Chaffinch and Greenfinch (12 February 2003)
Chaffinch Fringilla coelebs is one of those species that gains its breeding plumage head colour by abrasion, rather than by growing new feathers before the breeding season. In the male, each feather is slaty-blue at the base, with a buff-coloured tip. As always with feathers, the lighter the colour the weaker it is, so the tips gradually wear off during winter. According to a study of Polish birds quoted by Ian Newton (New Naturalist - Finches), Chaffinch crown feathers lose about 1% of their length per month, but this rate of loss steps up to 7% per month in April and May. British Chaffinches are paired by April and have chicks in May so I assume that Polish birds breed somewhat later than ours. Many birds actually scratch their heads to accelerate the abrasion and the onset of their bright blue colour.
These couple of photos show the variation in the abrasion of the buff-edged head feathers of the males. They have not got their really blue bills so the hormones haven't started racing yet.
Several other species gain their head coloration the same way: Reed Bunting Emberiza schoeniclus, Brambling Fringilla montifringilla, Yellowhammer Emberiza citrinella, Fieldfare Turdus pilaris and Siskin Carduelis spinus amongst those that we most frequently handle. All of these are birds that tend to flock together, and I guess that looking less obviously male during the autumn and winter will tend to reduce aggression between individuals, yet this neat ecological adaptation allows them to attain breeding plumage without the effort of having to grow any different feathers.
At home this afternoon I had a male Greenfinch Carduelis chlorus with an interesting tail. The left-hand photo shows, at a quick glance, that it looks pretty much like a perfect adult tail, rounded and in good condition, but the right-hand photo reveals that it is a first-year bird (age 5) that has moulted all of its tail except for the third (from the middle) feather on both sides (t3), paler (brownish rather than greyish), more pointed, abraded tips, etc. In fact the tips of these feathers are just visible on the photo of the closed tail.