My apologies for the delay in participating in this discussion. I agree with
Andrew's comments on this issue.
Phillip Veerman is also correct in saying that some arid-zone bird species have
short feathers or bare skin to increase convective heat loss. The most notable
of these species in Australia is the Emu which has very short feathers and
well-vascularised skin on the head and neck (known physiologically as heat or
thermal window). In addition, it is important to recognise that the most
important part of the body to keep cool is the brain. The Emu is the bird
species with the smallest brain relative to body size, its brain has a high
surface-area to volume ratio and so it is adapted to losing more convective
heat than a bird with a relatively smaller brain size. Interestingly, if emus
are near a water source on a hot day they will splash water over their head and
neck, which would result in evaporative cooling.
However, it is also important to note that arid zone areas can also become very
cold at night, so species have to adapt behaviourally, physiologically and
morphologically to avoid excessive loss of body heat. Therefore, it does not
suit small bird species to have large areas of bare skin under these
conditions. In fact, a number of studies have shown that very small birds
species that live in the arid and semi-arid zones tend to have a dense matrix
of downy body feathers at times of the year when there are cold nights,
presumably to keep them warm. In at least one Australian species (the
Spinifexbird) it appears that the downy feathers on the breast and abdomen are
also used to collect water droplets from the dew deposited on vegetation
overnight, thus providing a ready source of drinking water. This is documented
in the following paper:
Ambrose, S. J., Bradshaw, S. D., Withers, P. C. and Murphy, D. P. (1996). Water
and energy balance of captive and free-ranging Spinifexbirds (Eremiornis
carteri) North (Aves:Sylviidae) on Barrow Island, Western Australia. Aust. J.
Zool., 44: 107-117.
My own observations of behavioral tolerances of bird species to each other at
excessive ambient temperatures support what others have described on
Birding-aus. I published the following small paper of my own observations at
Hamelin Station, Shark Bay on this topic:
Ambrose, S. J. (1984). The response of small birds to extreme heat. Emu. 84:
242-43.
I don't have this paper in front of me, so I'm relying on my memory, but it was
of observations of birds seeking shade when the shade temp was about 47 C and
the temp directly in the sun was about 65 C. It was not a pleasant day, I tell
you!
Dr Stephen Ambrose
Director
AMBROSE ECOLOGICAL SERVICES PTY LTD
|
