The Common Tern is 31-35 cm (12-14 in) long, and a 6-9 cm fork in the tail, with a 77-98 cm (30-39 in) wingspan. It weighs 110-141 g. Breeding adults have pale grey upperparts, very pale grey underparts, a black cap, orange-red legs, and a narrow pointed bill, red with a black tip, or all black. The upperwings are pale grey. The rump and tail are white. It is unlike the Arctic and Roseate Terns in which the tail protrudes beyond the wings. In non-breeding adults the forehead and underparts become white, the bill is all black or black with a red base, and the legs are dark red or black. The upperwings have an obvious dark area at the front edge of the wing, the carpal bar.
Habitat and Distribution
The Common Tern breeds across most of Europe, with the highest numbers in the north and east of the continent. There are small populations on the north African coast, and in the Azores, Canary Islands and Madeira. Most winter off western or southern Africa, birds from the south and west of Europe tending to stay north of the equator and other European birds moving further south. The breeding range continues across the temperate and taiga zones of Asia, with scattered outposts on the Persian Gulf and the coast of Iran.
Like all Sterna terns, the Common Tern feeds by plunge-diving for fish, from a height of 1-6 m (3.3-19.7 ft), either in the sea or in freshwater lakes and large rivers. The bird may submerge for a second or so, but to no more than 50 cm (20 in) below the surface. When seeking fish, this tern flies head-down and with its bill held vertically. It may circle or hover before diving, and then plunges directly into the water, whereas the Arctic Tern favours a `stepped-hover` technique, and the Roseate Tern dives at speed from a greater height, and submerges for longer. The Common Tern typically forages up to 5-10 km (3.1-6.2 mi) away from the breeding colony, sometimes as far as 15 km (9.3 mi). It will follow schools of fish, and its west African migration route is affected by the location of huge shoals of sardines off the coast of Ghana; it will also track groups of predatory fish or dolphins, waiting for their prey to be driven to the sea's surface. Terns often feed in flocks, especially if food is plentiful, and the fishing success rate in a flock is typically about one-third higher than for individuals.
The eggs may be laid on bare sand, gravel or soil, but a lining of debris or vegetation is often added if available, or the nest may be rimmed with seaweed, stones or shells. The saucer-shaped scrape is typically 4?cm (1.6?in) deep and 10?cm (3.9?in) across, but may extend to as much as 24?cm (9.4?in) wide including the surrounding decorative material. The peak time for egg production is early May, with some birds, particularly first-time breeders, laying later in the month or in June. The clutch size is normally three eggs; larger clutches probably result from two females laying in the same nest. Egg size averages 41 mm ? 31 mm (1.6 in ? 1.2 in), although each successive egg in a clutch is slightly smaller than the first laid. The average egg weight is 20.2 g (0.71 oz), of which 5% is shell. The eggs are cream, buff, or pale brown, marked with streaks, spots or blotches of black, brown or grey which help to camouflage them. Incubation is by both sexes, although more often by the female, and lasts 21-22 days.
Calls and Songs
The Common Tern has a wide repertoire of calls, which have a lower pitch than the equivalent calls of Arctic Terns. The most distinctive sound is the alarm KEE-yah, stressed on the first syllable, in contrast to the second-syllable stress of the Arctic Tern. The alarm call doubles up as a warning to intruders, although serious threats evoke a kyar, given as a tern takes flight, and quietens the usually noisy colony while its residents assess the danger. A down-slurred keeur is given when an adult is approaching the nest while carrying a fish, and is possibly used for individual recognition (chicks emerge from hiding when they hear their parents giving this call). Another common call is a kip uttered during social contact. Other vocalizations include a kakakakaka when attacking intruders, and a staccato kek-kek-kek from fighting males.