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Royal Tern
Thalasseus maximus
Order
CHARADRIIFORMES
– Family
LARIDAE
Authors: Buckley, P. A., and Francine G. Buckley

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Food Habits

Adult Royal Tern, non-breeding plumage; Florida, October

Feeding

Main Foods Taken

Fish, augmented by crustaceans, particularly shrimp. Prey items eaten by adults presumed similar to those fed chicks in colonies. Great variability in relative proportions of taxa taken associated with seasonal, oceanographic, climatic, weather, stochastic, and zoogeographic factors, such that rarely do any 2 studies report same results of diet analysis. See Breeding: young birds, below. Comments apply to boreal S. m. maxima, unless otherwise stated.

Microhabitat For Foraging

Forages over shallow (≤10–20 m; PAB) inshore salt water, singly or in groups of 2 or 3, but if large schools encountered, hundreds may gather in single feeding flock. Makes feeding flights up to 100 m from shore, parallel to beach. Forages heavily in back bays and lagoons and tidal creeks, less than on open nearshore ocean. May fly considerable distances from colony when foraging. Freshwater feeding almost unknown unless blown inland by storms. Frequency of diving diminished when water turbid. In plume of Mississippi River, among other terns whose presence correlated with lower salinity (presumably from freshwater inflow) and higher chlorophyll- a concentrations (Ribic et al. 1997).

Food Capture And Consumption

Feeds mainly by plunge-diving from 5 to 10 m into upper 1–2 m of water column; feeds by day and by night (Buckley and Buckley 1972a, for example, noting occurrence of Loligo squid, probably captured at night in Virginia–N. Carolina colonies), but nocturnal feeding unstudied. Rarely eats carrion along shore. Some fish, but especially small molting blue crabs (Callinectes sapidus), picked from surface of water in oblique, shallow dives; technique also used in tropical waters to catch flying fish (Exocoetidae). Prey carried singly crosswise in bill. Kleptoparasitism on adults bringing fish to chicks locally common. Adults significantly more adroit, selective, and successful per unit time foraging than juveniles: adults hovered precisely, plunged neatly and rapidly, and left water quickly, while juveniles circled hesitatingly over and over and made frequent intention movements to dive before actually plunging—often obliquely, sometime even falling onto water’s surface (Buckley and Buckley 1974). Erwin (1977) reported mean time between foraging bouts in Chesapeake Bay, VA, adults of 2.5 h, while those in Cape Fear River in N. Carolina fed every 46 min. Older adults had fewest bouts/d but with longest duration, foraging least during low and flood tides than did younger adults, but only in 1 of 2 study years (Wambach 2000, E. Wambach and S. Emslie unpubl.)—typical of high variance in food studies. Erwin (1977) also noted each adult in Virginia bringing in 17 prey items (averaging 15.4 g)/100 min, and after subtracting portions eaten by adults, chicks received on average 3.6 prey items/10 h.

Kleptoparasitism

Successful intraspecific kleptoparasitism not uncommon between boreal S. m. maxima adults returning to colony with prey in bills or by large chicks stealing food from smaller chicks or even from returning adults (sometimes by lunging at birds flying low overhead). Generally, especially as juveniles improve flying abilities, Royal Terns outfly most kleptoparasites, including conspecifics. On wintering grounds, Magnificent Frigatebirds (Fregata magnificens) harass adults and young, but only very rarely successful with juveniles. Austral S. m. maxima, by contrast, routinely preyed on by Kelp Gulls: thievery related to tern chick’s age (hence, size of prey brought in), being highest in chicks >20 d old at edge of colony. Successful attempts (defined as removal of even part of fish from tern adult or chick’s bill) ranged between 10 and 12% in 2 yr of study, while robbing efficiency (defined as number of successful attacks/total robbing attempts) ranged between 54 and 58%. Nonetheless, observers unable to ascertain if these levels of pressure adversely affected productivity owing to overall low chick mortality in colonies from starvation during study years (Quintana and Yorio 1999; see also Breeding: immature stage, below).

Diet

Generally opportunistic forager, nearly all studies commenting on lack of dietary consistency from year to year or even colony to colony within same year. Given that plasticity, surprising that numbers in California apparently tightly tied to those of Pacific sardine (Sardinops sagax); when that fish’s populations crashed in early 1950s, Royal Tern all but disappeared. Even more interesting, at same time that sardines were being replaced by northern anchovy (Engraulis mordax), Elegant Terns moved in, switching abundance completely with Royals, latter becoming rare, while former becoming numerous and regular breeder in s. California (Schaffner 1986). That situation persists today (Burness et al. 1999).

Chick diets: In Virginia–n. North Carolina, fishes in 5- to 10-cm range predominated, as in S. Carolina, where more tropical fish taxa occur. Dominant fishes include silversides (Menidia spp.), anchovies (Engraulidae), menhaden (Brevoortia spp.), and a variety of others according to seasonal and habitat availability (Buckley and Buckley 1972a, Blus et al. 1979, McGinnis and Emslie 2001). In s. North Carolina, chicks by 14 d old fed small (<1 bill length [ca. 64 mm]) fishes, but by weeks 6–8, larger fishes plus shrimp and crabs (Wambach 2000). Prey varied across breeding season and between years, opportunism likely determining much of prey composition, but anchovies accounting for >75%. In Virginia, mean prey capture/min 0.17 ± 0.11 SD and mean mass of prey 15.4 g (Erwin 1977). Also in s. North Carolina (same area as Wambach’s [2000] study), chicks fed 41% sciaenids/sparids/ Mugil, 30% clupeids/carangids/ scombrids, and 9% engraulids (McGinnis and Emslie 2001). On Pacific Coast, Schaffner (1985) suggested that recent breeding of boreal S. m. maxima in California might reflect changes in relative abundance of its preferred sardine prey, or perhaps a switch in its diet to other recently available inshore sardine analogs such as topsmelt (Atherinops) or even to northern anchovy—so favored by Elegant Terns there. No data on winter diet.

In Argentina, these findings mirrored with austral S. m. maxima, especially shifts to local analog species and great dietary variability: in 1 yr, 65% of diet came from engraulids (30% the following year, when clupeids increased to 40%). Other local fishes (bluefish [Pomatomus spp.], butterfish [Stromateidae], and drums [Sciaenidae]) filled in the balance, doubtless also opportunistically (Escalante 1985, Quintana and Yorio 1997).

Nutrition And Energetics

No information.

Metabolism And Temperature Regulation

Boreal S. m. maxima’s basic metabolic rate: 217.3 kJ/d (sample of 3 birds; Ellis 1984).

Drinking, Pellet-Casting, And Defecation

Drinks salt water from surface of water by skimming (Buckley and Hailman 1970). Often excretes salt from salt glands while incubating. Adults and young defecate on nest, giving colony conspicuous whitewashed appearance and a strong odor of ammonia. Pellets of fish scales/bones regurgitated around nest.