The Little Tern is a small, slender and elegant marine tern (total length 20–28 cm; mean adult weight approximately 53 g for males, 49 g for females [Higgins & Davies 1996]) with narrow wings and a fairly long and deeply forked tail. Sexes do not differ in plumage or size.
Adults in breeding plumage have a diagnostic head-pattern, consisting of a neat black cap and tapering loral stripe isolating a triangular white patch on the forehead that tapers to a point above the rear of the eye, and with the rest of the head and neck white. The rest of the upperparts are largely pale grey, with a contrastingly white rump, uppertail-coverts and tail, and a white line along the upper edge, and a thin black line along the bottom edge, of the folded primaries, and with slightly darker grey outer primaries. In flight the upperwing appears pale grey with a white trailing edge and thin black leading edge to the outerwing. The underparts are wholly white. The bill is bright yellow with a small black tip, the eyes blackish, and the legs and feet bright orange (Gochfeld & Burger 1996; Higgins & Davies 1996).
Adults are similar to breeding birds except the head-pattern differs, with the forehead, forecrown and anterior lores white-washed with grey, leaving a dark band extending from in front of the eyes to the nape, and that merges into dark spotting on the rear-crown. Non-breeding birds further differ by a pale-grey rump, uppertail-coverts and tail (concolorous with the rest of the upperbody), a less deeply forked tail, and a narrow dark cubital bar on the folded wing. The bill is also black, and the legs and feet duller, orange-brown (Gochfeld & Burger 1996; Higgins & Davies 1996).
Juveniles are like non-breeding adults but the black band on the head is narrower and duller; the white forehead, anterior lores and crown are washed brown; the mantle, back, scapulars and tertials are marked with narrow white scaling and bold brown U-shaped markings; there is a thin dark tail-band; and the upperwing-coverts are patterned much like the scapulars, and there is a dusky cubital bar on the upperwing. The bill is dark brown with a darker blackish tip and base, and the legs and feet are brownish orange (Gochfeld & Burger 1996; Higgins & Davies 1996). Little Terns are typically gregarious throughout the year and generally seen in small flocks but sometimes forming large flocks, for example a flock of 1600 birds was observed one November, at Sawtell, NSW, and thousands have been observed roosting at some sites, such as off Caloundra. Little Terns usually breed in small colonies (of up to 50 birds), sometimes with other species of terns, including Fairy Terns, but will also breed solitarily. Non-breeding birds have been observed to roost separately from breeding birds. Little Terns have been observed to roost with Fairy Terns, Whiskered Terns, Common Terns (S. hirundo ) and White-winged Black Terns (Chlidonias leucopterus) and other waders. Typically they forage in loose flocks, or at least with numbers of birds foraging in same area, particularly near breeding or roosting sites. They are also known to often forage singly, or, occasionally, in small tight flocks. Little Terns forage in association with other species, including Fairy Terns, Crested Terns (Thalasseus bergii ), shearwaters (Puffinus species) and cormorants. Occasionally individuals have food stolen by Silver Gulls (Larus novaehollandiae ) or Kelp Gulls (L. dominicanus ). The species is usually quite vocal and often heard calling while foraging, or around colonies, with the usual flight call a shrill, repeated kik. Colony members cooperate to mob predators, when they are quite vocal (Bolger 1984; Gochfeld & Burger 1996; Higgins & Davies 1996; Hill et al. 1988; Hulsman 1974; Owen 1990, 1991; Reside et al. 1989; Rogers 1977; Secomb 1994; Serventy et al. 1971; Starks 1992).