Hi Evan and all,
This is a longish post so anyone not interested in Glossy
Black-Cockatoos' feeding habits should delete now!
Evan raises some interesting questions. The question of why Glossies
only seem to eat from certain trees or shrubs while ignoring others
laden with seed capsules is a perennial one and I would like to
suggest an alternative theory to Evan's, which I'll get to shortly.
As a bit of background, in the Upper Blue Mountains their usual food
plants are Allocasuarina littoralis and A. distyla, the latter being
more of a shrub than a tree and often forming dense stands in
heathland. The places where I usually watch Glossies are in these
areas rather than the Lower Mountains where Evan and Neil see them
and where A. torulosa (Forest Oak) is is a commonly used plant. From
my experience Glossies prefer seed capsules that are fresh and still
a bit green. Older, browner cones are ignored. For this reason the
birds move around locally to wherever there are good stands of plants
with a new crop of fresh but fully developed seeds. On any given
plant, most of the seeds present tend to be too old rather than
under-ripe, although Evan's suggestion of the birds sampling the
seeds to find out if they're suitable is still a valid one.
Like Evan, I have often noticed that particular plants are thoroughly
worked over with a dense litter of chewings underneath, while similar
plants full of seeds that appear to be suitable, are ignored. It has
been shown that the GBCs on Kangaroo Island are able to select the
most productive trees to feed from (I don't have the paper in front
of me so can't give any further details). Here in the Blue Mountains,
I guess they could be selecting trees on the basis of seed
productivity, nutritional value or as Evan suggests, ripeness.
However every time I watch Glossies feeding I can't help thinking of
an alternative reason for this patchy feeding: that perhaps it's
simply a consequence of the way they feed. Anyone who's watched
Glossies knows that they can sit quietly for hours at a time chomping
away on seeds, climbing from one branch to the next, working their
way gradually through the canopy or shrub layer. To fly from one tree
to another takes unnecessary energy when there's a good supply of
seeds at hand. However if they're disturbed or if they've exhausted
the suitable seeds in that tree, they will fly up with a flash of red
tails, uttering those unmistakable calls and ultimately move to
another tree or another site, where they may commence feeding again.
So could it be that the only reason some trees appear to be favoured,
is because that's the tree they happen to have been feeding in for an
extended period? And the reason some trees have been abandoned is
because they happened to be disturbed and moved away? The reason some
trees or shrubs are ignored is simply chance and the fact that their
food resources are abundant in the Blue Mountains (in the absence of
recent bushfires) as opposed to some other areas, perhaps? It could
be that their limiting resource, at least in this area, is suitable
tree hollows for breeding or perhaps food supplies in poor years
(bushfire years?) acting as a bottleneck.
There are just ideas for tossing around and I suspect the situation
might be a combination of both theories. It would be an interesting
exercise to analyse seed capsules from the preferred trees vs ignored
or briefly tried trees, testing for things like state of ripeness,
number of seeds per capsule, and nutritional value of the seeds, to
see if there's a difference.
I'd be interested in any further thoughts and comments.
Carol
At 10:16 AM +1100 31/3/08, Evan Beaver wrote:
Birders,
I spent a long time trying to find my first pair of Glossies, and read
almost everything I could find to rty and help me out. Turns out the
best way to find them is to just follow Neil Kirby around and wait for
them to drop in and say g'day.
Since then a family group of 3 have been roosting in a gully near
home, so I hear them almost every morning as they head out to what
ever feeding tree they're using.
The reading I did, and some personal observations suggest that they
have a sensitive taste mechanism, and only eat casuarinas that are
ripe and nutritious. Further, when a tree comes on, they pretty much
eat every capsule on it, littering the ground with the spent corpses.
Near home there are 4 good casuarina that I've seen the birds feeding
in, but not for a while now. All 4 are in heavy seed and look to me
like good eating. The strange part is, a few nuts have been chomped,
but not many. I've observed this in a few places and developed a
theory.
The birds have a good memory, and can remember the location of many
trees they have previously eaten from. The seed capsules, like fruit
on a tree, get ripe as they develop. So, the Glossies keep a picture
of the trees they like, and every now and then, check how they're
going by spending some time tasting the seeds. To support this the
seed pods from the 'tasting sites' aren't nearly as thoroughly chomped
as when they're feeding. In this way they can keep track of good
feeding sites, and how they're going, and possibly even plan ahead,
with some idea of how long it will be before a given tree is ready to
go.
Not sure how to test this theory; double blind trials with wild
animals are difficult to organise. Has anyone else observed similar
behaviour? Comments otherwise?
Evan Beaver
Lapstone, Blue Mountains, NSW
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Carol Probets
Guided birding in the Blue Mountains & Capertee Valley
PO Box 330
Katoomba NSW 2780
Web: http://www.bmbirding.com.au
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