Are feathers like hair, in that once they're grown the colour can't be
changed?
More or less. However, feathers can change their appearance, in some cases
very dramatically, both due to wear (abrasion) and fading.
In which case, for a change to breeding plumage the feathers must be shed,
and then a new one grown in it's place, or do they grow the new feathers
before the old ones fall out?
The new feathers grow from the same follicles that contained the old
feathers. The new feather basically pushes the old feather out.
Birds can change into "breeding plumage" either by growing in new feathers
or simply through wear of the old feathers. The former is illustrated by a
Curlew Sandpiper, which replaces most of its grey and white plumage with
feathers which are largely black and red. The latter is illustrated by a
male House Sparrow, which gets its black throat when the pale tips of the
feathers wear off. Strictly speaking, there is no change in plumage in the
case of the Sparrow, just a change in appearance due to wear.
Some birds do both. An interesting case is the Red-necked Stint which moults
grey feathers into black and rufous feathers with broad grey tips. So
initially there is little change in overall colour until the pale tips wear
off, revealing the bright colour underneath. This is even true of its red
throat, since the new red feathers have broad white tips which have to wear
off before the red becomes visible.
Cheers, Chris.
Chris Corben
www.hoarybat.com
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