Monday, 17 August 2015

Spring Plumage

Diamond Firetail Stagonopleura guttata in bright adult breeding plumage
Spring is ever nearer in the south-east Australian bush and many of the birds, especially the passerines, are beginning to breed. Which means that they are now in full breeding plumage and looking their best.

Over the past weekend I was part of a team mist-netting birds at The Charcoal Tank Nature Reserve in south-western New South Wales, as part of a long-term study of birds in the area, organised by Mark Clayton. And it was clear by the condition of the birds' plumages that they were fit, healthy and ready to breed.

Diamond Firetail 
There was a flock of Diamond Firetails feeding on grass seeds along the edge of the reserve and they were dazzling as their red rumps flashed through the grasses. The bird shown here is an adult in full breeding plumage, with all the feathers on the head and breast clean and fresh. For these are the feathers which any potential mate or competitor would see when assessing its fitness. However, the flight feathers, the primaries secondaries and their coverts were a bit tatty, with chips and worn edges. For these feathers would have been moulted and re-grown at the end of the previous breeding season, and they will be again at the end of the current season, but for now they are still perfectly functional and there is no need to replace them yet. Birds are almost constantly moulting part of the their plumage at any time of the year, and for the moment it is the bright signalling feathers that need to be in top condition.  

White-plumed Honeyeater Lichenostomus penicillatus
It is not only their feathers which are spruced up for breeding. The white-plumed honeyeaters which we caught, also adopt shiny dark colouring to their bills - they are duller and greyer when not breeding. This adds impact to their dark eye and the slight fringe of dark feathers on their neck, next to their white-plume, which the dark colouring emphasises. By the time the birds are half-way through the breeding season, the best of these colours will have begun to fade.

Chestnut-rumped Thornbill Acanthiza uropygialis
Even the little-brown-jobs, the small dull coloured birds adopt brighter plumage. This chestnut-rumped thornbill had a particularly well marked face pattern, in relative terms, and it's only when we see them up close, in this case in the hand while ringing/banding them, that we can appreciate the detail of these tiny birds' markings. Although to a bird's eye, they see things across a wider spectrum than us, these markings probably also signify the birds health, fitness and hierarchy in the flock, only more demurely.

Chestnut-rumped Thornbill - face
The sexes of all three previous species have similar plumages to one another of their species, they are monochromatic to our vision - but probably not to their own. For birds can see ultra-violet colours and in some species this has been found to be used in their markings to differentiate between the sexes. Do these species sport such colouring?

Golden Whistler Pachycephala pectoralis - male
Some of the brightest birds in the study were the male golden whistlers, and if their plumage is enhanced with ultraviolet colouring they must be amazing to see through a whistler's eyes. This species is dichromatic - there is a colour-difference between the male and female. The female is an almost overall grey colour, which is fit for purpose, as she incubates the eggs and broods the young. That involves a few weeks of her life each year, spent sitting on a nest set amongst grey-barked branches, so the grey colouring acts as camouflage while she is on the nest. The species has evolved for the females to be camouflaged while the males are brightly coloured to show off their fitness and hence value for a female to mate with, or for another less-brightly coloured male to avoid.

Even in her grey plumage, the female can be seen to be healthy and fit to breed as her feathers are clean and fresh. She too is fit to breed.

Golden Whistler - female

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