Here is my personal arrangement of the part number 9 of The Carnival of the Animals.
"The Cuckoo deep in the woods" (far less famous than "The Swan".)
Finding a "good" cuckoo sound wasn't easy.
In the orginal music, the composer send the clarinetist in the backstage, and the two pianos are playing pp.
Any reed stop makes a funny klaxon effect, like an old cartoon !
Very funny but far from the effect I was searching...
Finaly, I've chosen the Flute Harmonique : a good sound, but not coming from far away ...
In a real church, I think the good solution would be to play the cuckoo sound with small choir organ.