The vexed question of Canada
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.
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.
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
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'.