The following extract from Ian Rowley's 'Behavioural Ecology of the Galah,
Eolophus roseicapillus, in the Wheatbelt of Western Australia' (Surrey
Beatty, 1990) may be of interest, perhaps not bringing you much closer to An
Understanding although suggesting that Ian Temby was on the right track:
SCARRING, BILL STROPPING and EYE WIPING
Most of the trees used by the Galahs for nesting were smooth-barked
eucalypts, and it was characteristic that trees in use had an area of this
bark removed. These areas, which I call Scars, were created by the nesting
Galahs that chewed the bark and stripped it off, thus exposing the cambium
which dried to a woody texture. Galahs are the only species that work on
these areas and then for only a small part of the year so that they increase
in size slowly and only a very few trees are completely ring-barked and
killed by this activity. The suggestion that present generations of Galahs
are ensuring a good supply of nest hollows for their descendants by
effectively "ring-barking" the tree is not an acceptable explanation because
it involves a degree of forward thinking not known in animals.
Early naturalists suggested that these Scars are made in order to deter
predators trying to reach the nest hollow, in particular, the large varanid
lizards. No such lizard has been seen to miss its footing on the Scar and
there is little likelihood of it providing such protection because these
bare areas are seldom more than half a metre wide and would easily be avoided.
Each year the Scar is freshened up by the owners of the hollow who chew at
the edges of the surrounding bark: I call this Bark Chewing. When a bird
works on its Scar, it sometimes intersperses Bark Chewing with bouts of Bill
Stropping: the bird rubs alternate sides of its mandibles on the bare area
as if stropping a razor. Intense Bill Stropping is sometimes so vigorous
that the bird rests the points of the carpel joints on the Scar to steady
itself. Birds, particularly males, also wipe the sides of their faces on
the Scar and in the process the crusty periophthalmic ring of skin makes
contact and leaves behind a dusting of fine powder, resembling talc. They
also bill the preen gland and a secretion from there could leave a smell but
I was not able to determone whether there is any scent involved in this Eye
Wiping behaviour which appears to have many of the characteristics of
mammalian marking procedures and which may serve as a visual sign that the
hollow is currently occupied by a pair of Galahs. If the ownership of the
hollow changes, the new occupants continue to use the existing Scar.
It was not always easy to tell whether a bird was Bark Chewing or Bill
Stropping from a distance; both activities are carried out on parts of the
tree other than the Scar and only Eye Wiping is confined to that place.
Observations from the hide showed that males were active at the Scar five
times more often than females (215 of 258 records) and that this imbalance
was most pronounced once incubation had started.
Bill Stropping is also performed in other parts of the tree and it is likely
that it has a function besides refurbishing the Scar. The mandibles of
parrots grow continually throughout their life and stropping may be an
essential part of bill maintenance, especially if the birds are feeding on a
soft diet. Furthermore, the action of Bill Stropping is quite noisy and may
well serve as a means of communication between the pair-member who is
off-duty and the bird that is incubating; such reassurance would save the
sitting bird from the panic exit which sometimes results after conspecifics
land in the nest tree.
Birds Australia Conservation & Liaison,
Australian Bird Research Centre,
415 Riversdale Road,
Hawthorn East, VIC 3123, Australia.
Tel: (03) 9882 2622. Fax: (03) 9882 2677.
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