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Mike and others,
On a handful of occasions over the past couple of
years I have seen small numbers of Corellas in the Lyndhurst area,
generally within a couple of kilometres south of the Westernport Highway/ South
Gippsland Highway junction. On the only occasion I got a good enough look
for a species identification the birds I checked were Little Corellas - no
guarantees on the other occasions. A couple of weeks ago a lone Little
Corella flew over my home at Glen Waverley. In recent years I have
recorded both Corella species in streamside woodland along Watsons Creek near
Kangaroo Ground, on Melbourne's outer north-eastern suburban fringe - it's
become a standard five minute stop on our annual Twitchathon route. I've
seen Crested Pigeons on Ringwood Public Golf Course, and twice in the past
twelve months on a sports oval in Templestowe. Maybe invaders taking
advantage of human interference with the environment, but still a nice change
from turtle-doves.
And I do sometimes wonder about the dynamics of
birds' distributions. What we see at any given time is a snapshot of the
product of whatever influences have been in place recently, and the fossil
record does show major climatic and resultant habitat changes which must have
profoundly changes the distributions of the species around at the relevant
times. It seems probable that the different fire regimes introduced by
Aborigines would have changed vegetation patterns across much of the continent,
presumably to the benefit of some species and the disadvantage of others.
Perhaps some of the southward range extensions of rainforest pigeon species such
as Topknot and White-headed Pigeon, suggested by more frequent records of these
species in Gippsland where they were known only, if at all, as very rare
vagrants could be related the the onset of anthropogenic climate
change.
However, what about more ephemeral fluctuations of
range? Is it reasonable to assume that some species' ranges might undergo
periodic or episodic expansion/ contraction phases, impermanent in effect,
over periods of, say, centuries. Particularly severe events such as
droughts or bushfires might lead to local extinctions, only for those areas to
be recolonised decades or centuries later when wandering parties of dispersing
young or surplus adults discover new, unoccupied aras of suitable habitat.
If these sorts of events do occur naturally - and our datasets are probably too
limited in terms of duration (and comprehensiveness and reliability of data from
any more than a few deacades ago) to provide any definite evidence, they might
provide some hope that local extinctions we have observed, such as the loss of
Grey-crowned Babblers and Southern Stone-curlews from southern Victoria, and
Crested Bell-birds from north-eastern Victoria, might be reversible in the long
term, provided suitable conditions and corridors for recolonisation are
available and provided healthy populations of those species are maintained
within dispersal range. Certainly there is a genetic diversity issue when
isolated populations are lost, but given the erratic climate and propensity for
catastrophic fires that affects much of temperate and arid Australia, and the
tendency to nomadism in so many of our bird species, I wonder whether such range
fluctuations and occasional local extinctions might have been part of the
natural economy of some species from well before European settlement. I
remember doing some research for a history assignment many years ago which
suggested that, at the time Melbourne was settled, the two common cockatoos of
the area were Long-billed Corellas and a red-tailed (probably Glossy)
Black-Cockatoo. They have gone (although the Long-bills may be making a
resurgence) - what were the range limits of the Sulpur-crested, Gang-gang and
Yellow-tailed Black-Cockatoo were in those days?
I hasten to add that I do not profess or wish to
encourage complacency about our declining species, especially those declining
across broad geographic or habitat fronts. But I think we can tend to
assume subconsciously that what we observe to-day is more constant or organised
than it really is. The capacity of native species to expand their range in
response to favourable conditions, whether or not anthropogenic in origin, might
provide fruitful ground for research into techniques to halt and reverse the
range contractions of other native species. Any thoughts? My
apologies if I'm treading on old ground that has been thoroughly tilled
before.
Regards,
Jack Krohn
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