When Lee Tiah Khee was at Singapore’s Japanese Garden in Jurong around noon one day in October 2008, three Oriental Honey-buzzards (Pernis ptilorhyncus) flew to a tree above him.
One image in particular showed newly emerged wing/tail feathers as well as a few worn out through were and tear. The image was sent to field ornithologist Wang Luan Keng who sent in her analysis:
“Wing moult: Seems like this bird has arrested its primary moult before it migrated to Singapore. Hence you notice that the outer two flight feathers are rather worn out (birds usually replaced its inner primaries and progress outwards). This usually happens to big birds that have so many feathers to replace that they cannot do so in one annual cycle. Birds usually do not overlap energetically-costly events such as breeding, moulting and migration.
“So it will breed, then replace whatever feathers it could, migrate south and return to its breeding ground to again breed first and replace the remaining old feathers. And at the same time, start another new moult cycle.
“This usually creates a situation where you might see at least 2-3 feathers growing from different places. This is a strategy that many big birds use to shorten the duration needed to replace its old feathers before the migratory season starts.
“Tail moult: Birds usually moult from the centre and progress outwards in both directions. In this buzzard, the central feathers look new, probably recently replaced, the next 3-4 feathers are old feathers (one looked as if it might be at least 2 years old but it might just be a very worn out 1st year feather).”
Feathers undergo wear and tear and need to be replaced regularly. As new feathers develop, they push out the old ones and this is termed moulting. Moulting does not involve all the feathers at the same time. Otherwise the bird would be naked for a short period and extremely vulnerable to predators.
So feathers are gradually replaced and as one is fully replaced, a neighbouring one is shed. It may take one to a few weeks to fully replace a wing feather.
Moulting is also responsible for the immature plumage changing to that of an adult, or provides a breeding plumage for the males.
Image by Lee Tiah Khee.
This post is a cooperative effort between www.naturepixels.org and BESG to bring the study of bird behaviour through photography to a wider audience.