Birds and feathers
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The statute
It is unlawful "at any time, by any means or in any manner, to pursue, hunt, take, capture, kill, attempt to take, capture, or kill, possess, offer for sale, sell... any migratory bird, any part, nest, or egg of any such bird".
Possession sits in the list
Not as a lesser offence or an afterthought, but between killing and offering for sale. The statute treats having it the same way it treats getting it.
And "any part" is broad
It extends to "any product, whether or not manufactured, which consists, or is composed in whole or part" of the bird or any part, nest or egg. A feather in a craft project is covered by the same words.
Finding is not an exception
There is no provision distinguishing a feather that was moulted from one that was plucked. The practical reason is evidentiary: nobody can tell which it was from the feather.
Nests and eggs too
An abandoned nest and a broken eggshell are both named. This is the one people find hardest to believe and it is the plainest text in this property.
Which species are covered
The list of protected migratory birds is long and includes most native species a beachcomber will encounter. Assuming a common bird is not covered is the wrong way round.
There are permits
For specific purposes, held by specific people and institutions. They are not a thing a collector obtains casually, and that is the point.
So the practical rule is simple
Photograph the feather. Leave it. It is the one find on a beach where the legal position is unambiguous and the temptation is high.