“The male Crimson Sunbird (Aethopyga siparaja) above looks like it is just completing its first post-juvenal moult. The crimson plumage has yet to be uniformly developed as seen HERE.
“A newly hatched male Crimson Sunbird is naked. The first feathers to develop are the downs, soft and fluffy. Next come the juvenal feathers, the first true contour feathers. As the juvenal feathers grow (juvenal moult), they push out the downs, which may drop off or temporarily attached to the developing juvenal feathers.
“In the case of a juvenile male Crimson Sunbird, there would be no trace of crimson feathers. These crimson feathers appear in isolated patches around the head, back, throat and upper breast only when the bird develops its breeding plumage – see HERE.
“Moulting: The upper tail coverts (feathers covering the base of flight and tail feathers) and the outermost tail feathers are still growing. You can still make out the growth bands on the tail feathers (below). One dark and one light band makes it a day of feather growth, so you can actually count how many days the bird will take to replace one feather.
“The reddish edge to the tip of the upper tail coverts is quite interesting and was never mentioned before by authors of published text (above). Robinson (1927) described the upper tail coverts as metallic violet. I have no opportunity to examine a male Crimson Sunbird, so have not seen this coloration before.
“The oil gland (aka preen gland or uropygial gland), the white pimple-like structure below the yellow rump feathers can be seen clearly in the above image. Birds use the oil from the preen gland to maintain the suppleness of the feathers and prevent them from getting brittle (works like a conditioner). The oil was previously thought to be the waterproof agent for the feathers but now it seems that it is only indirectly so. The actual waterproofing was done by making sure that the barbs and barbules are well interlocked with each other, providing a tight surface on the feather which is not only crucial for providing lift but also to prevent water from wetting the feathers.”
Richard Lim (images) & Wang Luan Keng (text)
12th February 2016
(NOTE: Wang Luan Keng is a field ornithologist. She runs The Nature Workshop that undertakes nature education and outreach programmes to increase awareness of biodiversity and nature conservation.)