The Little Heron (Butorides striatus) chick rescued from the Bukit Timah campus and kept in captivity for about a month, allowed for me to observe feather development.
The literature says that at hatching the chick is naked and blind. So the image of the rescued chick (left), taken on 3rd November, was at least a week old, if not more. It was covered with feathers. On closer examination, the soft down feathers known as natal downs, that develop at the time of hatching, were still present. These covered the entire body but very obvious around the head and neck (left, arrowed).
These downy feathers provide the chick with an insulating cover during the period when it cannot fly.
The natal downs are replaced by the juvenal plumage, the first true contour feathers. These true feathers are initially covered with a thin feather sheath, looking like the tip of a knitting needle. At this stage they are known as pin feathers. As the pin feathers grow, they push out the natal downs, to expose the feather vanes.
The image above was taken on 6th November. Note the abundance of natal downs on the head and pin feathers on the side. Some of the natal downs are seen at the tips of the pins. The image below, taken on 16th November, shows the absence of pins, the juvenal feathers having fully emerged from their sheaths. However, there are still some traces of natal downs on the head.
In the image below taken on 8th November, the wing feathers are breaking out of their sheaths and slightly more than the upper halves of the feather vanes have unfurled. The natal downs, earlier seen at their tips, are generally absent.
By 20th November, most of the wing feathers have fully emerged from their sheaths (below left). This means that the bird should be able to fly soon. However, not all feathers, other than the flight feathers, have been fully formed. Traces of natal downs are still visible and delicate white flakes of feather sheaths are still being shed (below right).
(Images by YC Wee except close-up image of developing wing feathers by Lin Yangchen and developed wing feathers by Chan Yoke Meng)
Martinez-Vilalta, A. & Motis, A. (1992). [Family Ardeidae (Herons)]. Pp. 376-429 in del Hoyo, J., Elliott, A. & Sargatal, J. eds. Handbook of the birds of the world. Vol. 1. Ostrich to ducks. Barcelona: Lynx Editions.