To: "'Geoff Shannon'" <>, "'Graeme Chapman'" <>, "'Mike Carter'" <>
Subject: Scrubwrens
From: "Stephen Ambrose" <>
Date: Mon, 30 Apr 2018 11:36:46 +1000
Hi Geoff,

Eye colour in birds is due to the presence or absence of pigments in the iris 
(mainly melanins and lipochromes) and the refraction of light.  Birds have two 
forms of melanin:  (1) eumelanin, which gives rise to dark black, brown or grey 
colorations, and (2) pheomelanin, which gives rise to lighter yellowish to 
reddish colorations. Blue eyes are a result of the absence of melanins in the 
iris. In humans, blue eyes are a result of a mutation of the HERC2 gene, 
resulting in an inhibition of melanin production in the eye, but in other 
primates with blue eyes (e.g. lemurs) there is no mutation to the HERC2 gene.  
This indicates that blue eyes in humans and distant primates is a phenotypic 
convergence.  As far as I am aware, the genetics of eye coloration in birds is 
not well understood, but Galvan & Salvano (2016) discuss it to some extent. 
This paper also describes the biochemistry of melanin synthesis in the cells of 
the integument in birds, the whole paper can be downloaded online:

In comparison, the neural and hormonal mechanisms of melanin production in 
vertebrates have been quite well studied and there is a good summary of them in 
Bentley, P.J. (1982, Comparative Vertebrate Endocrinology, 2nd ed., Cambridge 
UP, Cambridge). Melanins are produced by special cells called melanocytes in 
the skin’s epidermis, the middle layer of the eye (the uvea), the inner ear, 
vaginal epithelium, meninges, bone and the heart. Melanin production is 
stimulated by increased levels of melanocyte-stimulating hormone (MSH) 
circulating in the bloodstream. The eyes (and in some lower vertebrates, the 
pineal gland) pick up visual cues related to daylight length and intensity of 
light. Light hitting the retina stimulates receptors that transmit nerve 
impulses to the hypothalamus in the brain. These, in turn, inhibit the normal 
inhibitory effects of nerves that connect the hypothalamus with the pars 
intermedia of the pituitary gland, and this causes the pituitary to release 
MSH. At shorter daylengths and lower light intensities, the retinal impulses 
are weaker or less frequent, so the inhibitory effects of the hypothalamus on 
the pituitary gland are strengthened and MSH secretion is reduced.

It appears that MSH can also stimulate testosterone production in male birds, 
either directly or by acting on other hormone cycles, and in turn, testosterone 
can act directly on pigments within the integument, including the eye. 

The blue eye colour in the Spotted Scrubwren could be the result of a gene 
mutation early in the evolutionary origin of the species, given that blue eyes 
do not appear to be present in other scrubwren species, and it has been passed 
onto subsequent generations. But the apparent absence of other eye colours in 
the Spotted Scrubrwen suggests that blue eyes have a functional advantage and 
there has been natural selection that favours this coloration. Alternatively, 
it could be diet-related; perhaps there is something in the diet of Spotted 
Scrubwrens which inhibits production of melanins or other pigments in the eyes, 
which is not found in the diets of scrubwrens in eastern Australia.

One thought that has crossed my mind is that the Spotted Scrubrwen is found 
more generally in environments where the intensity of light and ambient 
temperature are likely to be greater than in forested and coastal environments 
along the east coast of Australia. For a species that spends a lot of time 
foraging on or close to the ground, there would be a lot of reflected sunlight 
reaching the eyes, so you would expect Spotted Scrubwrens to have pigmented 
eyes to protect them from harmful UV radiation. But dark objects absorb all 
wavelengths of light and converts them into heat, so the object gets warm.  
Therefore, you would expect pigmented eyes to heat up more quickly than those 
that are not pigmented.  So perhaps blue eyes in the Spotted Scrubwren reduce 
the risk of the eyeballs (orbits) becoming too hot (and cooking) when exposed 
to intense light conditions, especially when in coastal dune scrubland. But if 
this is true, why don’t you find blue eyes in other species (e.g. fairy-wrens) 
in similar environments? And why do Spotted Scrubwrens in forested environments 
of South-western WA (as opposed to coastal scrubland and heathland) have blue 
eyes?  These two points indeed expose a weakness in my hypothesis, but perhaps 
it is just one adaptive pathway to surviving those environmental conditions, 
whereas other species have found other adaptive mechanisms. 


Stephen Ambrose
Ryde NSW

-----Original Message-----
From: Birding-Aus  On Behalf Of 
Geoff Shannon
Sent: Sunday, April 29, 2018 5:10 PM
To: Graeme Chapman; Mike Carter
Cc: ; Dr. Richard Schodde; Stephen Ambrose
Subject: Re: [Birding-Aus] Scrubwrens

Just to add picture, male and female Tasmanian Scrub- wren  April 2018. 
Interesting discussion.
I am interested if anyone has good reference to the physiology / biochemistry 
etc on eye colour changes. Is it just age or are there other factors? There has 
been some discussion with Brown Thornbills ability to change colour seasonally 
or even acute stress. I do not have references. Thanks 

Geoff Shannon 

On 27/04/2018, 4:04 PM, "Birding-Aus on behalf of Graeme Chapman" 
< on behalf of 
> wrote:

    Hello Mike,
    Thank you so much for replying to my request. I'm attaching my K I image 
that should have been attached to the original but was removed somehow in 
    I'll agree, your bird's eye has a greenish tinge but it is also rather dark 
and dull and in my opinion, a probable young bird.
    I've been looking at the books on this one and what a can of worms! In 
HANZAB you can take your pick in the text on soft parts and in the plate, it 
shows maculatus with a yellow eye, which is wrong. The new CSIRO "guide" opts 
out altogether on iris colour in the text and the plates are really too small 
be of any use on this subject. What a pity this book wasn't published as a 
concise handbook in A4 format. They obviously have all the information but it 
has been compromised by shoehorning into too small a space.
    What I am fairly sure about is that age is a factor here as it is with many 
of our small birds - we know so little because so few of our birds have been 
studied in detail.
    I have a wide range of pics of this species and I'm attaching a few examples
    The first two are Brown Scrubwrens from Tasmania. The first one is an adult 
male at the nest - I would describe that iris simply as yellow.
    The second bird at the same nest I always assumed to be a female (on 
plumage) but it has an olive coloured eye - such dull colours are usually 
characteristic of younger birds and it is much more likely to be a helper than 
the adult female, or if it is the female it is a younger bird. I don't expect 
the sexes to have different coloured eyes as adults. This eye colour reminds me 
of your K I bird.
    The next pic is what I regard as a classic frontalis  and I would describe 
that as pale yellow - virtually all east coast birds are like this, maybe a bit 
darker as you go north.
    The last is the old "Buff-breasted Scrubwren" of the 1926 Checklist from 
northern NSW, slightly darker yellow.
    I guess what I am on about here is the difference between "Spotted" and 
"unspotted" birds. All the Spotted ones I've seen have the pale blue eyes, 
which gives them a totally different (to me) look.
    So. how long do they take to become adult ( by eye colour )? I'd say at 
least two years.
    I guess I'm on the same bandwagon as my recent comments on Eastern 
Whipbirds. Most of the books get that one wrong and show adults with brown 
eyes, whereas in fact they are cream. How long it takes nobody knows but it's 
likely to be similar to the Grey-crowned Babbler which is four years.
    What really started this interest was the years I spent with Ian Rowley 
studying corvids and choughs, both of which can be aged by eye colour, a very 
handy indicator when you are looking at life history. We worked with birds we 
banded in the nest, so we KNEW how old they were. Getting to the Australian 
Raven's nests was interesting, I can't even lift a rope ladder any more, let 
alone climb one.
    Spotted Scrubwren from Kangaroo Island - eyes pale blue 
    Brown Scrubwren male at nest, eyes yellow. I assume this is an adult.
    Brown Scrubwren at nest, probable immature, eyes olive.
    White-browed Scrubwren, Gloucester NSW. eyes pale yellow. Virtually all 
east cost birds are like this,
    White-browed Scrubwren. Tooloom northern NSW subsp.laevigaster -  eyes 
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