Extinct Giant Cro of New Zealand WS

Forest and Bird, Issue 313, 1 August 2004, Page 36

Extinct Giant Cro of New Zealand

WS

BRIAN GILL

—BRIAN GILL,

traces the appearance and

disappearance of New Zealand's ravens.

tch any film or television drama from Australia, Britain or North America, and the chances are you will hear in the background of outdoor scenes the cawing of crows. Members of the crow family are found almost worldwide, but New Zealand and the polar regions are among the few substantial areas without them, though it was not always so. Our only crow today is the rook Corvus frugilegus, introduced from Britain in the 1860s and 1870s. It is mainly limited to hill country and arable farmland in Hawke's Bay and Fd Canterbury where its populations are controlled by poisoning. It is not widely known that we once had native crows of our own that are now extinct. Like other crows, they had a long, stout bill, and their plumage was probably black. ‘Crow’ is a general term, either for members of the family Corvidae, or, more particularly, for those in the genus Corvus. The largest of the latter are called ‘ravens, and the family includes jays and magpies (but not Australian ‘magpies, which are in a different family). The New Zealand crows were discovered as ‘fossils. In 1892, H.O. Forbes of Canterbury Museum found fossil bones of a large crow in sand dunes on Chatham Island. He described these as belonging to a new species called Corvus moriorum. A year later he named a slightly smaller species, now called Corvus antipodum, based on fossil bones from the North Island. The bones of both species are large, as crow bones go, so we are justified in calling these extinct birds ravens. Raven bones have since been found around much of the New Zealand coast, mostly in sand dunes and never more than about 25 kilometres inland (see map). Along with sites on the Chatham Islands, these sites are all

known, or assumed to be, Holocene in age; that is, deposited since the last ice age, which ended about 10,000 years ago. Most are probably younger than about 6500 years old, when the present sea level was established. Older raven bones, up to several tens of thousands of years old, have been found at Oamaru and in limestone caves in the

northwest of the South Island. Some of the bones are found at coastal archaeological sites, in Moriori cooking middens on the Chatham Islands and in prehistoric Maori waste in the North and South Islands. This human association proves that the ravens were not extinct before the Polynesian settlement of New Zealand began about 750 years ago. Raven bones are absent from several wellstudied inland sites, such as the caves of the Waitomo limestone region, and Earnscleugh Cave in Central Otago. This suggests that the ravens preferred open habitats and forest margins. The east coasts of both main islands, where dense forest was less abundant, may have suited the ravens best, and they may have been specialised coastal scavengers.

hatham Islands ravens were slightly ; larger on average than those from the South Island, which in turn tended to

be slightly larger than North Island birds. For example, the humerus (one of the major wing bones) was 80-87 millimetres long in the Chathams, 78-83 millimetres in the South Island and 74-82 millimetres in the North Island. The variation on the mainland was probably clinal average size increased gradually from North to South as happens in many New Zealand birds. This follows Bergmann’s Rule, whereby in birds and mammals that vary in size geographically, size tends to be larger among the populations in cooler areas, i.e. towards the south in New Zealand. Larger animals are better able to conserve their body heat in colder areas. We recognise two species of ravens, a mainland form (antipodum) and a larger insular form on the Chatham Islands (moriorum). There are several living crows and ravens in Australia, New Caledonia and the Solomon Islands. New Zealand ravens were the largest of all these crows. The humerus is only 71-75 millimetres long in the Australian raven Corvus coronoides, the largest Australian crow, which weighs 540-820 grams. However, in European populations of the northern raven C. corax, the humerus is 88102 millimetres long. New Zealand ravens would have weighed about 0.9-1.0 kilograms when fully grown. Northern ravens weigh up to 1.4 kilograms, and are often cited as the world’s largest songbird. However, that title apparently belongs to the thick-billed raven C. crassirostris of Ethiopia which is larger,

although its weight is unknown. There are several other large ravens, such as the whitenecked raven C. albicollis of southern Africa, which weighs around 0.9 kilograms. The unrelated superb lyrebird Menura novaehollandiae of Australia can weigh around 1.1 kilograms. Compared with these, the Chatham Islands raven was probably the world’s fourth or fifth-largest songbird. he wing bones of New Zealand ravens fully developed relative to the leg bones, suggesting that they were strong fliers. There was no reduction in the wing, as evident in weak-flying birds such as the kokako or saddleback. The tendency towards flightlessness, displayed by so many ancient New Zealand land birds, was not shared by the ravens, not even the isolated Chatham Islands population. The wing bones of New Zealand ravens were shorter than those of the northern raven of Europe, but the lower leg was equal or longer in New Zealand birds. Thus New Zealand ravens were relatively long-legged, perhaps as an adaptation to spending more time on the ground, walking or running. Apart from the leg bones, there is nothing unusual about the relative size of the raven bones, one to another, or their shape. They were typical crow bones which suggest that New Zealand ravens were fairly ordinary crows. During the isolation of New Zealand ravens from their overseas ancestor, natural selection must have favoured those that kept flying despite the lack of mammalian predators. Flight may have been essential in the ravens’ ability to succeed as coastal scavengers.

Within the last 20 years, studies of DNA have revolutionised understanding of bird taxonomic relationships. Among the new findings is the remarkable and unsuspected notion that the world’s songbirds fall into two major groups: the corvidans (Parvorder Corvida) which originated in the Australasian region, and the passeridans (Parvorder Passerida) which arose elsewhere. The crow family belongs in the Australasian group. The origin of the corvidan songbirds presumably goes back some 90 million years to the Cretaceous when the Australian region (including New Zealand) broke away from Antarctica and drifted north. The ancestral species that gave rise to the Corvida probably reached Australia from Asia 55-60 million years ago. There was an ‘adaptive radiation’ of these birds into the local songbird groups of the Australasian region. When Australia got closer to southeast Asia, a successful crow-like species must have crossed the gap and spread widely to give rise to the Northern Hemisphere crows, magpies and jays. The closest living relative of New Zealand ravens is not immediately obvious. The five Australian Corvus represent a secondary radiation from an ancestral Corvus that reinvaded Australia. Species of Corvus in New Zealand, New Caledonia and the Solomon Islands must also descend from an ancestral Corvus that reinvaded the area, most likely the same one that gave rise to Australian crows. The Australian raven C. coronoides is the likely closest relative of the New Zealand species, being the largest Australian Corvus and one that is widespread

in southeast Australia. New Zealand ravens probably developed following the invasion of New Zealand by C. coronoides or its ancestor. This took place sufficiently long ago to allow development of the slightly increased size of the New Zealand birds. There is no evidence that any European ever saw a New Zealand raven. So they are among the birds eliminated by prehistoric Maori activity, which included forest clearance, the introduction of mammalian predators, and direct hunting for food. It is puzzling that our ravens should have died out when crows in general are renowned for intelligence, adaptability and association with human settlements. This is especially so considering the size of their wing bones shows that New Zealand ravens flew well. At Marfell Beach, Marlborough, raven bones are not present in Maori middens at greater relative frequency than in nearby natural deposits, suggesting that overhunting was not a main cause of extinction. Instead, it is plausible that New Zealand ravens nested on cliff ledges, or had come to nest elsewhere on or near the ground, and that their eggs and nestlings were prey for the southeast Asian rat (kiore, Rattus exulans) that Maori introduced. Irruptions of this rat would have spread throughout the country, posing a threat to the eggs and young of many birds, if not to the adults themselves.

Curator of Birds at Auckland War

Memorial Museum, has a particular interest in extinct birds. He has written several books, including New Zealand's Extinct Birds with Paul Martinson (Random Century, 1991) and New Zealand's Unique Birds with Geoff Moon (Reed, 1999).