I've now found the bit in which Jill does her anti-hen rant, and it's quoted below.
"I have said little or nothing about our hens, as they are Mummy's concern and not mine, though actually I could write a book about the way they play Mummy up. I consider them the most soul-less of animals that will not respond to overtures of friendship, and for that reason I will not have anything to do with them in life. They are at their best when dead. Also I think there is something rather repulsive about them." (Jill's Gymkhana, 1st edn, 1949, pg 87)
I did blink a little when I read it: it's quite amazingly strident. It doesn't to me read like something that Jill would say, and it's one of the more understandable abridgements. The piece does however, seem to have the ring of bitter personal experience, so I wonder if Ruby Ferguson had hideous hen experiences of her own? I doubt we'll ever know. I can't see "The Use of Hens as Plot Devices in Children's Literature" having a future as a PhD subject, but you never know.