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Re: New Zealand's Giant Eagle Showed Rapid Size Gain
On 7/1/05 7:45 am, "Ronald Orenstein" <email@example.com> wrote:
> The largest known raptor on New Caledonia is the Swamp Harrier, which is
> probably still undersized by comparison to the extinct flightless
> galliform; and NC is also an ancient continental remnant with many unique
> plants, so I don't think age is an issue. On climate, I was referring both
> to the ecogeographic "rule" in which homeothermic animals in colder regions
> tend to be larger, and to the idea that a large aerial predator of ground
> birds is perhaps more likely in a country whose climate and soil led to the
> development of extensive open areas at higher elevations.
Just some added info - according to Worthy & Holdaway (2002), the
habitat of _Harpagornis_ was the open lowland forest of the South Island of
New Zealand, predominantly the eastern half of the island. Before human
settlement, there was actually very little in the way of open countryside in
New Zealand below the tree line, where eagles may have nested, but probably
didn't hunt significantly (the larger moa were all lowland forms - the
upland moa [_Megalapteryx_] was one of the smallest, about the size of a
turkey). The proportions of _Harpagornis_ (relatively short, broad wings)
are also much more in line with a forest predator. _Harpagornis_ is thought
to have been absent from the North Island - a single bone is known from
there, but was found in a midden and is thought to have been a trade item.
The forests of the North Island were much thicker, maybe too much so for
such a large bird. The other predatory birds present in New Zealand were a
large harrier (_Circus eylesi_), which was considerably larger than the
Swamp Harrier, though I don't have an exact size on hand, sorry, the New
Zealand falcon (_Falco novaeseelandiae_), which was a small bird predator,
and the laughing owl (_Sceloglaux albifacies_), which was mainly a predator
of small birds and invertebrates. It has also been suggested that the
flightless and deeply weird adzebill (_Aptornis_) may have been a predator.
The Swamp Harrier is an open-land specialist, and didn't appear in the
country until after human settlement.
The harrier may have been a potential competitor with the eagle, but the
owl and falcon would have probably been trafficking in a different
size-range of prey from the start. The adzebill is a trickier case - like
many an omnivorous bird (such as the ostrich), I guess it may have worked on
a 'swallow first, decide if it was edible later' basis.