Black-billed gull (Larus bulleri)
|Size||Length: 35 – 38 cm (2)|
Wingspan: 90 cm (2)
|Weight||190 – 270 g (2)|
The black-billed gull is classified as Endangered (EN) on the IUCN Red List 2006 (1).
Adapted for sustained, soaring flight, the black-billed gull has a compact body with long wings and a fan-shaped tail to help control movement in strong winds. With typical colouration for a gull, the black-billed gull can be distinguished by its black bill, legs, feet and wing tips, against a background of grey on the wings and back and white on the head, neck, breast, underside and tail (2) (3).
Endemic to New Zealand, the black-billed gull is found in the north of South Island and south of North Island and on nearby Stewart and Snares Islands (2) (3).
This species is found on the small islands of braided rivers, the margins of lakes, in parks, and on wet lawns, sheep pastures and ploughed fields. It is found both in coastal regions and further inland, typically moving closer to either the coast or to towns during the winter, after breeding inland (2) (3) (4) (5).
The black-billed gull is a social and gregarious species, forming large flocks to forage and to breed. They are forced to travel long distances to feed each day as the flock can clear an area of food within a day. They scavenge less than many other gull species, but will follow ploughs to pick up earthworms and insects from the freshly-turned farmland. The birds feed from lakes and rivers as well, selecting small fish and aquatic invertebrates, as well as picking flying insects out of the air above the water (2). Food is spotted during soaring flight, and the gulls will descend on areas such as tussock grassland to eat moths, calling noisily as they do so (3).
During the breeding season the black-billed gull returns to the same site it visits every year and pairs begin to build deep depressions of twigs and grass during October. The female lays between one and three eggs on lake edges or in braided rivers, and these are incubated by both the male and female for about 22 days continuously. After hatching, the chicks are fed by their parents until they fledge around 26 days later. The family stays at the breeding site until fledging unless disturbed when they may abandon the nest as soon as the majority of their eggs have hatched, moving on to coastal habitats for the winter. The young are able to breed after two years, but more normally will not pair up until three or four years of age (3).
Despite a substantial decline on South Island over the past ten to twenty years, the black-billed gull remains more common on South Island than North Island, where it has actually expanded its range since the 1970s. It is hoped that its overall declining trend has reversed recently, partly due to a change in agricultural practices that has seen more insects and worms being revealed by ploughing. The black-billed gull has developed a habit of following ploughs and aiding farmers by picking out insect pests. However, the breeding habitat of the bird continues to decline in size. This reduction in habitat is the cause of the drop in gull numbers over the last 30 years as land is increasingly turned over to agriculture. Additionally, breeding habitat is currently threatened by the spread of exotic lupins which clog the waterways (2) (5).
The New Zealand Government has launched the New Zealand Biodiversity Strategy to respond to its obligations under the Convention on Biological Diversity. This includes management of wetland habitats by protecting them from development, as well as by planting appropriate plant species in areas recovering from disturbance in order to stabilise the soil and reduce fertiliser run-off that can encourage the growth of exotic species, such as lupins, at the expense of native species. The water level must be maintained and a water care code has been established to ensure that users of wetland habitats are able to act responsibly (6).
For further information on this species see Del Hoyo, J., Elliott, A. & Sargatal, J. eds. (2001) Handbook of the Birds of the World Volume 3: Hoatzin to Auks. Lynx Edicions, Barcelona.
For more information on this and other bird species please see:
- BirdLife International:
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- Endemic: a species or taxonomic group that is only found in one particular country or geographic area.
IUCN Red List (May, 2006)
- Del Hoyo, J., Elliott, A. and Sargatal, J. (2001) Handbook of the Birds of the World Volume 3: Hoatzin to Auks. Lynx Edicions, Barcelona.
New Zealand Birds (May, 2006)
- Burger, J. and Gochfeld, M. (1997) Use of space by nesting black-billed gulls, Larus bulleri: Behavioural changes during the reproductive cycle. Australian Ornithology, 96(2): 73 - 80.
New Zealand Government Department of Conservation (May, 2006)
The New Zealand Biodiversity Strategy (May, 2006)