Reading room participation is a great way to take a fresh look at
Silver Chair.
And at Glimfeather, in particular.
Much of the humour about Tu-whoo'ing or not tu-whooing when dealing with owls, is to do with humans imitating owls, and not the other way around. Whilst it might be cheesy for a movie Glimfeather to speak how we consider owls to speak, the fact is that Glimfeather is an owl, a proper owl, who acts and eats like one. And would speak like one, rather than an average conspirator or politician, even if he goes to an owlish parliamentary meeting.
Humour in SC is also in the interactions between different characters. EG when Glimfeather escorts Jill and Eustace to meet Trumpkin, who is going deaf, to see what help they can get. Trumpkin's deafness may make funny dialogue, but it might also be a problem for Narnians and would-be missing prince seekers. A few well-placed tu-whoos, like a speech impediment, would certainly illustrate exactly why not greeting Caspian at once has left Eustace and Jill out on a limb, and why Trumpkin, through no fault of his own, might have become a bit too much like Governor Gumpas over the years in dealing with business.