Demography and Populations

Unless otherwise noted, all data pertain to boreal S. m. maxima .

Age At First Breeding; Intervals Between Breeding

Buckley and Buckley (1972a) suggested 2–3 yr, and band recoveries confirm that most remain on wintering grounds for at least first 2 winters (J. Weske pers. comm.). Plumage details subsequently suggested first breeding probably most frequently in birds’ third and fourth calendar years, with only occasional attempts in second calendar year (Buckley and Buckley 1976, PAB). But S. Emslie’s (pers. comm.) unpublished analysis of recaptures of breeding adults in N. Carolina colonies—following J. Weske’s decades of banding there—indicates that majority do not begin breeding until 5–6 yr old (only handful of 3- to 4-yr-olds present), comparing favorably to European Sandwich Tern (S. s. sandvicensis; Smith 1975).

Clutch

See Breeding: eggs, above.

Annual And Lifetime Reproductive Success

No systematic data on hatching/fledging rates for any population, or on rates of egg or chick loss. Incidental observations suggest that egg and chick mortality normally very low, and hatching and fledging success quite high. Most mortality of young probably takes place postfledging (Buckley and Buckley 1972a, 1974). Presumed that most adults attempt to breed annually, absent any data to contrary. Still, because colonies often wash out, mean adult successful breeding frequency possibly less than annual. Lifetime success unknown, but perhaps not far from 1.0 × mean number breeding year.

Number Of Broods Normally Raised Per Season

Only one brood known in all populations.

Proportion Of Females That Rear At Least One Brood To Nest-Leaving Or Independence

Unknown, but likely very high, as most mortality in boreal S. m. maxima colonies in Virginia–N. Carolina was postfledging. Across several years, of 12,000+ chicks, only 15 found dead, astonishingly few even allowing for indeterminate number removed by scavengers (Buckley and Buckley 1972a).

No published survivorship data. However, C. Collins (pers. comm.), from sample of 36 chicks banded in Orange Co., CA, found overall mean survival rate of 82%, with 86% in year 0–2, 96% and 92% in year 2–4, and 77–69% in year 4–7. Mortality heavy in first winter in boreal S. m. maxima, where most common cause of mortality (entanglement in fishing lines) dis-proportionately responsible for deaths of first-winter birds (Buckley and Buckley 1974). S. Emslie, J. Weske, and M. Browne (unpubl.) believe that cohort survival of Virginia–N. Carolina juveniles was strikingly lower in ENSO (El Niño-Southern Oscillation, commonly shortened to El Niño) years when abnormally warm tropical waters present young with increased feeding difficulties; cf. ENSO effects on Elegant Terns in California (Schaffner 1986).

Oldest known North American individuals 27–28 yr (S. Emslie pers. comm.); findings comparable to Sandwich Tern. However, excessive aluminumband wear in Royal Terns mandates replacement after 7–8 yr (J. Weske pers. comm.), so reliable longevity data from stainless steel or incaloy bands is still needed.

Diseases

Almost no information. Locke et al. (1974) reported one death by aspergillosis.

Body Parasites

No data.

Exposure

No published records of death from exposure, although heat-related death of chicks and adults reported from Virginia (M. Byrd pers. comm.). Chicks in low-lying colonies subject to tidal flooding probably killed in times of high water. Austral S. m. maxima reportedly suffers egg loss from heavy rains, and low-lying boreal S. m. maxima colonies not uncommonly swept clear of nests and eggs in spring and storm tides (Buckley and Buckley 1972a).

Anthropogenic

Juveniles suffer heavy mortality from variety of causes, chief among them fishing-related activities (hooked by bait-casting fishermen; diving on lures; struck by cars on bridges where people fishing). Banding recoveries embrace wide range of mortality sources (Buckley and Buckley 1974).

Predation

See Behavior: predation, above. Overt predation on adult Royal Terns all but unknown. In boreal S. m. maxima, eggs frequently taken, especially on periphery, by Laughing Gulls, Ruddy Turnstones (Arenaria interpres), Fish Crows (Corvus ossifragus), and Boat-tailed Grackles (Quiscalus major). Boreal S. m. maxima are amazingly docile when confronted by marauding gulls, rarely even lunging or moving away from one unless ≤30 cm. Most predation occurred when terns flew up from the colony on disturbance by passing boat, etc. Then gulls would remain on ground and economically run from egg to egg, puncturing one after another, which they would then return to consume at leisure as terns will not incubate punctured eggs. However, Laughing Gulls never seen taking any chicks (Buckley and Buckley 1972a).

In austral S. m. maxima colonies, egg predation by Kelp Gulls (99% on peripheral nests; n = 454) was major factor limiting reproductive success (Yorio and Quintana 1997). Snowy Sheathbills (Chionis alba) and Dolphin Gulls also minor peripheral nest-egg predators; chicks also rarely taken in this population. Escalante (1985) also noted adult skua (sp.) killing adult austral S. m. maxima, possibly only solid instance of adult Royal Tern killed by another bird. Black-crowned Night-Herons (Nycticorax nycticorax), notorious tern chick predators, probably also take Royal Tern chicks, even regularly, but no data at hand.

Competition With Other Species

Not found, but sought. Work by S. Emslie and colleagues with boreal S. m. maxima in N. Carolina confirms overlap with Sandwich Tern in prey species taken but not in size classes, as well as differential percentage representation of fish families in diets of the 2 species (see Food habits, above). No firm data that competition for nesting sites or space limits Royal Tern populations, although intense interactions between austral S. m. maxima and Kelp Gulls may exact energetic or time budget cost leading to reduced tern productivity (Quintana and Yorio 1998).

Initial And Subsequent Dispersal From Natal Site

After fledging, juvenile and adults normally remain in general vicinity of colony for several weeks before beginning true dispersal, which might take them hundreds and even thousands of kilometers from colony area, frequently northward. Remain as late as late Nov in New England, Dec in s. New Jersey, and later still farther south, eventually moving south to true winter quarters (see Migration, above).

On Pacific Coast, disperse northward postbreeding to San Francisco Bay (although increasingly uncommon north of Morro Bay and wholly unknown north of California), currently with “counts [in s. California] seldom exceeding few dozen birds, and . . . none over 100” (M. A. Patten in litt.).

On Atlantic Coast, moves in numbers north to Long Island, NY, with nonhurricane peaks (to 1,600) at Cape May, NJ, in Oct (Sibley 1997) and 114 along 30 km of beach between Shinnecock Inlet and Georgica Pond, Long Island, 29 Sep (Cooper 1998). Hurricane counts north of Cape May larger still, with 600+ at Fire I. Inlet, Long Island, 12 Sep 1960 (day of Hurricane Donna; Buckley 1960), when almost 100 found between Martha’s Vineyard, Nantucket, and Monomoy, MA (Veit and Petersen 1993).

Van Velzen (1968) analyzed band recoveries from Virginia colonies, finding none recovered before Nov, and most wintering in Florida or Caribbean south to Venezuela, with several 1-yr-olds still in Colombia and Panama in May–Jun. Farthest recoveries Trinidad and Panama, with one well inland in central Guatemala, perhaps crossing to or from Pacific. Recently, S. Emslie, J. Weske, and M. Browne (unpubl.) found 62% of 4,000 N. Carolina-banded winter recoveries from Florida, 23% from the Caribbean region, and 15% from se. U.S., Mexico, and Central/South America.

No published data on distances between natal and first-breeding sites although J. Weske (pers. comm.) indicates strong tendency for birds to return to breed in natal colony or nearby.

Fidelity To Breeding Site And Winter Home Range

No published data on winter-site fidelity, and almost none on breeding-site fidelity in any S. m. maxima population. In N. Carolina, regional breeding philopatry estimated at 85%, but in some sites (e.g., Cape Fear River) as high as 94% (Maness and Emslie 2001). Color-marked individuals known to move between colonies between years in Virginia. Conversely, entire colonies routinely desert site and move to another when disturbed, especially before eggs have hatched (Buckley and Buckley 1972a).

Home Range

Few published data. In boreal S. m. maxima, color-marked foraging adults seen 24–40 km south, 40 km southeast, and 65 km north of breeding colonies (Buckley and Buckley 1972a).

Numbers

Most recent comprehensive summaries of S. m. maxima Royal Tern breeding populations those of Buckley and Buckley (1984a) for area between the N.–S. Carolina border and Maine–Canada border; Clapp and Buckley (1984) for area between Texas–Mexico border and N.–S. Carolina border; Sprunt (1984) for Bahamas; and van Halewyn and Norton (1984) for West Indies, updated by Chardine et al. (2000). Data on austral S. m. maxima scarce, with breeding proven in Argentina and Brazil, suspected in Uruguay (Escalante 1991).

Buckley and Buckley (1984a) estimated 4,734 pairs (following data in pairs) breeding in Maryland (134) and Virginia (4,600) in 1983; southward, Clapp and Buckley (1984) reported additional 62,532 (1976–1983) as follows: 17,029 in N. Carolina, 11,200 in S. Carolina; 1,000 in Florida, 250 in Mississippi; 10,590 in Louisiana, and 22,463 in Texas. In all areas, colonies on islands, often made from dredge spoil, frequently at inlets, and sometimes in very remote locations (Chandeleur Is. off Louisiana, Laguna Madre in Texas). Colonies change sites regularly, and numbers vary greatly year to year. Unless whole-area surveying done by aircraft, major colonies easily missed.

Sprunt (1984) believed few pairs bred in Bahamas, with nesting confirmed from only a handful of sites between 1967 and 1981: Great Inagua (12–20), Schooner Cays off Eleuthera I. (9), South Rocks off Conception I. (14), with 75 on South Cay southeast of Andros I. largest known colony. We believe that until simultaneous aerial survey censuses are done, preferably by helicopter, true breeding population for “greater” Bahamas and elsewhere in West Indies can only be glimpsed dimly. For example, Buden (1987) noted regular breeding on Little Water Cay, Caicos I., in the 1970s.

Van Halewyn and Norton (1984) estimated <1,400 pairs as then-current West Indian breeding population. Only colonies of consequence 1,000 at unnamed island(s) off ne. Yucatán (probably on Campeche Bank [Howell and Webb 1995] and possibly Cayos Arcas and Alacran Reef [Paynter 1955]); <500 discovered in late 1970s on Battures des Malmanoury, isolated islet off French Guiana (Tostain et al. 1992); 155 on Culebra (east of Puerto Rico); and 100 on Las Aves off Venezuela. Perhaps another 100 pairs thinly scattered throughout West Indies at only 15 sites between 1950 and 1983, and van Halewyn and Norton (1984: 183) describe Royal Tern breeding in Caribbean as “irregular in location, year, and number of pairs.”

Chardine et al. (2000) added a few more Bahamas sites (Cat I., Rum Cay, Hawksbill Cay), Jamaica (Pedro Cays), and some specific sites in U.S. Virgin Is. (Dog Cays, Flat Cays, Pelican Cay, Turtledove) and Anguilla (Anguillita and Scrub Is.) but did not materially change van Halewyn and Norton’s (1984) assessment. Chardine et al. did estimate current West Indian breeding population at 450–800 pairs, down from van Halewyn and Norton’s <1,400. However, Chardine et al. miscopied size of French Guiana colony as “100 pairs,” not about 500, so their revised total should read 850–1,200 pairs, still down from 1984 numbers, but within sampling error.

Only other known area of boreal breeding S. m. maxima is in Gulf of California (= Sea of Cortez), where Everett and Anderson (1991) estimated 8,000–10,000 pairs on main site, I. Rasa. Known to have bred at only 4 other locations in Baja California: Laguna Ojo de Liebre; I. San Roque; I. San Jorge; Is. Ballena in Laguna San Ignacio; and new site, Montague I. (Palacios and Mellink 1993). Possibly other sites used in Gulf of California, and Everett and Anderson (1991) offered no estimate for entire area’s breeding population; 10,000 pairs safe minimum, but without doubt I. Rasa colony normally largest in area, probably by orders of magnitude, and most important one. Coincidentally, Rasa also supports largest known Elegant Tern colony in world (Burness et al. 1999) and one of largest of Heermann’s Gull (L. heermanni) colonies, so global importance obvious. Additional breeding sites (colony size and occupation data lacking) reported from other locations in Sonora and Sinaloa (Friedmann et al. 1950, 1957), Is. Tres Marias. (I. Isabel; Am. Ornithol. Union 1998), and, apparently overlooked by almost everyone, on Caribbean islet called Fowl Key in Cayos Cochinos off Honduras (Monroe 1968). Breeding probable in similar locations along Belize coast (suggested by Howell and Webb 1995), and perhaps elsewhere in w. Caribbean.

Austral S. m. maxima known with certainty to breed at few locations. For years suspected of breeding in area between coastal Brazil and Patagonia, but not until 1969 that first nests found in Patagonia’s Chubut Province. As recently as mid-1980s (Escalante 1985), breeding still unproven for Uruguay and Brazil, and although strongly suspected, some instances perhaps only immediately postfledging juveniles accompanied and fed by parents. However, breeding finally demonstrated in Brazil (Olmos et al. 1995), and many believe species could be widespread along largely unknown barrier beaches of several se. Brazilian states. No estimates for population and colony sizes for entire austral S. m. maxima population.

Data on S. m. albididorsalis breeding sites and populations lacking, beyond reference to areas where known or suspected breeders.

Trends

Generally, boreal S. m. maxima populations holding constant, although 1988–1990 decline of 33% reported in N. Carolina (N. Carolina colonial waterbird database unpubl.). In West Indies, no detectable population change, although data sparse (Chardine et al. 2000). Few systematic, useful population data from most parts of range, and some ‘declines’ probably only untracked shifts between colony sites, or temporary movements out of area.

No information beyond likelihood that extremely extended parental care of (normally single) juvenile well into its second calendar year (Ashmole and Tovar 1968, Buckley and Buckley 1974) and single-egg clutches indicate slow development of skills needed for difficult prey capture. Frequent washouts of low-lying colonies often result in re-laying, at same site or new one. Colonies deserted easily in egg stage, but not later. Suitable habitat on east coast of North America limiting, with most now being provided by dredge-spoil islands. Adult mortality probably not limiting, but that of young in first winter may be (see Causes of mortality, above). Food probably not limiting, but colonies need sites on isolated islands, free of mammalian predators, especially at/near inlets/passes with high fish productivity.