On Fri, Nov 15, 2002 at 11:02:10AM +1100, wrote:
> Can you tell me what the purpose of the colour spectrum change at
> different angles would be? For example, I viewed this PBD from a
> downward angle.
> Would the "from above" angle cause the purple, and "on the same level"
> cause the green?
> Would the purple colour deter predators such as raptors, thinking its
> poisonous? And the green deter land based animals?
The PBD's speculum is barely visible when wings are closed suggesting
its not a signal to predators. Its definitely a signal to other PBDs
and I would guess used for multiple purposes including perhaps courtship,
agonistic displays, alarm and maintaining contact in flight.
The variation in colour with viewing angle may be a consequence of
the mechanism used to produced the colour, and not have any advantage.
Green pigments are found only in one group of birds (Touracos) - all
other green feathers are structural colors.
But its also possible this iridescence enhances the visual signal.
It is also possible it signals fitness to potential mates - if for
example parasites, disease or poor nutrition impairs the iridescence.
PBD's vision presumably extends into the near-ultraviolet so signals
visible to them may not be apparent to humans.
I've cc'ed this to birding-aus as someone must know more - Mallards are
closely related to PBDs and presumably well-studied.
Andrew
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