Question: what was the fact about the bird feather.

Adam Washington answered on 12 Nov 2019:

The simple experiment is that, if you take almost any white or blue bird feather and soak it in water, it will become transparent. If you then that the feather back out of the water and let it dry, it will go back to its original colour.

The feathers themselves are made of keratin, the same thing as our fingenails, which is largely clear. However, the barbs contain tiny holes which are between 100 nm and 400 nm in size. These holes are around the same size as visible light, so the light scatters off of these holes, changing the feathers from clear to opaque white (or blue, if all the holes are on the smaller side). Soaking the feather in water causes the holes to fill up, so there’s nothing to scatter off of and the feather turns translucent. Drying out the feather empties the holes and brings the colour back.

As an interesting aside, this simple experiment goes back to Robert Hooke’s early microscope studies in the 1600’s.