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Pacific Black Duck

To:
Subject: Pacific Black Duck
From: (Andrew Taylor)
Date: Fri, 15 Nov 2002 12:14:21 +1100
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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