By watching eaglets over the last month or so, I discovered pin feathers. Mind you, I would have discovered them much earlier had I done more reading — or kept chickens.
A pin feather is just a feather during its formation. Normally, a feather is composed of non-living material, but when growing, it is enclosed in a protective keratin sheath and is supplied with nurturing blood. What is seen is the long whitish tube extending from a partially completed feather — a somewhat strange sight.
The problem with spotting pin feathers on a recently hatched chick is that the chick is usually deep inside the cup of a nest which is, itself, located high above eye level. By the time the chick emerges, the feathers have already formed.
However, I occasionally watched a nest of Bald Eagles from high on a distant bank, which placed my eye almost at the same height as the shallow nest. Further, the eaglets would occasionally spread their wings in practice flight, and in doing so, display some pin feathers on their wings.
I set the stage with a picture taken in mid-June (2021/06/14). I had used it earlier to illustrate that Bald Eaglets had mainly dark brown plumage, and so were readily confused with Golden Eagles. Most of the feathers shown here had already formed.
However, in a picture taken a week earlier (2021/06/06) white extensions can be seen from some flight feathers. At the time, I thought these were the feather shafts without realizing that they were actually enclosing keratin sheaths and were called pin feathers.
A picture of an eaglet’s spread wing in mid-June (2021/06/14) shows pin feathers on both the flight feathers and dramatically on the underwing coverts.
However, by mid-July (2021/07/11) all feathers look completely formed. Will flight be far off?
I include this picture of the two chicks, just because I like it (2021/06/29).
I then searched through my pictures of nestlings. The only one where I was looking down into the nest was one of robins. The pin feathers are apparent on the heads of the chicks (2016/07/14).