfeld 1991; Appendix 2), late May in Gulf of Maine (Kirkham 1986, Hall 1999), mid-May–early Jun in Gulf of St. Lawrence (Chapdelaine et al. 1985, Razurel 1996), 10 Jun in Labrador (Todd 1963), late Apr–early May in lower Great Lakes (Peterjohn and Rice 1991, Levine 1998; Appendix 2), mid-May–early Jun in upper Great Lakes (Shugart and Scharf 1983, Robbins 1991), early–late May in southern prairie provinces (Switzer et al. 1971, Fox 1976), mid-Jun at Great Slave Lake (Sirois et al. 1995). Laying dates at northernmost sites dependent on ice breakup (Vermeer 1973, Sirois et al. 1995). Egg dates from early May in Texas (Pemberton 1922), and early May–Aug in Netherlands Lesser Antilles (Voous 1965, van Halewyn 1985, A. del Nevo).

At most sites, 75–90% of nests initiated in period of 20–25 d after earliest eggs are laid, with modal date 5–11 d and median 7–14 d after earliest laying (Safina et al. 1988, 1989). Smaller numbers usually continue to initiate clutches up to 60–70 d after first laying; late pairs include young birds and pairs re-laying after failure at same or other sites. Late nests predominate at a few sites (Burger and Gochfeld 1991). Most eggs hatch 22–27 d after laying; most chicks fledge 22–29 d after hatching (see Hatching, Fledgling stage, below); fledglings fed at breeding site for 10–20 d before dispersing (Nisbet 1976). Coastal colonies south of Cape Cod, MA, vacated by mid-Aug, except occasionally when large numbers of birds re-lay and chicks are fed until early Sep; colonies in Gulf of Maine, Gulf of St. Lawrence, and prairie provinces usually occupied until late Aug or early Sep.

Second/Later Broods Per Season

After losing eggs or chicks, many pairs re-lay 8–12 d later (Nisbet 1978a, Arnold et al. 1998, Wendeln and Becker 2000). Occasionally, a few pairs lay and incubate second clutches while still feeding chick(s) from first brood (Hays 1984a, Wiggins et al. 1984). These second clutches often laid when only 1 chick remains from first brood and may then function as insurance against loss of last remaining chick. In rare cases, chick(s) may be raised to fledging from second broods (Hays 1984a, D. J. Moore), but evidence that chicks from first brood survived in these cases is equivocal (Nisbet 1994).

Nest Site

Selection Process

Male initially establishes nesting territory. During 1–5 d prior to egg-laying, male and female explore territory together, scraping in several different places (House-hunting); female lays egg in selected site (Palmer 1941, Cullen 1956, ICTN).

Microhabitat

Nests on ground, primarily in open areas with loose substrate (sand, gravel, shell, or cobble), but with scattered vegetation or other cover in which chicks can shelter within each nesting territory (i.e., distributed on spatial scale of 1–2 m). Such areas found on many coastal or lake islands, and interdune areas on barrier beaches. Most nests are 0–5 m above high-water mark; highest sites occupied first and late birds tend to settle at or below high-water mark (Nisbet et al. 1984). At some sites, early-nesting birds select nest-sites before vegetation starts to emerge, presumably using knowledge of vegetation from previous year (Severinghaus 1983). Where partly vegetated areas not available, will nest on islands that are mostly rocky, open sand or gravel, fully vegetated (preferably with grazed or otherwise stunted vegetation), on algal mats or lines of tide wrack in salt marshes, or on floating mats or muskrat houses in freshwater marshes (Voous 1965, Nickell 1966, Peck and James 1983, Burger and Gochfeld 1991, Cuthbert and Timmerman 2001). Where natural sites not available, nest in many artificial sites, including dredge spoil islands, confined disposal facilities, derelict piers and barges, breakwaters, bridge abutments, navigation cells, floating rafts, platforms, snowpiles, and gravel roofs (Scharf 1981, Maxwell and Smith 1983, Shields and Townsend 1985, Dunlop et al. 1991, Cuthbert and Timmerman 2001); in Europe even on a traffic roundabout on a busy highway (Bouwmeester and van Dijk 1991). In s. Caribbean, mainly on bare sandy substrates (LeCroy 1976, van Halewyn 1985); also reported nesting on mud mounds formed by nesting Greater Flamingos (Phoenicopterus ruber; Voous 1965).

Site Characteristics

In quantitative studies of substrate selection, occupied areas with 10–30% cover in N. Carolina (Soots and Parnell 1975), 20–40% cover in Ontario (Blokpoel et al. 1978). In New Jersey salt marshes, nested mainly on small islands (0.44 ha) with mixture of cordgrasses (Spartina alterniflora and S. patens) and with <12% wrack, but most nests were on wrack (Burger and Lesser 1978, Burger and Gochfeld 1990). In another study in New Jersey salt marshes, nests were on thicker and lower mats of dead vegetation and with shorter grass than random points (Storey 1987b). Compared to Forster’s Terns nesting on similar mats in Virginia, Common Terns nested at higher elevations, and on smaller mats with shorter grass (Storey 1987a). Habitat use by Common and Forster’s Terns in mixed colonies also compared by Dennis (1996). In an interdune site in New York, Common Terns nested in areas with 10–25% cover of beach grass (Ammophila breviligulata); nest sites were more open and farther from vegetation than those of Roseate Terns (Burger and Gochfeld 1988a, 1991). Where range overlaps with Arctic Tern, Common Tern generally nests on islands closer inshore, and uses nest sites with more vegetative cover (Palmer 1949, Chapdelaine et al. 1985, Kirkham 1986, C. S. Hall), but no quantitative comparisons.

In experimental studies, nests were closer to edges of artificial mats than were Black Skimmer nests; only terns nested on narrow mats (1 m wide; Burger and Gochfeld 1990). Preferred areas of dried grass to open stony areas (Severinghaus 1982), and areas of gravel with scattered vegetation and driftwood to areas of open gravel or broken concrete (Richards and Morris 1984). None of these studies, however, investigated the fine-scale preference for edges of small patches of vegetation suggested by observational studies (e.g., Austin 1929, Marples and Marples 1934, Blokpoel et al. 1978). See also Conservation and management: management (below), for successful attempts to attract Common Terns to modified sites.

Nest

Construction Process

Usually started by male as part of territorial advertising. Male bends forward and scrapes hollow in substrate with feet; female may follow and scrape in same hollow. Pair may make several scrapes at different locations within nest territory before female selects one for laying. On rock, concrete, or wood substrates, seeks natural depressions or grooves. After eggs laid, nest material gradually added throughout incubation. Each time bird leaves nest, it walks slowly away, picking up loose material and throwing it backwards over shoulder (Sideways Nest Building; Palmer 1941). Sitting bird picks up items within reach and adds them to nest, so that substantial nest may be formed by time of hatching.

Structure And Composition Matter

Usually constructed of dead vegetation, tide wrack, etc.; sometimes shell fragments, bones, stones, or plastic items if vegetation not available. Nests on piers or breakwaters may have no material except for concrete or wood chips. When threatened by rising water, adds material to nest very quickly, gathering material from edge of territory or even outside (Burger 1979, Storey 1987a): capable of raising eggs 2–5 cm within 1–2 h (Burger and Gochfeld 1991, ICTN).

Dimensions

Widely variable depending on substrate and availability of loose materials within territory. Measurements of 45 nests at Bird I., MA, at time of hatching: external diameter 18 cm ± 6.2 SD; diameter of nest cup 9.8 cm ± 1.6 SD; depth of nest cup 2.6 cm ± 0.5 SD; height of rim above surrounding substrate 1.3 cm ± 0.5 SD (n = 15) for nests on sand, 2.4 cm ± 0.9 SD (n = 15) for nests on tide wrack, and 7.6 cm ± 3.6 SD (n = 15) for nests on bed of pond (subject to flooding). Largest of latter had external dimensions 70 × 38 cm, height 15.5 cm, wet mass 630 g, dry mass 482 g (ICTN). For 47 nests in New Jersey salt marshes at unspecified dates: width 16.7 cm ± 2.4 SD; height above substrate 9.8 cm ± 2.7 SD; nests on Spartina alterniflora wider and higher than those on S. patens (Burger and Gochfeld 1991). Height above substrate for 32 nests in New Jersey salt marsh measured after hatch 3.4 cm ± 1.1 SD (Storey 1987a).

Microclimate

Water-vapor pressure surrounding eggs maintained at about 20 torr, calculated from rate of water loss and water-vapor conductance of eggshells (Rahn et al. 1976; see Eggs: eggshell characteristics, below). Mean egg temperature not precisely measured, but about 37°C (Rahn et al. 1976, J. Arnold).

Maintenance And Reuse Of Nests, Alternate Nests

Quickly repairs nest if damaged during incubation, especially if integrity of nest rim is breached (Burger 1979). Nest not maintained after chicks hatch; not used after chicks are ambulatory (ICTN). If first egg removed when laid, some pairs continue laying in same scrape, while some pairs move to new scrapes; some deserted scrapes are taken over by other pairs (Arnold et al. 1998, González-Solís et al. 1999a).

Eggs

Shape

Subelliptical. Third egg in clutch (C -egg) distinct from first (A -egg) and second (B -egg), with smaller radius of curvature at large end (Gemperle and Preston 1955).

Size And Mass

Typical average dimensions 42 mm × 30.5 mm, volume 20 ml, fresh mass 21 g. A -egg usually slightly larger than B -egg, and both distinctly larger than C -egg, in breadth, volume, and fresh mass; differences in length usually only slight (Gochfeld 1977, Nisbet 1982, Custer et al. 1986, Bollinger 1994). Also vary with parental age (Nisbet et al. 1984), date of laying (Nisbet and Welton 1984, Arnold et al. 1998, González-Solís et al. 1999b), site (Nisbet et al. 1984, Safina et al. 1989), and year (D. J. Moore). Compared to eggs in first layings by same birds, eggs in repeat layings are smaller when first clutches are lost late in season, but similar or larger when first clutches are lost early (Nisbet and Cohen 1975, Wendeln and Becker 2000). At Bird I., MA, fresh masses vary from 14.5 to 25.6 g; occasional mini-eggs (frequency about 0.05%) weigh only 5–9 g; one exceptionally large egg measured 54.2 mm × 33.7 mm and weighed 32.1 g (ICTN). Volume and fresh mass predicted by equations V = 0.502 LB 2and M = 0.524 LB 2, where V = volume (ml), M = mass (g), L = length (cm) and B = breadth (cm; Nisbet et al. 1984, Moore et al. 2000). Fresh density 1.058 g/ml (Rahn et al. 1976). Eggs from Netherlands Lesser Antilles smaller than those from continental North America, especially in breadth (Voous 1957, van Halewyn 1985). For large series of measurements from Europe, see Watson et al. 1921, Schönwetter 1967, Glutz von Blotzheim and Bauer 1982 .

Color And Surface Texture

Smooth, nonglossy, with finely granular surface; waxy coating of dried mucus when fresh. Ground color cream, buff, or medium brown, sometimes tinged with green or olive; finely marked with streaks, spots, blotches, or fine lines of black, brown, or gray. C -eggs often paler than A - and B -eggs, and markings tend to be denser around greatest diameter and towards large end, sparser towards small end; A - and B -eggs are more uniformly marked (Preston 1957). Rare eggs (<0.1%; ICTN) unmarked greenish blue, sometimes chalky in texture. Eggs from Netherlands Lesser Antilles consistently paler than those from North America (Voous 1965).

Eggshell Characteristics

Mass of pre-1947 eggshells 1.13 g ± 0.14 SD (n = 108 eggs from New England; ICTN), 1.17 g ± 0.11 SD (n = 69 eggs from e. Canada; G. A. Fox), 1.14 g ± 0.11 SD (n = 39 eggs from w. Canada; G. A. Fox). No data on shell thickness before 1947, but shells collected in Great Lakes and e. U.S. in 1980s had mean thickness in range of 188–205 µm (Custer et al. 1986; Weseloh et al. 1989; Karwowski et al. 1991, 1992). Mean density 1.80 g/ml, water vapor conductance 4.30 mg/d/torr, total pore area 0.36 mm2(Rahn et al. 1976). Rate of mass loss (A -eggs) during natural incubation 122 mg/d ± 21 SD (n = 112; Rahn et al. 1976). A -egg shells heavier and thicker than B -egg shells, which in turn are heavier and thicker than C -egg shells (Nisbet 1978a, 1982). Shells of eggs collected in 1969–1973 thinner and lighter by 4–15% than pre-1947 or post-1980 shells (Fox 1974, Morris et al. 1976, Nisbet 1978a, R. Morris, ICTN); however, shell thickness less affected by DDE than in other fish-eating birds subject to same levels of contamination (Switzer et al. 1971, 1973). Main effect of DDE was to change composition and structure of shell, resulting in reduced porosity and gas exchange, and frequent breakage (Fox 1974, 1976).

Composition

In 30 eggs from Yarmouth, MA, in 1973, yolk mass made up 28.3–29.2% of contents, lipids 9.5–9.9%, water 74.4–77.6%; average proportions similar in A -, B -, and C -eggs (Nisbet 1978a). In 114 eggs from 8 sites in upstate New York in 1986–1989, lipids made up 8.3–10.1% of contents, water 74.9–83.4% (Karwowski et al. 1991, 1992). Percentages of yolk, lipid, albumen, and total dry matter decreased with increasing mass of contents (Nisbet 1978a). Shells of pre-1947 eggs contained 39.1% calcium (13.2 mg/cm2), 18.0% magnesium, and 0.27% phosphorus; contamination with DDE decreased percentage and density of calcium and increased percentage of phosphorus (Fox 1974).

Clutch Size

Usually 2–3, occasionally 1 or 4, very rarely >4 eggs. In some reported cases, mean clutch sizes may have been biased by inclusion of incomplete clutches or by partial losses; for large data sets, see Cooper et al. 1970; Peck and James 1983; Severinghaus 1983; Erwin and Smith 1985; Safina et al. 1988, 1989; Burger and Gochfeld 1991; and other sources in Appendix 2 . In cases where nests were visited daily or near-daily, means tend to be higher (e.g, Morris et al. 1976, Nisbet et al. 1984, Chapdelaine et al. 1985). Modal clutch size usually 3; in some sites and years 70–90% of clutches have 3 eggs (Nisbet and Drury 1972, Morris et al. 1976, Nisbet and Welton 1984). In other sites and years, modal clutch size is 2 and mean may be as low as 2.1 (Appendix 2). At Bird I., MA, mean clutch size in nests visited daily was 2.90 in 1970–1985 (n = 932), but 2.48 in 1991–2000 (n = 8,388; ICTN). Clutch size also varies with parental age (Nisbet et al. 1984, in press), laying date (Nisbet and Welton 1984), and habitat (Safina et al. 1989). Clutch size sometimes, but not always, smaller in repeat layings than in first layings of same pairs (Nisbet and Cohen 1975, Wendeln and Becker 2000).

Frequency of supernormal clutches (≥4 eggs) varies among colony sites, from 0.1–0.3% at Bird I., MA (ICTN) to 4.9% at Windermere Basin, Lake Ontario (D. J. Moore). In some cases, 4 eggs are laid in same nest; in others, eggs are displaced by tidal flooding or other agents and rolled into nests by sitting birds (ICTN). Clutches of 5–7 eggs may arise by same processes; larger clutches (up to 10 eggs) may result from human interference. Supernormal clutches were more frequent (up to 10%) at some sites in period 1894–1921 (Mackay 1895, Jones 1906, Massey 1916, Bent 1921, museum collections).

Egg-Laying

Eggs most frequently laid in late afternoon or early evening (J. M. Arnold), but data scanty. Male feeds female during egg-laying period; female rarely leaves nest site except to drink and bathe (Nisbet 1977). Laying intervals 1.5–1.9 d between A - and B - and between B- and C -eggs, lengthening during season in some studies but not in others (Nisbet and Cohen 1975, Courtney 1979, Bollinger et al. 1990). If first egg lost, some but not all females continue to lay full clutch (i.e., lay supernumerary egg), often laying rest of clutch in new scrape and sometimes in new territory (Heaney and Monaghan 1995, Arnold et al. 1998).

Incubation

Onset Of Broodiness And Incubation In Relation To Laying

Both female and male sometimes sit on A -egg on day of laying, but incubation irregular and incomplete until last egg laid (Nisbet and Cohen 1975). Incubation attentiveness (% time with 1 bird on nest) usually increases to 70–90% by clutch completion and to 98% within 3 d thereafter (Courtney 1979, Bollinger et al. 1990).

Incubation Patches

Three incubation patches on both male and female, with total area about 37 cm2. Defeathering completed during egg-laying period; refeathering started late in incubation (Courtney 1979).

Incubation Period

In absence of disturbing factors, mean incubation periods (defined here as interval between laying and hatching of same egg) about 23.1 d for A -egg, 22.3 d for B -egg, and 21.7 d for C -egg, shortening significantly during season as attentiveness increases (Nisbet and Cohen 1975, Courtney 1979). When predation causes nocturnal desertion (see Behavior: predation, above) or other factors lead to inattentiveness, mean incubation periods increase to 24–31.5 d, individual eggs hatching after intervals of up to 33 d (Nisbet 1975, Courtney 1979, Nisbet and Welton 1984, C. M. Adams).

Parental Behavior

Both sexes incubate, females slightly more than males (32.0 vs. 26.9 min/h) during daytime (Wiggins and Morris 1987). Female does most of incubation during night; attendance of male drops to <10% by 21:00 h. Male incubates more during first and third hours of daylight, while female forages (data from Germany; P. H. Becker and H. Wendeln). Incubation stints vary from <1 min to several hours, shorter in morning (ICTN); no quantitative data from North America. Changeovers usually protracted as departing bird performs Sideways Nest Building (see Nest: construction process, above). Attentiveness greatest during cold rain or midday heat. Heat-stressed birds perform gular fluttering. When air temperature >30–35°C, sitting bird cools eggs by flying to water, dipping feet and belly, and returning to nest (Grant 1981, Nisbet 1983a). Flies off nest to defecate (ICTN). Egg-rolling behavior variable, but often attempts to roll eggs into nest from within about 15 cm (Palmer 1941, Marshall 1943). In controlled test, birds did not distinguish their own eggs from others (Saino and Fasola 1993). Adults remove foreign objects and broken eggs from nest within a few minutes (ICTN).

Hardiness Of Eggs Against Temperature Stress; Effect Of Egg Neglect

Most eggs hatch even when regularly deserted at night (e.g., 89% hatched when mean incubation period lengthened to 27.8 d; Nisbet 1975). 44% of eggs hatched after being removed from nest when fresh, held at 11–16°C for 11 d, then returned to nest (J. M. Arnold). Embryos may be more sensitive to cold after pipping, but no clear documentation. Sensitivity of eggs to solar heating when unattended during day not reported.

Hatching

Preliminary Events And Vocalizations

Star-shaped cracking usually appears near large end of egg about 3 d before hatching. Pip-hole usually appears about 24 h before hatching. Chick persistently squeaks once pip-hole is open.

Shell-Breaking And Emergence

Chicks hatch at all times of day or night. Chick enlarges pip-hole to form ring around large end, then pushes cap off; entire process takes about 1 h. Hatching intervals average about 1.0 d between A - and B - chicks, about 1.5 d between B - and C -chicks, typically lengthening during season so that interval between A - and C -chicks often ≥4 d (Nisbet and Cohen 1975, Courtney 1979, Bollinger et al. 1990).

Parental Assistance And Disposal Of Eggshells

Parents give no physical assistance, but are restless during hatching, frequently standing and adjusting eggs. Parent removes hatched eggshells, usually within 15 min, flying off with each piece and dropping it 20–100 m away.

Hatching Success

Main causes of hatching failure are predation and flooding; these occur sporadically, but often reduce hatching success to zero or near-zero (Austin 1948, Gochfeld and Ford 1974, Courtney and Blokpoel 1983, Burger and Gochfeld 1991). In absence of predation and flooding, hatching success typically 85–95% (sources in Appendix 2); sometimes >97% (Nisbet et al. 1984, Safina et al. 1988, Rossell et al. 2000). Lower values of hatching success usually result from breakage, disappearance, and/or embryonic mortality; sometimes associated with parental inattentiveness (Morris et al. 1976). Very low hatching success (24–61%) reported at freshwater sites in 1969–1973, associated with DDE contamination and defective eggshells, and in some cases deaths of embryos after pip-ping (Switzer et al. 1971, 1973; Fox 1976, Morris et al. 1976).

Young Birds

Condition At Hatching

Chicks nidifugous, semi-precocial with eyes open, covered in thick down; capable of standing and taking food within 1–3 h.

Growth And Development

Development of embryo described and illustrated by Hays and LeCroy (1971). Chick hatches with 0.5–2.0 g remaining in yolk sac (Hart 1998); can live for 2–3 d without food if not chilled (ICTN). Egg tooth retained for 3–5 d (LeCroy and Collins 1972). Usually offered food within 1–2 h of hatching, but takes only a few small items during first day. Growth curves, based on daily measurements from hatching to fledging, reported as follows: wing (LeCroy and Collins 1972, LeCroy and LeCroy 1974, Ricklefs and White 1981, Chapdelaine et al. 1985, Safina et al. 1988); tail (Ricklefs and White 1981); culmen (Szulc-Olechowa 1964); tarsus (Cymborowski and Szulc-Olechowa 1966); body mass (LeCroy and Collins 1972, Langham 1974, LeCroy and LeCroy 1974, Nisbet 1975, Nisbet et al. 1978, Chapdelaine et al. 1985, Safina et al. 1988, Klaassen 1994). Body mass data used for comparative studies of growth in several other studies (Nisbet 1978a, Custer et al. 1986, Safina et al. 1989, Bollinger at al. 1990, Galbraith et al. 1999, Hall 1999, Nisbet et al. in press), without reporting of complete growth curves. Wing grows approximately linearly from days 12 to 25; chick fledges when wing about 70% of adult length (LeCroy and Collins 1972, Ricklefs and White 1981, Safina et al. 1988).

Mean body mass at hatching about 15 g; mass in-creases approximately linearly from 25 to 100 g during days 3–14, reaches plateau of 100–130 g during days 17–25, often declines slightly before fledging. Grouped data for body mass growth fitted to logistic equations by Ricklefs and White (1981), Langham (1983), Chapdelaine et al. (1985), and Hall (1999): growth constant K 0.21–0.34 d–1, inflection point ti 5.4–9.5 d, asymptote A 93–136 g. Other measures of growth reported include linear growth rate (linear slope of mass vs. age during days 3–14; Nisbet 1978a, Custer et al. 1986, Hall 1999, Wendeln and Becker 1999a, Nisbet et al. in press), logarithmic growth rate (slope of ln[mass] vs. age during days 0–10; Bollinger et al. 1990), mass at age 15 d (Courtney and Blokpoel 1980), and asymptotic mass (mean mass during days 17–25; Hall 1999, Nisbet et al. in press). These measures of growth vary according to site, year, brood size, hatching order, hatch date, egg mass, parental age, and parental body condition (Nisbet 1978a; Courtney and Blokpoel 1980; Langham 1983; Nisbet et al. 1984, in press; Custer et al. 1986; Safina et al. 1988, 1989; Hall 1999; Wendeln and Becker 1999a).

Development of plumage described by Cymborowski and Szulc-Olechowa (1966) and Nisbet and Drury (1972). Body composition and organ masses of chicks at hatching reported by Ricklefs (1979) and Hart (1998). Changes in body composition and organ masses during chick development reported by Ricklefs and White (1981) and Klaassen (1994); same authors analyzed and reported energetics of growth. Changes in relative sizes of skeletal elements during embryonic and post-hatching development analyzed and reported by Cane (1993).

Chick Survival

Predation and flooding often important causes of chick loss, sometimes reducing chick survival to zero or near-zero (Nisbet and Welton 1984, Safina et al. 1989, Harper and Harper 1997, Barbour et al. 2000). Cold, foggy, or wet weather reported as important causes of chick mortality in Maine, New Brunswick, and Québec (Palmer 1938, Power 1964, Amey 1998), but not elsewhere except in combination with nocturnal predation. Otherwise, main cause of chick deaths is starvation, falling successively on C - and B -chicks in each brood as result of asynchronous hatching (Bollinger et al. 1990, Bollinger 1994). In some sites and years, most A - and B -chicks grow well and survive to fledging; C -chicks are marginal, some dying and others fledging in poor condition, so that productivity is in range of 2.0–2.5 fledglings/pair (Nisbet 1978a; Nisbet et al. 1978, 1984; Rossell et al. 2000; Appendix 2). In other sites and years, most C - and B -chicks die and A -chicks are marginal, so that productivity is in range of 0.5–0.9 fledglings/pair (Nisbet and Drury 1972, Nisbet et al. in press). More frequently, B -chicks are marginal and productivity is in range of 1.1–1.8 fledglings/pair (sources in Appendix 2). In addition to site and year variation, survival of marginal chicks depends on parental age (Nisbet et al. in press), parental quality (Bollinger 1994, Wendeln and Becker 1999a), hatch date (J. M. Arnold), and egg mass (Nisbet 1978a). Most chicks that die do so at ages 1–6 d (Nisbet 1978a).

Parental Care

Brooding

During first 4 d after A -chick hatches, female broods and/or attends chicks about 70% of time during day and 90% of time at night; male broods and/or attends 40–50% of time during day but only 10–20% of time at night (Wiggins and Morris 1987, Galbraith et al. 1999, P. H. Becker, H. Wendeln). Greater attendance by female persists at least through day 12 (Wiggins and Morris 1987). By day 6, chicks increasingly left unattended during day. By day 12, chicks attended only 20–60% of daylight hours, depending on weather, food availability, and brood size; single chicks and broods of 2 attended more than broods of 3 (Wiggins 1989). Parents brood assiduously during cold, wet, or hot weather. When air temperature >37°C, parents cooled chicks by belly-soaking: this protected chicks younger than 7 d, but older chicks died or lost mass (Nisbet 1983a).

Feeding

During first 3 d, male brings 60–95% of food items; female often brings very small items (Wiggins and Morris 1987, Galbraith et al. 1999). At Bird I., MA, males brought mean of 16.9 items ± 7.6 SD (n = 25) to broods of 1–4 chicks in 24-h period (Galbraith et al. 1999). Food items brought singly in bill; chick learns to take fish and swallow head-first within 1–2 d (Gochfeld 1979b).

Female brings more food after day 4. At 2 sites in Great Lakes, male continued to bring 60–90% of feeds, especially to single chicks, at least through day 12 (Wiggins and Morris 1987, Wiggins 1989, Moore 1993, Burness et al. 1994). At 2 sites on Atlantic Coast, however, males and females reported to bring food at similar rates (Kirkham 1986, Wagner and Safina 1989). Feeding rates (reported here as number of items brought/h by both parents) vary widely depending on fish size, brood size, chick ages, and other factors. At Peters I., Nova Scotia, mean feeding rate 18.7 items/chick/d (35.3/brood/d); rates increased until day 16, declining thereafter (n = 1,378 items; Kirkham 1986). At Stratton I., ME, feeding rates increased with brood size and chick age, from 0.80 items/h for single newly-hatched chicks to 2.85 items/h for broods of 3 chicks >10 d old (n = 1,800 items; Rossell et al. 2000). At Cedar Beach, NY, mean feeding rate (averaged over all brood sizes and ages) was 1.08 items/brood/h (n = 2,369 items; Safina et al. 1990).

At Port Colborne, Lake Erie, in 1982–1983, mean feeding rates during days 1–12 after hatching of last chick were 0.60 items/h to single chicks, 1.06/h to broods of 2, 1.67/h to broods of 3; rates increased to day 5 and then declined, but fish sizes increased after day 5 (n = 27 pairs; Wiggins and Morris 1987, Wiggins 1989). At Windermere Basin, Lake Ontario, in 1991–1992, feeding rates varied between 0.2 and 1.1 items/chick/h, without consistent patterns of variation with chick age, year, or season; fish sizes increased significantly with chick age (n = 2,460 items; Moore 1993). Other data on feeding rates reported by Courtney and Blokpoel (1980), Chapdelaine et al. (1985), Kirkham (1986), Burness et al. (1994).

Proportion of fish taken by kleptoparasites (other chicks and adults) varies among sites and years, from near-zero to at least 17% (D. J. Moore); highest for late-hatched chicks in dense colonies (J. M. Arnold). Parents defend small chicks against kleptoparasites, often successfully if both parents are present. Chicks soon learn to swallow fish quickly and/or run to cover; parents make little attempt to defend large chicks (ICTN).

Parents make little or no discernible effort to allocate food among chicks: usually feed chick that arrives first and/or begs most conspicuously (ICTN). Hence, strong tendency for oldest and largest chick(s) to be fed first until satiated, resulting in successive loss of younger chick(s) unless food is superabundant (Bollinger et al. 1990).

Single parents can raise 1 or even 2 chicks to fledging if mates are lost after hatching (Nisbet et al. 1978).

Nest Sanitation

See Behavior: self maintenance, above.

Cooperative Breeding

Apparently rare, but not investigated at sites where supernormal clutches most frequent (see Eggs: clutch size, above).

Brood Parasitism

No evidence of interspecific nest parasitism; occasional mixed clutches with Roseate Terns result from egg displacement after laying (ICTN). Intraspecific nest parasitism not investigated, but probably infrequent because 4-egg clutches are rare even at sites where modal clutch-size is 3 eggs. Adoption of chicks from other nests fairly frequent at some colonies (Saino et al. 1994), rare at others; possibly related to nest density and/or food shortage (ICTN). At Bird I., MA, 10–20% of pairs adopt chicks and some are raised to fledging; most adoptions involve chicks <48 h old and take place during hatching of adopters’ own chicks (M. S. Friar, ICTN). At Port Colborne, Lake Erie, however, most adoptions were of chicks 3–10 d old and adopters’ own chicks were 1–11 d old; about half the adoptees’ were raised to fledging (Morris et al. 1991).

Fledgling Stage

Departure From Nest

Chicks start to wander away from nest at age 2–3 d, seeking cover within territory. Except for a few at edge of colony, chicks remain in territory until fledging. Age at fledging varies widely, from 22 to >29 d, probably dependent on condition (Nisbet and Drury 1972, Burger and Gochfeld 1991). After first flight, fly to and from outer edge of colony; fed at first mainly in territory, later mainly on beach or rocks. Unless occupied by dense vegetation, territory serves as rendezvous for family for 10–20 d after fledging; used for chick-feeding and roosting (ICTN). After 2–4 d, chick starts to fly out over water unattended by parents; practices dipping down to surface, touching water, and picking up floating objects; starts to dip head in water after 4–6 d, but does not catch fish for itself until 20–30 d after fledging. After 6–10 d, chick starts to accompany parents to feeding territory or other fishing grounds. Chick stands on shore or elevated perch and waits to be fed, or follows parents, begging, landing on water to take fish. Families disperse away from colony site after 10–20 d (Nisbet 1976).

Growth

Most growth completed prior to fledging. Primary feathers continue to grow for about 20 d thereafter; bill probably continues to grow also (ICTN).

Association With Parents Or Other Young

Parents continue to feed young at colony site until dispersal, although chicks often left alone while parents forage; siblings may be fed at different locations (ICTN). No evidence for brood-splitting; families leave colony site together and remain together throughout dispersal and staging periods (see Migration: migratory behavior, above). Juveniles accompany parents to feeding grounds, or may wait at staging sites for parents to bring food.

Immature Stage

Juveniles remain less proficient at fishing than adults into Sep (Källander 1991). Not known whether families remain together during migration and early winter, but adults migrate farther south than first-winter birds and are largely separated by Dec (Hays et al. 1997). Most young birds remain in winter quarters or migrate short distances north during second calendar year (see Migration: timing and routes of migration, above). LeCroy (1976) reported several 1-yr-old birds accompanied by adults at breeding site in Venezuela; no similar reports in North America. Most 2-yr-olds migrate north to breeding areas and explore natal or other colony sites (Wendeln and Becker 1998).