Re: Birds' eyes

To: Stephen Ambrose <>
Subject: Re: Birds' eyes
From: James Davis <>
Date: Thu, 26 Mar 1998 16:45:00 +1000 (EST)
I have a few questions.

On Thu, 26 Mar 1998, Stephen Ambrose wrote:

> I suspect that the violet eye colour in male Satin Bowerbirds is a signal
> that it?s sexually mature. While the colour may be as a result of food
> pigments in the diet, I think it is more likely to be hormonally-influenced.
What is the colour of male and female juveniles?

> The main male sex hormone, testosterone, binds to receptors in cells that
> are involved in sexual display, producing the secondary sex characteristics
> necessary to attract the attention of females. Testosterone levels are
> genetically-influenced (e.g. hereditary levels, age of maturity),
> environmentally-influenced [e.g day-length, rainfall, diet (composition and
> quantity)], behaviourally-influenced (e.g. presence and behaviour of
> females, position in social hierarchy) and the bird?s condition (health,
> parasite-loading and age).

Nicely put, however this arguement is a proximate explanation. Could there
be another reason why purple/blue was evolutionarily choosen?
> Interestingly, male White-winged Choughs have white-pale yellow eyes as
> immatures, but become orange-red as they become adults. During the breeding
> season, displaying males have bulging red eyes, presumably as a result of
> forcing extra blood into blood vessels of the eyeballs. Imagine this
> happening in humans!!!!!!

Is the red colour of an unexcited male due to blood or a pigmented iris?

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