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:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4848976/
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.
Cheers,
Stephen
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
birding-aus.
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.
Cheers
Graeme
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
yellow
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