Behavior

Walking, Hopping, Climbing, Etc

Stands and walks on territories; runs or flies at territorial intruders. Chicks run away from intruders or predators; fledglings and adults usually fly when rapid movement necessary. Perches on rocks, posts, rails, or boats within and above nesting or feeding territories. Frequently stands on buoys or other floating objects (e.g., on kelp beds in British Columbia; Campbell et al. 1990).

Flight

Buoyant in air, body rising slightly with each downbeat of wings. Capable of taking off vertically even in still air. Agile, makes rapid turns and swoops during aerial pursuits, or when catching flying insects; frequently hovers over nesting territory, or over schools of fish. When commuting with fish, flies within 60 cm of surface against head wind, but 10–30 m above surface with following wind. Rarely flies higher than 100 m except when migrating (Alerstam 1985). Mean airspeed 8.4 m/s (30 km/h) when flying downwind or crosswind, 12.2 m/s upwind (n = 26; Wakeling and Hodgson 1992), 12–15 m/s when migrating at night (n = 17 flocks; Alerstam 1985).

Swimming And Diving

Aerial plunge-diving is main method of food capture; hovers or holds position against wind until suitable prey is located, then swoops down, either leveling out to pick prey from surface, immersing head and part of body, or folding wings to penetrate up to 50 cm below surface (Abb. 117 in Glutz von Blotzheim and Bauer 1982). Does not dive from surface. In breeding areas, only occasionally lands on water, except to bathe. In winter quarters and on migration, occasional records of flocks resting on water (Sinclair 1982).

Preening, Head-Scratching, Bathing, Anting, Etc

At breeding colonies, spends much time bathing in shallow water close to shore, often forming compact flocks with conspecifics and other terns. Splashes into water from air, dips head below surface, shakes wings and ruffles body-feathers; after 1–2 min vigorous bathing, flies to shore and preens. For detailed analysis of preening, including relationships among preening movements and between preening and other activities, see van Iersel and Bol 1958 . Head-scratching direct. Sometimes scratches head or preens body-feathers in flight. Anting not reported, although ants abundant at some breeding sites. Allopreens only during incubation: off-duty bird frequently preens outer wing- and tail-feathers of sitting mate (ICTN).

Sleeping, Roosting, Sunbathing

Sleeps with bill tucked into scapulars. Occasionally sleeps by day while standing in resting flocks, or while incubating eggs; nocturnal sleep not described. Female usually incubates or broods at night (see Breeding: incubation, below); male rests nearby or at edge of colony. During daytime, off-duty birds sometimes rest standing in nest territory, more frequently in club at edge of colony, usually close to water line. Away from breeding sites, commonly rests in flocks of hundreds or thousands on islands, beaches or sand flats, often at end of barrier beaches adjacent to inlets. Staging birds around Cape Cod, MA, rest by day at about 20 sites, but roost in only one or two major concentrations, to which they fly ≥50 km (Trull et al. 1999). Birds wintering in Trinidad rest by day in flocks on beaches, boats, jetties, or pilings, roosting mainly on platforms or boats at anchor (Blokpoel et al. 1982, 1984; Morris et al. 1982; Erwin et al. 1986). Birds wintering in Guyana and Brazil roost on mudflats, sand bars or barrier beaches, arriving after dark and leaving before dawn (Trull 1984, Hays et al. 1999). Sunbathing not reported; does not seek shade even in hottest part of day.

Daily Time Budget

No detailed studies in North America. In Lake Ontario, time spent foraging by adults feeding chicks estimated as 8.0 h/d (Moore 2001), or 7.1 h/d for birds with 1 chick, increasing to 10.2 h/d for birds with 4 chicks (Courtney and Blokpoel 1980). At Bird I., MA, males feeding small chicks brought 45–65% of feeds during first 5 of 16 h of daylight (Galbraith et al. 1999). At Port Colborne, Lake Ontario, rates of mate-feeding significantly lower in afternoon (11:30–15:30) than in early morning or evening (Morris 1986). At West End Beach, NY, food delivery rates highest in morning and evening (Burger and Gochfeld 1991). Types of prey brought to chicks sometimes vary during day (Safina et al. 1990, Burness et al. 1994), without major changes in feeding rates. At West End Beach, NY, food delivery rates highest during 2 h prior to high tide (Burger and Gochfeld 1991). Otherwise, no published studies of relationship between chick feeding and tidal cycles in North America, although foraging behavior is known to depend on tidal currents outside breeding season (Braune and Gaskin 1982a).

Physical Interactions

Occasional intense fights resulting from intrusions into nesting territories; fighting birds (usually males) fence with bills, or hold each others’ bills and wrestle; one bird may seize the other’s neck, wing, or tail. Bird returning to territory and finding intruder present will dive from air and peck intruder in back or head. Adults rarely injured in fights, but 2 cases of fatal stab wounds in back, puncturing thorax or lung (Palmer 1941, ICTN). Parents also attack adults or chicks that attempt to steal fish from their young.

Adults vigorously attack alien chicks that wander into their territories, or approach their nests. Females more prone than males to attack alien chicks (J. Schwerin). Aggressive behavior suppressed when parents own chicks are hatching; parents then may accept, and adopt an alien chick that they find in their nest (see Breeding: brood parasitism, below).

Adults respond aggressively to any tern exhibiting abnormal behavior, e.g., when injured, tangled in vegetation, or struggling in water. Adults hover in flock over stricken bird, giving alarm calls and diving on it, sometimes pecking it. This behavior exploited in nineteenth century by collectors and market-gunners, who sometimes killed dozens or hundreds hovering over an injured bird in water (Brewster 1879, Bent 1921).

Communicative Behavior

Displays described and illustrated by Cramp (1985), based largely on ethological studies by Cullen (1956, 1960). Assertive or aggressive displays usually feature Bent posture: head bent down with bill towards ground, wings open and partially spread, tail raised. In aggressive interactions, bird faces antagonist, presenting black cap; in courtship, bird faces to one side, turning cap away from mate. Corresponding appeasement display is Erect posture: bird stretches neck and points bill upwards, twisting neck so that black cap is concealed from opposing bird. Bent posture used in low-intensity defense of nesting or feeding territories. When intruders approach closer, territory owner adopts Slant posture, with neck stretched forwards and upwards, bill slightly raised, carpal joints slightly open; this often alternates with Bowing, with head tilted downwards and with loud Advertising calls. When pair defends territory together, especially when confronting another pair, Slant and Bowing are elaborated and synchronized. In High Intensity Threat, standing male raises and spreads wings, raises head, and gives loud kee-arrr call; often followed by attack if opponent does not withdraw. Except for coordinated Slant/Bowing by pairs, male does most territorial defense. During incubation, males responded with threat displays and even physical attacks to mirror placed 20 cm from nest (i.e., to unfamiliar individual 40 cm away); females rarely did so (Pearson 1996).

Two birds contesting airspace sometimes perform Upward Flutter, stereotyped aggressive display in which both birds flutter vertically upwards up to 100 m, lower bird with neck twisted and bill pointing up, upper bird with neck arched and bill pointing down. Males defending feeding territories usually respond to all intruders who approach territory, even when 100–200 m away. Male flies down from perch and low over water directly at intruder, with deep wing-beats; intruder usually turns immediately and flees, pursued from behind and below (Nisbet 1983d).

Territoriality

Nesting territory selected initially by male soon after first arrival, then used for courtship display. Territory defended intermittently against birds that occupy neighboring sites; final boundaries not established until egg-laying or even later. Territory then defended vigorously against neighbors, intruders, and wandering chicks. Chicks fed in parents’ territory until fledging and sporadically for 10–20 d thereafter (Nisbet 1976). Pairs usually return to same site in successive years if both survive and return synchronously (Austin 1949, González-Solís et al. 1999d).

Territory size and nest spacing widely variable depending on local numbers, available space, and substrate. Mean density typically 0.06–0.5 nests/m²(Austin 1929, Nisbet and Drury 1972, Maxwell and Smith 1983, Richards and Morris 1984, Storey 1987a), but as high as 3.1 nests/m²on an artificial platform (Hatch 2001); at Bermuda, pairs are spaced over wide area of Hamilton Sound, each pair defending single islet (D. B. Wingate). Nests often aggregated within available area because of preference for nesting at edges of patches of vegetation (Austin 1929, Marples and Marples 1934). Mean nest-spacing (nearest-neighbor distances) 87–514 cm (n = 55 colonies) in New York, New Jersey, Maryland, and Virginia, declining from about 300 cm in colonies of <30 pairs to about 100 cm in colonies of >300 pairs (Erwin and Smith 1985, Safina et al. 1989, Burger and Gochfeld 1991). At Los Roques, Venezuela, nests clustered with mean nearest-neighbor distances 2–5 m within clusters (LeCroy 1976).

In some areas, pairs defend linear feeding territories of 150–250 m along shoreline (Nisbet 1983d, Kirkham 1986). Territories require perches such as rocks, posts, docks, or boats. Territories defended throughout breeding season, but used most consistently in pre-laying and incubation periods. One male at Bird I., MA, held same feeding territory for at least 7 yr (ICTN). No evidence for defense of territories in winter quarters, but some transitorily defend feeding locations during post-breeding period (Reed et al. 1982).

Individual Distance

Not precisely measured, but about 30–40 cm (ICTN). Possibly larger in winter quarters when perching on boats or cables (see photographs in Morris et al. 1982, Trull 1984).

Mating System And Sex Ratio

Socially monogamous with biparental care. Trios and female-female pairs recorded, but apparently rare (ICTN). No precise data on sex ratio, but equal numbers of males and females among oldest birds and no greater tendency for old males to be mated to younger females than vice versa (Nisbet et al. in press). Unmated males conspicuous throughout breeding season at some sites, but this may reflect greater activity or earlier arrival of males rather than greater numbers. Supernormal clutches of 4–7 eggs (indicative of female-biased sex ratios and multi-female nesting associations in other larids) infrequent (0–5%) in North American colonies at present day, but more prevalent (up to 10%) at times in past, especially during 1890–1920 (see Breeding: eggs, below).

Courtship Displays And Mate-Guarding

Displays described and illustrated by Cramp (1985), based largely on ethological studies by Cullen (1956, 1960). Much early courtship and mate selection occurs in aerial displays. In Low Flight, male carries fish low over colony, followed by one or more females; in High Flight, male and female ascend in wide circles to altitude of ≥100 m. Both feature Pass (female passes low over male in aerial equivalent of Erect posture) and Glide (both birds descend together in shallow, zigzag glide).

Ground displays include Greeting Ceremony (female in Erect posture and male in Bent posture), Parade (male walks in circles around female or in arcs ahead of her), and Scraping (one or both birds scratch hollows in substrate with feet). These behaviors may be repeated with several different partners, but over 3–8 d pairs become established and female returns repeatedly to male’s territory, with displays becoming more coordinated.

Early in courtship period, male carrying fish may attract one or more females who beg for it, either in Erect or Hunched Food-begging postures. Male may approach female and proffer fish, but withdraws it when female approaches; female may snatch fish and fly off with it. At this stage, some males masquerade as females to obtain fish, which they then use in display (ICTN). As potential mates become familiar with each other, male becomes more willing to relinquish fish; when pair fully established, male starts to bring fish regularly and feed female (Courtship-feeding, or Mate-feeding). At this point, pair starts to spend most of day on feeding grounds, often in male’s feeding territory, where female rests on rock or other perch while male fishes and carries food to her (Nisbet 1977, Kirkham 1986). This behavior lasts for 5–10 d (Honeymoon Period); male’s close attendance on female at this time probably functions as mate-guarding. Female catches little or no food for herself during this period, but body mass increases from about 120 g to about 180 g during 20-d period prior to egg-laying (Wendeln and Becker 1996, Wendeln 1997b). Between 8 h and 4 d before egg-laying, female returns to nesting territory and then spends all her time there, leaving only to drink and bathe, while male catches and brings prey (Nisbet 1973a, 1977). After laying first egg (and ovulating second), some females solicit food from other males and may accept food and/or copulate with them (see Extra-pair copulations, below). Mate-feeding declines abruptly after last egg is laid (Nisbet 1973a, Morris 1986, Wiggins and Morris 1988, Blanchard and Morris 1998, Moore et al. 2000), but male occasionally brings fish to nest during incubation and female sometimes takes them.

Copulation

Males start to give Precopulatory display as soon as females visit them in nesting territories, but females rarely accept mounting until pair is well established. Male approaches female with breast extended, head raised and drawn back; then walks in decreasing arcs in front of her. If unreceptive, female walks away or turns away; if receptive, she stops and adopts Hunched Food-begging posture. After slow approach, male flutters onto back of female. Male may remain mounted for 30–60 s, giving Copulation calls, before starting to wag tail from side to side, then bends tail to one side under female’s to make cloacal contact, spreading or flapping wings to maintain balance. If undisturbed, male may remain mounted and attempt cloacal contact up to 10 times over 2–3 min period. Copulations occur from about 10 d prior to first laying, peaking between 3 d before first laying and 2 d after, declining abruptly after second egg is laid (Cullen 1956, Morris 1986, Wiggins and Morris 1988, Blanchard and Morris 1998, ICTN). Wiggins and Morris (1988) reported peak frequency as about 0.2 copulations/h (3/pair/d, many with multiple cloacal contacts), but Gochfeld (1980a) reported mean frequency as 7.2/pair/d during week before egg-laying. No close temporal relationship between mate-feedings and copulations (Wiggins and Morris 1988, Blanchard and Morris 1998). Most copulations occur on nesting or feeding territories, but some in clubs adjoining colony. Interference with copulation infrequent (<10%; Burger and Gochfeld 1991).

Unmated males frequently perform Precopulatory display with mated females and occasionally attempt forced copulations, but these rarely, if ever, successful. Unmated males sometimes land near unattended chicks, who crouch to avoid attack; males then may attempt to copulate with the chicks, apparently mistaking crouching of chicks for receptive crouching of females (ICTN).

Pair Bond

Mate fidelity high (Austin 1947), but no precise data from North America. In study in Germany, 66% of pair bonds retained from year to year; 18% broken by death and 15% by divorce, the latter associated with late arrival of 1 partner (González-Solís et al. 1999c, 1999d). Birds that lose eggs or chicks and renest in same season invariably retain same mates (González-Solís et al. 1999d). Longest documented pair bond 14 yr (ICTN). Pairing strongly age-assortative (Nisbet et al. 1984, González-Solís et al. 1999d). At Bird I., MA, most birds aged 3–9 yr had mates within 1 yr, birds aged 10–14 yr had mates within 2 yr, and birds aged 18–23 yr had mates within 5 yr (Nisbet et al. 1984, in press).

Extra-Pair Copulations

Some males solicit extra-pair copulations away from nest and a few females accept them, usually in exchange for fish during egg-laying (ICTN). No information on extra-pair paternity.

Degree Of Sociality

Gregarious throughout year. Breeds in colonies typically with tens or hundreds of pairs, but ranging from a few (rarely singly) to thousands (Burger and Gochfeld 1991); a few coastal colonies have exceeded 10,000 pairs (see Demography and populations: population status, below). Freshwater breeders feed singly or in small scattered groups and coastal breeders often do so, but coastal breeders also feed in flocks of tens to hundreds of birds over predatory fish or other features that make schooling prey available near surface (Duffy 1986, Safina 1990). Degree of sociality tempered by defense of nesting territories and feeding territories (see Spacing, above), but breeders re-aggregate in flocks for resting, roosting, and bathing.

Breeders perform 2 kinds of synchronized flight activity (Marples and Marples 1934, Palmer 1941, Cullen 1956). In upflights, many or all birds in a breed-ing colony rise simultaneously into the air, giving alarm or advertising calls. In panics or dreads, many or all birds suddenly fall silent and fly rapidly away from nesting area low over the water, rising and starting to call after 5–30 s. Panics occur many times/d in early stages of settlement in spring, but gradually become less frequent as season progresses (Marples and Marples 1934, Cullen 1956, Morris and Wiggins 1986). Upflights and panics sometimes occur in response to non-threatening and threatening predators, respectively (see Predation: response to predators, below), but most have no obvious external stimuli; best interpreted as false alarms, resulting from fear of potential predators when on land and tendency to respond immediately to sudden movements by neighbors (Cullen 1956).

Staging, migrating, and wintering birds rest and roost in flocks up to 20,000 birds on open beaches and sand flats (see Habitat, above). Commonly feed in flocks (Braune and Gaskin 1982a, Blokpoel et al. 1984, Trull 1984, Erwin et al. 1986).

Play

Juveniles and adults occasionally perform series of swooping dives, pulling out just before hitting water. Juveniles learning to fish commonly pick up seaweed, sticks, etc., then drop them and occasionally catch them in air (ICTN).

Nonpredatory Interspecific Interactions

Forms mixed-breeding colonies with Arctic and Roseate Terns and Black Skimmers (Rynchops niger), less frequently with terns that are more segregated by habitat (Andrews 1990, Burger and Gochfeld 1991). Coastal colonies often adjacent to breeding Laughing Gulls; lake colonies often adjacent to breeding Ring-billed Gulls (Larus delawarensis). These and species such as Herring Gulls (L. argentatus) and Double-crested Cormorants are dominant over terns and often displace them from breeding areas as numbers of larger species expand (Drury 1973–1974, Nisbet 1973b, Kress et al. 1983, Cuthbert and Timmerman 2001). Terns and larger species co-exist, however, if islands are large enough to provide segregated breeding space, as in Lake Winnipeg (W. Koonz). In s. Caribbean, forms mixed colonies with Bridled (Sterna anaethetus) and other terns (Voous 1957, LeCroy 1976). Common Terns defend nesting territories, but not feeding territories, against other tern species (Kirkham 1986, ICTN). Ducks and shorebirds sometimes nest within Common Tern colonies and may gain benefits (e.g., defense against predators) from doing so (Vermeer 1968, Young and Titman 1986, Alberico et al. 1991).

Forms mixed feeding flocks with Arctic and Roseate Terns in the breeding season and during post-breeding dispersal (Braune and Gaskin 1982a, Duffy 1986, Safina 1990). No information on feeding associations in South America.

Kleptoparasitized by jaegers (Stercorarius spp.; Furness 1978, Bélisle 1998), Laughing Gulls (Hatch 1975), and Roseate Terns (Dunn 1973b), as well as conspecifics (see Food habits: food capture and consumption, above). Common Terns frequently steal fish from Arctic (Hopkins and Wiley 1972) and Least (ICTN) Terns. Coastal birds often feed over predatory fish such as striped bass (Morone saxatilis) and bluefish (Safina and Burger 1988). For feeding associations with diving birds and marine mammals, see Pierotti 1988; also see Food habits: flock size and foraging associations, above. Staging, migrating, and wintering birds rest and roost in mixed flocks with other terns and Black Skimmers (Harrington et al. 1986, Hays et al. 1997, Trull et al. 1999).

At breeding colonies, respond aggressively to species that are usually not egg or chick predators, including Osprey (Pandion haliaetus), Canada Goose (Branta canadensis), Rock Dove (Columba livia), common muskrat (Ondata zibethicus), and diamondbacked terrapin (Malaclemys terrapene); also to marine mammals such as dolphins (Stenella spp.) and seals (Phoca spp.) that swim close to breeding sites (Burger and Gochfeld 1991, ICTN).

Like other ground-nesting terns, vulnerable to predation: more than 30 bird species, 13 mammal species, and several reptiles and invertebrates recorded as predators in North America.

Kinds Of Predators; Manner Of Predation

In most or all cases, predation by one or a few specialist individual predators (Spretke 1998); hence, incidence of predation variable and sporadic. Most mammalian predators take eggs or chicks at night, following scent from nest to nest; a few take adults if these remain at nests (Austin 1948, Burger and Gochfeld 1991, Burness and Morris 1993). Norway rats (Rattus norvegicus) sometimes roll large numbers of eggs into caches under boards or bushes (Austin 1948); mustelids may kill many chicks and fledglings (Burger and Gochfeld 1991, ICTN); several records of Great Horned Owls (Bubo virginianus) killing hundreds of adults (Floyd 1925, Austin 1940); other predators rarely indulge in surplus killing. Great Horned and Short-eared (Asio flammeus) Owls take adults until these are no longer available, then take small and large chicks (Nisbet 1975, Nisbet and Welton 1984, Holt 1994). Black-crowned Night-Herons (Nycticorax nycticorax) primarily take pipped eggs and small chicks (Hunter and Morris 1976, Nisbet and Welton 1984, Shealer and Kress 1991). Herring and Great Black-backed (Larus marinus) Gulls rarely take eggs, but occasionally take numbers of chicks prior to fledging; they sometimes take fledglings, chasing them until they fall into water (Houde 1977, Whittam and Leonard 2000, ICTN). Peregrine (Falco peregrinus) and other falcons kill adults in the air, or take newly-flying fledglings (Nisbet 1992; J. G. T. Anderson). Ruddy Turnstones (Arenaria interpres) scavenge dead eggs; also sometimes take live eggs, walking among nests and taking eggs when unattended (Parkes et al. 1971, Farraway et al. 1986, Morris and Wiggins 1986). Ants (Lasius neoniger and Solenopsis molesta) enter pipped eggs and kill or blind chicks (Safina et al. 1994). Garter snakes (Thamnophis sirtalis) took chicks (Lazell and Nisbet 1972), as did land crabs (Geocarcinus lateralis; Voous 1957, D. B. Wingate).

Response To Predators

Mammals such as red fox (Vulpes vulpes), mink (Mustela vison), or Norway rat generally eliminate colonies established on mainland, and hence limit Common Terns to nesting on islands. At island sites, most important predators are nocturnal birds. In response to predation by Great Horned Owls, usually desert colonies at night for up to 8 h, resulting in deaths of chicks from exposure and enhanced predation by Black-crowned Night-Herons and ants (Nisbet and Welton 1984, Hébert 1985, ICTN). Duration of desertion events varies among individuals from a few minutes to all night, depending on stage of nesting cycle, proximity and activity of owl; apparently reflects and is adjusted to degree of threat to parents (J. M. Arnold). Desertion continues after owl is removed or ceases visits, gradually waning over period of 5–10 nights as terns learn of reduced threat (ICTN). Similar behavior described in response to predation by Short-eared Owls in U.S. and Germany, and by Long-eared Owls (Asio otus) in Germany (Holt 1994, Wendeln and Becker 1999b). Nocturnal predation by Great Horned Owls also caused inattentiveness to eggs by day, hence promoting diurnal predation by Ruddy Turnstones (Morris and Wiggins 1986). Nocturnal predation by Black-crowned Night-Herons sometimes leads to colony desertion by terns (Hunter and Morris 1976, Shealer and Kress 1991); at other times Common Terns attack herons and are able to limit effects of predation (Marshall 1942, ICTN). Shields and Townsend (1985) reported that a feral cat caused nocturnal desertion and nest abandonment. Otherwise, no information on behavioral responses to nocturnal predation by mammals, but high losses of eggs and chicks (Austin 1948, Shields and Townsend 1985, Burger and Gochfeld 1991, ICTN) suggest that Common Terns are unable to defend against such predation.

Responses to diurnal predators vary markedly, depending on identity and behavior of predator, stage in nesting cycle, and degree of habituation. In response to hunting Peregrine Falcon, Common Terns initially flee low over water (panics; see Social and interspecific behavior, above) and may evacuate colony site for ≥30 min (ICTN). Presence of hunting Peregrine disrupted well-established colony and delayed occupation by 8–12 d (Nisbet 1992). Merlins (Falco columbarius) and non-hunting Peregrines, however, cause upflights and a few Common Terns pursue and attack them, diving from above and behind (ICTN). Many other diurnal predators mobbed similarly, including Northern Harriers (Circus cyaneus), Short-eared Owls, American Crows (Corvus brachyrhynchos), Herring and Great Black-backed Gulls, and Great Blue Herons (Ardea herodias; Nisbet and Welton 1984, Burger and Gochfeld 1991, Whittam and Leonard 2000). Gulls that fly overhead in transit to other locations are usually ignored, but gulls that are hunting or approach low over the ground are attacked vigorously (Cavanagh and Griffin 1993, Whittam and Leonard 2000). American Crows usually driven away, but Northern Harriers and specialist gulls sometimes persist and are able to take chicks at will. Muskrats, snakes, and domestic dogs (Canis canis) are followed by clouds of terns which hang overhead, giving Alarm calls (ICTN). Common Terns attack many non-threatening species such as doves, shorebirds, and egrets, but do not respond aggressively to Ruddy Turnstones or other shorebirds that walk among their nests, even when turnstones take eggs (Morris and Wiggins 1986). Responses to land-crabs and ants not reported; appear able to keep ants away from pipped eggs in absence of owl predation (Nisbet and Welton 1984).

Common Terns frequently attack human intruders, diving from behind, pecking their heads, and defecating on them (Burger and Gochfeld 1991, Nisbet 2000). Birds with little experience of humans initially circle high overhead, giving Alarm calls, but approach closer and press home attacks as they become accustomed to visitors. Intensity of attacks increases during incubation and peaks at time of hatching of earliest eggs, waning rapidly as chicks of earliest breeders grow beyond first few days of life (ICTN). Adults discriminate among individual humans, attacking familiar individuals more intensely than strangers (Burger et al. 1993). Attacks on humans more intense, but attacks/individual less frequent in larger or denser colonies (Erwin 1988, Burger and Gochfeld 1991).

In response to adults’ Alarm calls, chicks 0–3 d old crouch motionless in or near nest; chicks >3 d run to cover and crouch under vegetation or other objects if available. Chicks >15 d may run 20–50 m away from human intruders and hide in dense cover. Behavioral camouflage also includes spacing of nests, cryptic coloration of eggs and chicks, dispersal of feces, and removal of hatched eggshells; no systematic studies of effectiveness or functions of these traits.

Most important anti-predator adaptation probably preference for nesting on islands, especially those lacking predatory mammals or reptiles. Islands far offshore generally have fewest predators, but may be less suitable for other reasons such as weather or distance from foraging areas (Hall 1999). In response to predation on adults, or heavy predation on eggs early in season, Common Terns often desert breeding sites and move en masse to alternative sites (Austin 1946, 1951; Gochfeld 1979a; Kress 1997). At other sites, however, they persist for many years despite very low breeding success owing to chronic predation or other factors (e.g., Burger and Gochfeld 1991, Harper and Harper 1997). Reasons for variation in response may be related to perceived threat to adults themselves, and/or availability of alternative sites. During twentieth century, many major colony-sites occupied for several decades, but few, if any, occupied continuously for 100 yr (sources in Appendix 1).