When I found this feather lying on the pavement I was sure it was from a goldfinch but its always good to have it clearly confirmed by a reference. And if its a bird (or even just a bit of a bird) you're in luck because the level of detail that's available is truly impressive. This book is 'Tracks and Signs of the Birds of Britain and Ireland' by Roy Brown etc.
Not only does it confirm that this is a primary feather from a goldfinch but, by its length, it is the 9th primary of 10 (i.e. the second one in from the tip of the wing) and its from the left wing. Furthermore you can find out when the adult moults its feathers. Mid July to mid October if you must know - unlike the juvenile which moults in August and September. (Note the wear and tear at the tip, by the way.)
Just about everything about birds has been inspected and detected, but you try and find out what the larva of the common or garden noon fly eats or how a tiny insect makes an oak leaf produce such weird and wonderful galls and you might as well give up now.