Food Habits

Main Foods Taken

Data from pellet and stomach content analyses, and observations of fish dropped at nests and prey transfers from adults to mates and chicks, suggest that Caspian Tern’s diet consists primarily or exclusively of fish; occasionally takes crayfish and insects (FJC).

Microhabitat For Foraging

Typically fishes along coasts, shorelines, inland lakes, rivers, lagoons, estuaries, and sloughs; less commonly on open sea (Baltz et al. 1979). Flies at heights ranging from 3 to 30 m over shallow (0.5–5 m) or sometimes deeper waters, often within ≤100 m of shore, but sometimes much farther out (Bergman 1953, Ferguson-Lees 1971, Whitfield and Blaber 1978, Bijlsma 1985; see Demography and populations: range, below). Terns radio-tracked in a freshwater environment in Ontario did not show predictable or shared for-aging patterns or fidelity to specific foraging areas (Sirdevan and Quinn 1997). On coast of s. Finland, however, fishing areas usually changed according to spawning places and migrations of fishes (Bergman 1953).

Food Capture And Consumption

From Bergman 1953, Ferguson-Lees 1971, and Whitfield and Blaber 1978, except where noted. Circles slowly with bill pointing down; sights fish, hovers, and usually completely submerges; in shallow water, may dive with little or no submergence (Suchantke 1960). Resurfaces 1–2 s after dive, flies toward breeding place with faster wing-beats; may swallow first fish and bring only second or third back to breeding place. At Lake St. Lucia, South Africa, where terns were observed throughout 1975–1976, fishing activity peaked in early morning, and all fish captured were swallowed on the wing. During Baltic breeding season, fished from dawn until dusk, occasionally into deep dusk if fish were spawning. At Grays Harbor, WA, fishing success (number of fish caught/total dives) was 42% (Smith and Mudd 1978); in Gulf of Suez, 26% (Bijlsma 1985); and in Baltic, 15% (Bergman 1953).

In Elkhorn Slough, CA, Caspians preyed on adult shiner perch (Cymatogaster aggregata) and northern anchovy (Engraulis mordax), though hatch-year young of these species were abundant and preyed on heavily by Common Terns (Baltz et al. 1979). In Finland, abundant but small fishes (8–12 cm) were seldom found in pellets, while no common fish species 12–25 cm long inhabiting foraging waters was absent from diet (Koli and Soikkeli 1974).

Robs other terns and small gulls in flight (Ferguson-Lees 1971). Infrequently observed eating carrion and scavenging dead fishes from net catches (Cunningham 1966, Whitfield and Blaber 1978), as well as hunting on beaches, apparently for inver-tebrates (Middleton 1987). Forms feeding territories, at least temporarily, in response to patterns of prey distribution (McNicholl 1990; see Behavior: spacing, below).

In Great Lakes (1963–1964), 74% of diet consisted of alewife (Alosa pseudoharengus); rainbow smelt (Osmerus mordax), yellow perch (Perca flavescens), and rock bass (Ambloplites rupestris) made up 25% (stomach contents of 22 Caspian Terns and fish recovered from 8 colonies, for a total of 169 fish recovered; Ludwig 1965; see also Breeding: parental care, below). In Lake Michigan, diet has changed little since 1960s; dominated by alewife and rainbow smelt (Ewins et al. 1994).

In n. Lake Michigan, fishes taken usually ranged from 5 to 15 cm in length. Frequency of prey delivered to young changed over breeding season: Smelt accounted for 51% of prey in Jun, 5% in Jul; alewife 31% in Jun, 90% in Jul. Temporal distribution of prey taken appeared related to seasonal distribution of prey species (Shugart et al. 1978).

In Lake Ontario, diet consists mostly of alewife, but during past 20 yr diet has shifted toward fewer alewife and smelt and greater numbers of rock bass and other Centrarchidae. Diet at Lake Huron colonies is consistently more diverse; proportion of alewife and rainbow smelt in diet declined steadily from 1960 to 1991, and centrarchid species now are more important (Ewins et al. 1994).

On Pacific Coast, prey of breeders near an estuary in San Francisco, CA, included jacksmelt (Atherinopsis californiensis, 33%), shiner perch (19%), and staghorn sculpin (Leptocottus armatus, 19%; Gill 1976). In Monterey, CA shiner perch was the dominant prey and ranged from 7.5 to 10 cm (n = 13); northern anchovy also taken (Baltz et al. 1979). In San Diego Bay, CA, topsmelt (Atherinops affinis) was the species most often found at nests (Ohlendorf et al. 1985). At Grays Harbor, WA, shiner perch was the most common fish collected in colony, and chum salmon (Oncorhynchus keta) and staghorn sculpin were strongly represented. Fish prey ranged from 9.8 to 29.6 cm (n = 30; Smith and Mudd 1978). At Bolsa Chica Ecological Reserve, s. California, northern anchovy was the most frequently observed species in diet, followed by Pacific sardine (Sardinops sagax); in general, marine species were most common in diet (Loeffler 1996). At Rice I., OR, most common prey during breeding season were juvenile salmonid species—e.g., coho (Oncorhynchus kisutch, 36%), chinook (Oncorhynchus tshawytscha, 30%), steelhead (Salmo gairdneri, 18%), and sockeye (Oncorhynchus nerka, 1%). As breeding season progressed and more fish species became available, diet gradually diversified, but salmonid species still represented majority of prey items taken (Roby et al. 1998).

In pellet studies in Great Lakes, Manitoba, and Finland, small proportions of nonfish items found—e.g., insect remains (beetles [Coleoptera], flies [Diptera], etc.), mussels, snails (Mollusca), crayfish (Crustacea), birds, mammals, and larid eggshells, down, or feathers (Vermeer 1973, Koli and Soikkeli 1974, Ewins et al. 1994, FJC). Terns may obtain snails and mussels incidentally through alimentary canals of fish prey (Koli and Soikkeli 1974), while other items may be scavenged from shorelines adjacent to breeding colonies, especially following storms (FJC).

See Feeding, above.

Little information. Two measurements (n = 2 adult birds) using double-labeled water of daily energy expenditure from breeding adults at Rice I., OR: 849 kJ/d and 1,240 kJ/d (Roby et al. 1998). In a study on gut passage time and prey assimilation in Caspian and Elegant (Sterna elegans) tern chicks, Caspian chicks excreted 90% of plastic markers they were fed in 33.5 h. Both species began to pass red carmine dye 2.5–5.8 h after ingesting treated fish, which coincided with time period for excretion of 1% of plastic markers. Caspians had slower gut passage time than Elegants, but mean energy assimilation efficiency was not significantly different between the 2 species; for Caspians, 80.2% ± 2.9 SD (n = 10 chicks; Chavez 1997).

When hot (ambient temperature >30°C, substrate >40°C), incubating or brooding Caspian Terns in Great Lakes reduce body temperature by tilting head so that direct sun does not strike black cap, raising feathers on back, gular-fluttering, flying to water and drinking, dipping feet and breast. Adults protect chicks or eggs from high temperatures by standing in nest cup to provide shade (FJC).

Drinks fresh water (FJC). Pellet-casting very common. Pellets contain fish bones and scales, and sometimes remains of insects, mollusks, crayfish, etc. (Neuman and Blokpoel 1997). When casting a pellet, tern stretches its neck, opens bill, shakes head, and then drops pellet on ground. Casts pellets 1–2 times daily. Nesting colonies commonly littered with fresh and disintegrated pellets (Bergman 1953, FJC).

Adults defecate before taking flight; typically walk >30 cm from nest to defecate. Chicks leave nest cup to defecate on nest rim or outside cup within 1–2 d of hatching (FJC).