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The vexed question of Canada Goose subspecies

Malcolm Ogilvie

Every winter three or four Canada Geese turn up on Islay in the flocks of Barnacles and Greenland Whitefronts. And every winter these birds arouse considerable interest among birdwatchers. That might seem strange, but these are not the large birds of English parks and gravel pits, but much smaller birds that probably qualify as genuine transatlantic vagrants, hence the interest.

The Canada Goose is a highly variable species which has long caused headaches for taxonomists. In the early 1950s, it was split up into 12 subspecies, of which two were believed to be extinct though one, the Giant Canada Goose, was later found still to exist. The 12 subspecies were based partly on known ranges, but more on examination of skins. At the time, it was thought that all the subspecies were separated geographically, at least on the breeding grounds if not entirely in winter. The introduced Canada Geese in Britain are among the largest and palest of the subspecies. There are also large, dark subspecies, some very small ones, no bigger than a Barnacle Goose, as well as some in between. These 12 subspecies are still referred to in many identification books, including recent ones.

Since the 1950s, however, things have got a bit messy. The numbers of nearly all the subspecies have increased greatly. In the 1950s, there were thought to be about 1.5 million Canada Geese in North America. The total now is put at 4.5 million! As a result, their breeding ranges have spread until several of them overlap. This has led to much inter-breeding and the occurrence of numerous intergrades which are virtually impossible to assign to a particular subspecies. Even as early as the 1970s, one researcher, working by Hudson Bay where two subspecies breed, was talking about a 'complex' of mixed subspecies as he was no longer able to identify many of the pairs he was studying.

The upshot of all this is that when looking at the vagrant Canada Geese on Islay, there is really very little point in attempting to identify them, based on illustrations in books, as belonging to a particular subspecies. Most book illustrations are still based on the original 12 races of the 1950s and do not take into account the intergrading that has gone on since. And live birds in wildfowl collections are no better as a guide, because their provenance may not always be certain.

Canada Geese are evolving before our eyes and it may be a long while before things settle down enough for any new taxonomic classification to appear. Thus, the only sensible approach to vagrant Canada Geese in Britain is to describe them in terms of their size (e.g. large, medium, small) and colouring (dark, medium, pale), plus any other feature such as a white neck ring or short or long bill, and leave it at that.

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Dr Ogilvie is a natural history writer and editor, formerly a research scientist with the Wildfowl and Wetlands Trust, and resident on the island of Islay since 1986. Until 1997, a member of the 'British Birds' editorial board and also one of the editorial team which produced 'Birds of the Western Palearctic'.