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Bill and others,
I'm sure there are complex mixes of factors that
find _expression_ in observed range changes. In the short term, individuals
displaced from destroyed habitat will turn up in apparently unsuitable, or at
least less suitable habitat within their dispersal capability. If they are
not capable of dispersal, they will probably die very quickly after their
habitat is lost. Dispersal presumably is fundamentally a response to
unsuitable habitat. Juveniles (of sedentary species at least) wouldn't
disperse from their natal territories if the habitat didn't become unsuitable,
the habitat in this instance including the presence of adult birds which
transform from caring parents to hostile and superior competitors as the
breeding urges for the next season swing into effect. I guess the
dispersing birds go as far as they have to in order to find somewhere at least
temporarily suitable, or they go as far as their physical endurance allows, and
stop somewhere unsuitable because they haven't the strength to keep
going.
I suspect the resurgence (or colonisation?) of
Rainbow and, to a lesser but increasing extent, Musk Lorikeets in the suburbs of
Melbourne at least partly reflects significant changes in the suburban habitat
over the past thirty or so years. I have a copy of the Goul League's
"Birds of Victoria: Urban Areas", a slim volume published in 1969 which was
given me when it had just been released (it was followed by six further volumes
dealing with other habitats over the next 14 years). It lists the Rainbow
Lorikeet as "Accidental" in the Melbourne metropolitan area, which accords well
with my recollections from those days. I remember my father mentioning
very occasional sightings around Parkville (Melbourne University campus during
the 1970s, and recall my own excitement when I saw a flock of three birds fly
over Hawthorn (an inner eastern suburb of Melbourne) in about 1980. I also
remember seeing Musk Lorikeets in the grounds of the primary school I attended
in Glen Iris (not far from Hawthorn) around the mid-late 1960s (what a
giveaway!). They're well and truly back now, but they were completely
absent throughout the 70s and early 80s. Now Rainbow and Musk Lorikeets
are both among the most numerous native bird species in the eastern
suburbs. Why?
There have probably been theses and papers
published on this phenomenon, which to my recollection now (nearly 10:30 on a
Tuesday evening, not my best time) I haven't read. If the following
thoughts have been published by someone else, maybe it's my sub-conscious at
work - please point it out to me and I'll happily go back and read the proper
source. However, for what it's worth, I think the suburban habitat is much
more Lorikeet friendly substantially because it became fashionable around the
late 60s/ early 70s to plant native trees and shrubs in suburban gardens.
This was probably a product of growing environmental awareness in a general
sense, along with, more specifically, growing awareness of native plants'
drought tolerance in the wake of a couple of good solid droughts around that
time. The survivors of those "fashionable" plantings, including also
street-trees planted by local Councils (such as Red Ironbarks, Spotted Gums,
Hymenosperums (none locally indigenous, but still "native"), Callistemons,
Melaleucas, etc have matured over the past twenty years, providing palatable and
profuse nectar flows for which the Lorikeets are able to compete
successfully. Other factors probably have also contributed, for instance
the number of suburban residents who find the colourful, active and noisy
Lorikeets sufficiently attractive (and have perhaps been to the Gold Coast on
holidays?) to put out supplementary feed.
The birds must be finding somewhere to nest, too -
maybe using artificial structures like hollow metal utility structures, or
taking advantage of hollows in mature exotic trees (at what age do hollows form
in things like elms and oaks, compared to native hardwoods like eucalypts?) or
planted (or remnant?) natives. Are there really fewer Starlings and
Sparrows around the eastern suburbs of Melbourne compared to twenty years ago,
as it seems to me there are (with absolutely no objective data to go on)?
If so, does that suggest that native hollow-nesters like Lorikeets and Rosellas
(or, on a less optimistic note, "flying rats", ie Mynas) are learning to
out-compete Starlings for nest-sites? Or is it more a reflection of less
ready availability of house cavities in predominently brick veneer structures
compared to the more common weatherboard buildings of the not so distant
past?
The fruit-bats - a vexed question down here at the
moment. Thanks to human intervention (purely coincidental - I don't think
anyone set out to create a flying foxes' paradise) that is what we have
made. Mature Moreton Bay figs are spread through inner suburban public
parks and gardens. Many inner and middle suburban gardens contain
fruit-trees - apples and pears (which the Lorikeets also relish, on my father's
highly annoyed authority) as well as soft fruits. Some of those trees are
relics of the wave of orchards which advanced outwards ahead of the residential
tide, only to be overtaken with each surge of population. I live in Cherry
Street, just round the corner from Orchard Street - and I have seen flying foxes
fly over my home, 20 km+ ESE from their base in the Botanic Gardens. It
may have been an accident of dispersal that brought the original colonists to
Melbourne (but surely other dispersing individuals had arrived here before, over
the decades or centuries, they just hadn't found conditions congenial enough to
allow them to settle), but given a tolerable climate, with allowance apparently
made by some animals developing migratory habits, a suitable roosting/ nursery
site thoughtfully provided by the other recent colonists, and a larder of
fruits, both familiar (at least to the species) and exotic but tasty - that
"accident" has led to establishment of a population which, without further human
intervention, would presumably remain "permanent" until the habitat, from a
flying fox's point of view, declines to become unsuitable. As to why those
individuals dispersed in the first place - there could be many reasons, natural
or anthropogenic. Sadly, habitat clearance could well have been a
factor. But it seems to me that at a taxon level, not necessarily an
individual animal's level, dispersal is a critical long-term survival
strategy.
Sorry for rambling so much, and for all the
brackets - it's late! I'd be grateful if anyone is able to point me in the
direction of any published references dealing with this issue.
Regards,
Jack
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