Leucocarbo onslowi (H.O.Forbes, 1893)

Common Name

Leucocarbo onslowi, commonly known as the Chatham Islands shag, has the following description.

Size Measurements

Adult Chatham Islands shags measure 63 cm (25 in) in length and weigh between 1,790–2,400 g (63–85 oz).

Non-breeding Plumage Base

This is a large pied shag with white feathers on the throat, breast, belly, and patches on the wing scapulars; all other areas are black or black-brown.

Breeding Plumage Sheen

During the breeding season, the head and hind-neck display a blue metallic sheen, the wings and mantle have a greenish sheen, and all other black areas have a purplish-blue sheen.

Breeding Adult Head Features

Adults have a prominent crest on the head, white filoplumes on the neck, a red gular sac, and orange-red caruncles on the face and mouth gape.

Eye Characteristics

Despite the common naming of the genus, the iris is brown, and the eye is surrounded by a violet eye-ring.

Bill and Limb Coloration

The bill is dark grey with a bright red interior mouth, and the legs and feet are dull pink.

Non-breeding Adult Appearance

Non-breeding individuals are similar in appearance to breeding birds but are duller overall, lack a crest and filoplumes, and have duller colored bare parts: caruncles are yellow, the eye-ring is blue, the mouth is orange, and the bill is light grey.

Geographic Endemic Range

The species is endemic to the Chatham Islands, an archipelago located approximately 700 km (430 mi) east of New Zealand.

Inshore Habitat Preferences

Within the Chatham Islands, this is an inshore species that rarely travels more than a few kilometers from shore, and it is most often found in sheltered waters such as bays and inlets.

Foraging Habitat

It also forages in the large brackish Te Whanga Lagoon.

Breeding Colony Nesting Sites

It breeds in colonies along coastal areas and around Te Whanga Lagoon, nesting on flat ground, slopes, or wide cliff ledges.

Known Colony Locations

Currently, there are thirteen known colonies of this species, located on main Chatham Island as well as Pitt Island, Star Keys, Rabbit Island, and North East Reef.

Colony Characteristics

All colonies are situated close to the high-tide mark, and a single colony can hold as many as 300 nests.

Roosting Behavior

The species roosts on rocks near shore or out at sea, and may use separate roosting sites for day and night.

Movement Patterns

It is non-migratory and has never been recorded outside of the Chatham Islands, though it does feed and roost on other islands within the Chathams group away from its breeding colonies, such as Mangere.