Hi Rob,
Your observation of the Yellow-rumped Thornbill was quite interesting, and I
would like
to offer a possible physiological explanation.
YRTs are usually found in small flocks, but you only refer to a single bird.
Does this
mean that it was indeed alone? If so, it may have been an unwell or injured
bird.
>From 1987-89, while I was working at the Australian National University, I
>studied the
energetics of Brown and Striated Thornbills in the Brindabella Range of the
ACT. In
particular, I was very interested to see if I could induce torpor in these
species at
low temperatures. I know some subscribers to Birding-Aus visited my study site
and saw
this work in progress.
Although I and my research team were not successful in inducing torpor, we were
able to
depress their metabolic rates considerably at low ambient temperatures, as well
as
having lowered cloacal temperatures. Some birds were quite listless and had
trouble
balancing on branches when they were released immediately after being in a
metabolic
chamber at low temperatures. The feathers were also fluffed up in these
instances,
making the birds look like a round ball of feathers (did your YRT look like
this?). So
their behaviour was very much as you described. These birds recovered after a
few
minutes, once they had warmed up, and flew away if we picked them up and placed
them in
the sun to warm them up. Sometimes, we put a thornbill into a calico bag before
it was
released, which I placed under my jumper for a few minutes to allow my body
heat to warm
it up.
I know that WA does not get as cold as the ACT in winter (I have lived in both
places!),
but anything the size of a thornbill will find WA winter temperatures cold, and
will
need to be feeding continually throughout the day to fuel a high body
metabolism.
Perhaps your YRT was not able to find enough food and so had a depressed
metabolic rate
(as we found in the ACT thornbills) and became hypothermic which, in turn,
affected its
behaviour.
Just a thought ... Shane may have other suggestions from a veterinary
perspective.
Regards,
Stephen
Dr Stephen Ambrose
Research and Conservation Manager.
******************************************************
Birds Australia (Royal Australasian Ornithologists Union).
Australian Bird Research Centre,
415 Riversdale Road,
Hawthorn East,
VIC 3123.
Tel: (03) 9882 2622.
Fax: (03) 9882 2677.
Email: S.Ambrose <> (at work)
(at home)
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