Variegation, part 3: spangles

This series is all about the contrast of light and dark we can see in the plumage of birds.  The spangle of the Lizard canary is a special form of variegation, unique amongst canaries, yet as we saw in Part 2 , common enough in the bird world.  Unfortunately there has been no scientific research into the spangling of the Lizard canary so we have to look elsewhere.  We’ve already learned something about melanin distribution from greenfinches and budgerigars; in this episode I hope to learn something from chickens.

The feathers you see at the head of this article show some of the remarkable patterns found in poultry feathers.  They make the Lizard’s spangles look relatively simple.  They were the subject of a research paper published in Nature (1).  It is not light reading; the terminology is unfamiliar, and the authors (a collaboration of scientists in the USA and Taiwan) have quite reasonably assumed that their readers will have specialist knowledge of melanogenesis (melanin synthesis).  Nevertheless it makes fascinating reading.  Yes, really.

The research focussed on the bio-mechanisms that produce patterns in plumage; it did not investigate the genetics of pigment synthesis.  That’s probably why I liked it; instead of incomprehensible genetic coding we get diagrams showing how spots, stripes and spangles are formed within a growing feather.  Unfortunately I can’t avoid scientific terminology altogether, so here is a short glossary:

  • Feathers grow from follicles in the skin.  The follicle is cylinder shaped, with a collar and a base.
  • The dark colours (black, grey, brown) in the feathers are known as melanins.  They take the form of pigment granules known as melanosomes located within specialised cells called melanocytes.
  • Melanocytes are produced by another group of cells known as melanoblasts or melonocyte progenitors.  The authors preferred the latter term, and that is the one I will use here.

It all starts with the melanocyte progenitors; they are the key.  Without them, melanogenesis cannot proceed and no dark pigments can be deposited in the growing feather.

Unlike a plant which grows from the base of the stem upwards and outwards, a growing feather starts with the tip and keeps growing until the base of the quill is formed.  Human hair has many similarities, but crucially, when the melanocyte progenitors stop working (as part of the aging process) the loss of pigment production is permanent. That’s why our hair turns grey in old age.  In birds’ feathers, some melanocyte progenitors are always retained at the base of the follicle so that pigment production can resume at the next moult.

A nascent feather emerges as a cylindrical structure with a shaft (the rachis, or quill) in the centre and a vane which unfurls on each side.  The researchers discovered that melanocyte progenitors move about in the follicle collar during feather formation.  The timing and location of melanin deposition can therefore change as the feather grows.  There appears to be a feedback system in operation that signals when and where melanocyte progenitors need to be active.  The study discovered that several factors affect the distribution of melanins and thus the colours and patterns they form:

  • If melanocyte progenitors are absent in the upper collar of the follicle, few or no melanins will be deposited in the emerging feather; light edges or banding will appear in the vanes.
  • If melanocyte progenitors are absent from one side of the collar, or in isolated patches, light zones or spots will appear in the vanes.
  • A follicle is capable of producing feathers that change from juvenile to sexually dimorphic adult patterns during regeneration. The action of ASIP appears to be a factor. (2).
  • Hormonal activity and seasonal changes can further influence the production of melanins (3).
  • Carotenoid pigments (4) and structural changes (5) can change the background colour against which the melanins are seen.

If we examine the above factors, the first two are critical because the process starts with the melonocyte progenitors.  It is their location and activity within the follicle that control the distribution of melanins, and thus create patterns in the feather.  The other factors can only modify their appearance.

Why am I so interested in such a technical subject?  Answer: because I believe it could explain how spangled feathers are formed.

Of the factors that produce light zones in a feather, the most straightforward is the first: the absence of melanocyte progenitors in the upper collar of the follicle.  This is what I surmise may be happening:

A juvenile Lizard does not have a light edge to its spangles; they look like continuous stripes running down its back.  That is not unusual in the bird world; it makes the young birds less conspicuous and gives them a better chance of survival.  Here is an example of a 2016 juvenile:

Juvenile clear cap Lizard canary 2016

The follicle is dormant until the first moult begins.  The melanocyte progenitors are located at the base of the follicle, and in most self-green canaries, migrate to the upper collar in readiness for melanin production.  In the Lizard, however, the signal for the melanocyte progenitors to migrate is delayed.  That means the tip of the new feather emerges before the melanosomes can be deposited in the developing vanes.  The result is a dark feather with a light fringe; in other words, a spangle (6).

The process of spangle formation that I have described here is conjecture, but if it works for chickens, it could well work for the Lizard canary.  The simplicity of the mechanism adds to its plausibility.  Spangle-like markings are so common in the bird world (as we saw with Stanley) that a relatively straightforward process is much more likely to be replicated across species than a highly complex one. Of all the feather patterns, spangling is actually one of the simplest.  It is only when you see the spangles arranged in symmetrical rows that the design looks so attractive.

Here is the same bird after the moult:

Mature clear cap male Lizard canary

The bird has been transformed.  If I am right, we now have scientific evidence that explains the process, but that does not diminish its beauty.  It is still a joy to behold.

Footnotes:

The genetic coding for spangles or other patterns was not discussed in the report, and I am not going to speculate about it here.

  1. Topology of feather melanocyte progenitor niche allows complex pigment patterns to emerge  by s.J. Lin et a ( Science. 2013 June 21; 340(6139): 1442–1445.)
  2. ASIP = Agouti signaling protein, which inhibits eumelanin, and enhances phaeomelanin production in the feathers.  This is most obvious in the brown-grey edges to the wing feathers of the Lizard canary, and the bronze tone in the ground colour of gold hens.
  3. For example the American goldfinch Spinus tristis goes through two major colour phases each year known as the nuptial and eclipse plumage.
  4. Carotenoids are the yellow, orange and red pigments we see in birds.  These colours are obtained from the bird’s diet e.g. colour feeding with canthaxanthin during the moult will turn a yellow canary orange.
  5. Structural colours are producing by the light-scattering effect of the feathers.  Examples include iridescence in the plumage of starlings and humming birds, and the ‘blue’ colour in budgerigars.  It is also evident in the tail of the chicken above.
  6. The colour banding in a Lizard’s spangle is actually more complex than that, but this explains the basic principle.

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