On Tue, Dec 12, 2006 at 12:19:16PM +1100, wrote:
Also, does anyone know whether crests are known to act as a "heat sinks"
for birds to regulate head temperature?
I can see two possible thermal advantages. Raised feathers may by
exposing the skin increase convective heat loss to the air. This could
be useful below 30C but it will quickly become useless as air temperature
approaches the bird's desired internal temperature (~40C) and would be
counter-productive at higher air temps.
Raised feathers might reduce heat gain from radiation by absorbing
sunlight before it reaches the skin with the air gap thermally isolating
the feathers. Essentially the feathers would act as shade.
Feathers can't act as heat radiators because they don't contain blood
vessels. The raised feathers might be a signal rather a thermal adaption.
At 40+C evaporative cooling is the only recourse for birds, and us.
Unlike us birds don't sweat, they evaporatively cool via their respiratory
system perhaps enhanced via panting or gular fluttering.
Humans will find huddling uncomfortable at 40C because it will impair
cooling via our skin. This isn't an issue for Ricki's huddled White-eyes.
If it didn't impair respiratory cooling, huddling could conceivably assist
at very high ambient temps by reduce heat flow from the environment.
High temperatures don't necessatily cause problems for birds. I recently
saw a surprising amount of bird activity at 41C (shade air temp) in
riverine vegetation in the Top End. Birds were calling, flying and
foraging at this temp - which might double their cooling requirements.
But these birds also had plenty of shade to avoid solar radiation,
humidity was not high so evaporative cooling was effective and they had
easy access to water so water loss was not an issue.
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