My thought processes about the physiological influences of eye colour in
scrubwrens continue …
The main hormone responsible for plumage coloration in birds is testosterone,
though other hormones are involved too. Luteinizing Hormone (LH) and
Follicle-stimulating Hormone (FSH), secreted by the anterior lobe of the
pituitary gland, are responsible for the maturation of the reproductive system
in both sexes, and trigger the release of testosterone (males) and oestrogen
(in females) into the bloodstream. The pituitary gland is triggered to secrete
LH and FSH when there are elevated levels of Melanocyte-stimulating Hormone
(MSH) in the bloodstream, after it has been secreted from the pars intermedia
of the pituitary gland.
In my PhD study in the early 1980s I found that some adult Spotted Scrubrwen
males had brown lores instead of black ones and I concluded at the time that
this could be caused by more dominant males suppressing full reproductive
maturation of these birds because of limited breeding resources. However,
Belinda Brooker also observed brown-lored male scrubwrens during her PhD study
at Shark Bay in the late 1980s, and some of these individuals were breeding
(see HANZAB). Interestingly, Rob Magrath’s research group (ANU) has never
found brown-lored adult male White-browed Scrubwrens in the Canberra
populations that it has studied, so the only places that brown-lored adult male
scrubwrens have been recorded are among WA coastal populations of Spotted
Scrubwrens. They may occur elsewhere and I would be interested to hear from
anyone if they do.
In my earlier email, I hypothesized that Spotted Scrubwrens had suppressed
levels of MSH because of a need to suppress secretion of aldosterone by the
adrenal gland, allowing them to excrete excess salt (sodium ions) from their
bodies. If that is true, it may also explain why there are brown-lored adult
males (not enough MSH to trigger testosterone levels that are necessary for the
full blackening of the lores), as well as reducing or shutting down melanin
production in the eyes (blue eyes). Maybe this phenomenon doesn’t apply to the
Canberra birds (and other WBSW populations in eastern Australia) because there
isn’t such a high salt load in their diet and thus such an urgent need to
excrete excess salt from their bodies. An even wilder theory than the one
proposed earlier, but still worth considering.
Cheers,
Stephen
Stephen Ambrose
Ryde NSW
_____________________________________________
From: Stephen Ambrose
Sent: Monday, April 30, 2018 3:25 PM
To: 'Geoff Shannon'
Cc: ; 'Graeme Chapman'; 'Mike Carter'; 'Dr.
Richard Schodde'
Subject: Re: [Birding-Aus] Scrubwrens
Hi All,
I spent a bit of time over the lunch break reading up on other functions of MSH
(melanocyte-stimulating hormone). Several research studies suggest that it has
an additional role, along with other hormones, in stimulating the secretion of
aldosterone, a hormone secreted by the adrenal gland. Aldosterone facilitates
the re-absorption of sodium ions across the walls of kidney nephrons back into
the bloodstream, thus assisting in the retention of salt in the body. When I
studied the ecophysiology of Spotted Scrubwrens for my PhD in South-western
Australia in the early 1980s, I found that free-ranging scrubwrens,
particularly those around the Eyre Bird Observatory, had very high sodium
turnovers – a very high dietary sodium (salt) intake and high sodium excretion.
This was because there was deposition of airborne salt in the coastal
landscape, probably picked up from the surface of the ocean by prevailing
winds, especially during winter storms. Salt deposited on the coastal dunes at
Eyre, also coated invertebrates that the scrubwrens were feeding on, hence the
high dietary intake of sodium. So these scrubwrens needed to excrete excess
salt from their bodies, rather retain it, and I’m assuming to do this they
needed to minimise the excretion of aldosterone. For this to happen, maybe
they need to minimise MSH secretion which, in turn, reduces or shuts down
melanin production in the eyes, giving them the blue colour. A wild theory, but
one worth considering.
Cheers,
Stephen
Stephen Ambrose
Ryde NSW
_____________________________________________
From: Stephen Ambrose
Sent: Monday, April 30, 2018 11:37 AM
To: 'Geoff Shannon'; 'Graeme Chapman'; 'Mike Carter'
Cc: ; 'Dr. Richard Schodde'; 'Stephen Ambrose'
Subject: RE: [Birding-Aus] Scrubwrens
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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