Together with terns, seagulls comprise the family Laridae. Gulls are long-winged birds of moderate size with rather short legs and webbed feet. The bill is bent down at the tip, the tail short and nearly always square-ended, and adult plumage generally consists of grey or black wings and back and a white body and tail. In many species the bill and feet are brightly coloured. Sexes are alike. Their usual dwelling places are coasts, harbours, rivers, and lakes, and here gulls feed primarily by scavenging. They also feed on shellfish and on insects and worms obtained from ploughed fields. They take fish in coastal and fresh waters, the eggs and young of other sea birds, and occasionally attack weak or helpless lambs and sheep.
Gulls are always gregarious and breed in colonies, and their nests are generally made of seaweed, sticks, and grass, which, depending on the species, may be placed on sandhills, shingle banks, cliff ledges, or even in trees or shrubs. Eggs are usually shades of green, brown, or blue, heavily marked with darker colours. Incubation is shared by both parents and the young when hatched are covered with a mottled down. Flight is strong, dextrous, and graceful. The birds spend a great deal of time on the water. They are quarrelsome, especially when feeding, and their voices are harsh with querulous yelps or mewlings.
Three species of gull occur in New Zealand; the large black-backed or Dominican gull (Larus dominicanus), the red-billed gull (Larus novae-hollandiae scopulinus), and the black-billed gull (Larus bulleri).
The black-backed gull is the most widely distributed. The bill of the adult is yellow with a red spot near the tip of the lower mandible, and the legs and feet are yellowish green. First-year birds have a black bill, dark-brown feet, and a mottled dark-brown and white plumage. Full adult plumage of black above and white below takes three years to develop. Nesting colonies of the black-back may be found on shingle banks, sand dunes, rocks, or even tussock grasslands, and they may be close to the sea edge, well up rivers, or occasionally on inland mountain sides.
Red-billed gulls are predominantly coastal in distribution and are rather more common and widespread in the North Island than in the South. In the adult the plumage of the back and wings is pearly grey and the wing tips are mainly black, the two outermost flight feathers each having a broad white band near their tips. Feet and eyelids are blood red like the bill.
Black-billed gulls are roughly similar in size and shape to the red-billed gulls, but have a daintier beak. In the adult the plumage of back and wings is pearly grey and the wing tips are mainly white with some black markings. The feet and legs are black with a reddish flush. Black-billed gulls are predominantly South Island birds and prefer inland lakes. Unlike the other two species, this one is confined to New Zealand.
In the immature stages of the red- and black-billed gulls, there is some brown mottling on shoulders and wings, and a stage is passed through when the young red-billed gull has a black bill and the young black-billed gull a red bill.
by Gordon Roy Williams, B.SC.(HONS.)(SYDNEY), Lecturer in Agricultural Zoology, Lincoln Agricultural College.