Circus dossenus
a.k.a Wood Harrier and Mime Harrier
Name: Circus
dossenus.
Phonetic: Sir-kus dos-sen-us.
Named By: Stors L. Olson & Helen F.
James - 1991.
Classification: Chordata, Aves, Falconiformes,
Accipitridae.
Species: C. dossenus.
Diet: Carnivore/Insectivore.
Size: Unavailable.
Known locations: The Hawaiian Islands of Oahu -
Barbers Point, and Molokai - Moomomi dunes.
Time period: Pleistocene to Holocene.
Fossil representation: Partial remains of two
individuals.
When
originally found, the fossils of the Wood Harrier (Circus
dossenus) were thought to be those of an Accipiter, a
bird that
belongs to the group more commonly known as goshawks and sparrowhawks.
This was mainly because the bones and wing proportions seemed to match
up best with the Accipiters, but further study into the remains led
to the 1991 conclusion that the bone actually belonged to a
harrier. Assigned as a species to the Circus
genus, C. dosseus
has since become known as the Wood Harrier, and occasionally the Mime
harrier. The dossenus name means jester, which is both a play on
words with the genus name of Circus, and the fact
that the describers
were ‘fooled’ into thinking that the bones were of an Accipter.
The
Wood Harrier is a likely example of insular dwarfism, the biological
phenomenon where animals become smaller as not to exhaust limited food
sources. Harriers did not develop independently upon Hawaii, so it
is likely that the ancestors of the Wood Harrier flew to the Hawaiian
Islands and then settled, with successive generations becoming
smaller. This seems quite plausible since in modern times another
species of harrier named C. cyaneus (a.k.a.
the Hen Harrier and
the Northern Harrier) is occasionally sighted around the Hawaiian
Islands, and though this may not be the ancestor to the Wood
Harrier, it establishes the precedent that harriers from other
locations can reach the Hawaiian Islands. On a side note, other
species of harrier are known to have colonised several islands in the
Pacific, with a good example being the Swamp Harrier (C.
approximans) being known in Australia, Fiji,
Vanuatru, New
Caledonia and New Zealand.
The
Wood Harrier appears to have been one of the smallest harrier species,
though aside from just growing smaller, the wings changed in
proportion so that became shorter and more in proportion to the wings
of an Accipiter. This is an indicator that the Wood Harrier adapted
to a different mode of hunting and ecological niche than that occupied
by other harriers. This is because harriers usually have long wings
that enable them to soar high over open spaces so that they can then
dive down on to ground dwelling animals such as reptiles, lizards,
snakes, small mammals, etc.
The
Hawaiian Islands however were a product of volcanic activity, which
means that they rose up out of the Pacific Ocean while remaining
isolated from other land masses. By being isolated, vertebrate
animals could only fly or swim there, and given their distance from
the continents where mammals dominated the land, only birds could
make it there. In addition, the lack of grazing animals meant that
the islands were covered in forests with very little in the way of
open spaces, so the Hawaiian Islands had neither the habitat nor the
prey animals that harriers were best suited to hunting.
In
order to survive, the first harriers had to change. By developing
proportionately shorter wings, Wood Harrier were better able to fly
through the forests without brushing the edges of their wings against
branches and trunks. In addition to this, the Wood Harrier likely
had to adapt to hunting the other birds of the Hawaiian Islands given
the lack of other prey animals that other harriers of the world
hunted. Some of these birds would have been secondarily flightless
(evolved from flying ancestors but losing the ability to fly over
successive generations), and it’s possible but not yet known for
certain, that Wood Harriers may have focused their attacks upon
these. An alternative would be Wood Harriers learning to attack other
birds while on the wing, and yet another alternative would be Wood
Harriers learning to catch and eat insects, which some birds of prey
like some falcons are known to do (Falco subbuteo
being one
example).
Unfortunately
as with so many Pacific Island animals (though continents are no
exceptions either), the Wood Harrier seems to have gone extinct as
a direct result of the first people arriving upon the islands. The
two factors that likely affected the Wood Harrier the most here was a
combination of habitat loss through clearing as well as the
introduction of Rattus exulans, better known as
the Polynesian rat
and Pacific rat. Harriers are known to nest on the ground, and so
far there is no evidence to doubt this mode of nesting for the Wood
Harrier. This means that both eggs and chicks would have been prey to
the rats, which are blamed for wiping out many species of Hawaiian
birds. With less and less young surviving to adulthood, and a
reduction in prey species, Wood Harriers simply could not survive.
The
harriers are a versatile group of birds of prey, and while the small
Wood Harrier was flying through the forests of Oahu and Molokai, the
Eyles’
Harrier of New Zealand grew up to be one of the largest
harriers ever known.
Further reading
- Descriptions of thirty-two new Species of Birds from the Hawaiian
Islands - Stors L. Olson & Helen F. James - 1991
(ISBN 0-935868-54-2).
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