Wednesday 6 May 2015

Colour - Beak Colouration

In early May birds are reaching the peak of their courtship and breeding dress here in Ireland.  A nice view here of a lingering Great Northern Diver (Common Loon, Gavia Immer) is a real treat.  In a couple of weeks this bird will presumably be back at it's breeding grounds in Iceland, Greenland or Canada.

In addition to having attained full summer plumage, it's bill is now totally black.  Hormones are responsible for both feather and bareparts adaptation for courtship and breeding.

Melanins and carotenoids are the main pigments involved in bill colouration.  The rhamphotheca is a thin sheath of keratin covering the surface of the bill and it is within this substrate that these pigments are laid down.  The actual combination and concentration of pigments is determined by hormones, diet and other factors.  The rhamphotheca is continuously growing and shedding, much like hair and nails in mammals, so colour is added gradually and is also generally lost gradually.  The pattern and colouration of the bills of individual Great Northern Divers in winter for instance may vary depending on individual feeding habits and their general condition, and this kind of variation is equally obvious among a flock of Starlings in the autumn.

The colour of the bill base of Common Starling (Sturnus vulgaris) in summer indicates its gender.  Pale blue for a boy (top left) and pale pink for a girl.  The colour wears and fades over the summer and is replaced with a melanin-rich black for the winter.  In an interesting study (HERE) it was found that a darker melanin-rich rhamphotheca is tougher than it's colourful summer equivalent so there are practical, survival reasons for this change.

Of course staining due to food or soil contamination can confuse matters.  Here the distinctive yellow bill of a juvenile Rose-coloured Starling (Pastor roseus) is almost entirely obscured by Blackberry juice (Rubus genus).

European Goldfinch (Carduelis carduelis) is colourful year-round and if anything appears to be much more in it's prime in the winter (when pairs may be forming from within social flocks).  While the bill of European Goldfinch is pale flesh coloured, with a black (more hard-wearing?) tip the black may be more extensive in summer.

I have looked at ultraviolet reflectance separately eg. HERE.  Probably the most interesting find I have made so far has been the lack of UV reflectance in the tip of the bill of Common Moorhen (Gallinula chloropus).

Emperor (Aptenodytes forsteri) and King Penguins (A. patagonicus, below) are also known to have UV reflectance patterns on their bills.

The appearance of bareparts colour can vary dramatically depending on lighting.  I have covered this in some detail HERE.  Translucency introduces additional challenges as illustrated below.

For more on beaks see HERE.

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