You can't do anything with the coffins on the right side of the room above the grate where Harper came out, once you've read the article there and picked up the bird drawing.
The coffins on the left side of the room, however, are a different story. Pull out one of them at random and see what you see.
Each coffin lid has a square slot with some odd little slots in it and a button. But one of them is different --
-- the coffin in niche 5, the middle one on the far left. This one has a picture of a bird in the square slot. Huh?
You're eventually going to have four of these puzzles to solve. For each one, you'll need three things: a plate like the one you found on the coffin in niche 5, a drawing showing the design you need to reproduce, and some sort of clue as to which coffin the design needs to go on.
If you put a plate on a coffin and solve the puzzle and nothing happens, you've got it on the wrong coffin. Push the square button to remove the plate and try another coffin.