How can I get the boucassier's feather?

You first need to find something to attract the boucassier.

Look in the store for that item.

You will first need to read the hints in the store section to figure out how
to open all of those locked cabinets.  Once you do, you should find a decoy
in the "storage cabinet."

The decoy won't make noise on its own -- you need to hit it against
something.

Try hitting it against the bell.

To get to the bell, you'll need to use the same basic trick of walking across
roofs that was described in the previous question.  Read that question if you
don't know what that trick is.

Have Fulbert stand in the gap just to the left of the bell.  He doesn't need
to climb the plant to get to the gap -- just have him walk up the stairs and
then to the gap.

Now Blount can walk across and ring the bell with the decoy.