Lesser Adjutant: Growth bars of feathers

“I went birding with friends at Parit Jawa, off Muar in Malaysia on 20th December 2011 and was able to see many Lesser Adjutant (Leptoptilos javanicus) storks feeding along the river mouth. My attention was drawn to a bird hunting along the riverbank (above). It had lots of prominent textures on its feathers. These ridge-like textures were seen all over the back and wings, see feather details (below). I found this rather interesting as I have never seen this on other birds before. I asked Ms Wang Luan Keng from the Raffles Museum of Biodiversity, National University of Singapore on the function of these ridges and here is her reply.

‘”‘The ridges on the lesser adjutant is actually the growth bars of the feathers. This bird has just replaced its feathers. They all looked fairly new, with little signs of wear and tear. The growth bars show a light and dark band – each light and dark bar correspondence to a 24-hour period of growth. The dark bars are derived from materials laid down during the day and the light bars during the night. So scientists actually use some growth bars to calculate how much time is needed to replace all the feathers. (An important consideration as the feather growth is energy demanding, as with reproduction and migration.) And if the width of the growth bands is regular and there are no fault bars, it implies the availability of the diet. This stork has got good supply of food, so the growth bands are very regular. Very interestingly, all the growth bars seemed to be in line, which is usually an indicator that all the feathers were replaced at the same time – this usually happens only during the first set of plumage. I do not know about storks but this is interesting as the bird in the photo is an adult, not a young bird.”

“I have attached a picture of another bird flying (above), showing the wing feathers fully spread out to form an aerodynamic structure to create a big lift for this huge bird. Its a wonder to observe how this big lanky birds can take off effortlessly and soar high up in the sky. Indeed it is the Lesser but no less greater in size and flying ability than most birds.”

Thong Chow Ngian
28th December 2011

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