

So, apparently Some species of hummingbird (sword-billed hummingbird in the pictures) use their feet for preening since their bills are so long and awkwardly-shaped. That makes me think of how deinonychosaurs might have used their toe claws to preen, almost like a dog. Perhaps therizinosaurs used their hand claws as well? That must've looked funny.
Images from BBC's Life of Birds, The Mastery of Flight.
Same goes for Theri as I don't quite know how limber they were. Unsure if they could reach everywhere with the claws. They COULD be social animals like Finches and preen each other where they couldn't reach though like how CJ suggested.
Interesting idea. Another thing birds do that I would like to imagine Dinosaurs doing is jumping into some water, ruffling their feathers and then jumping out to preen as what my Budgerigar and Zebra finches do. Perhaps flapping the feathers to dry them afterwards as does the finches.
I fully agree to Deinonychosaurs using claws to preen and sratch (For some weird reason, the ZT2 Velociraptor has that behaviour). I can almost them imagine sitting in a group and making social preening like primates.