Re: dino-lice

[Dann Pigdon <dannj@alphalink.com.au>]

On Thu, Apr 21st, 2011 at 9:03 AM, Vivian Allen <mrvivianallen@googlemail.com> 
wrote:

> On 20 April 2011 23:58, Dann Pigdon <dannj@alphalink.com.au> wrote:
> > It seems to be a one-way relationship. Amongst scavenging birds that 
> > regularly plunge their
> heads
> > into unmentionably nasty places, bare heads and necks are a distinct 
> > advantage (old-world and
> > new-world vultures, Leptoptilos storks).
> >
> 
> What's that actually based on, though? Other than the observation that
> they scavenge and they have bare necks. Then why (as Jason points out)
> do other bare-necked birds that don't scavenge exist? I mean, the
> implication that its easier to clean is intuititve, but the existence
> of exceptions suggests it could do with proper testing before it's
> counted as a definitive hallmark of scavenging.

Many bare-headed non-scavenging birds use brightly coloured skin or wattles for 
display purposes. 
This may occur when a lineage lacks the genetic basis for developing bright 
plumage, so brightly 
coloured skin takes over the role. Sometimes an outrageous set of wattles does 
the job even 
better than plumage would.

The yellow face of the Egyptian vulture, for instance, is achieved by the 
consumption of herbivore 
faeces that are rich in carotenoids. The body feathers (including a 
fully-feathered neck) are rather 
drab, as is their 'natural' skin colour.  If they don't get a regular supply of 
carotenoids in their diet, 
they lose their bright yellow face colouration.

Cassowary necks and heads also sport brightly coloured skin, which their rather 
drab body 
feathers otherwise wouldn't be capable of. The same seems to be true of ground 
hornbills - bright 
face/wattle skin colours, drab plumage.

The naked heads and necks of scavenging birds, on the other hand, are usually 
quite drab in 
colouration, so clearly their selective feather loss had nothing to do with 
display adaptations.

Naked heads and/or necks may also serve thermoregulatory purposes - both for 
scavenging and 
non-scavenging birds. A single adaptation after all can have more than one 
source of selective 
pressure.

-- 
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Dann Pigdon
Spatial Data Analyst               Australian Dinosaurs
Melbourne, Australia               http://home.alphalink.com.au/~dannj
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