Welcome to the forum Spadge.
Damned if I know, but it's a good spot. I suppose the obvious answer is that it was something like a Grebe or Gamecock - even EB's memory can't be faultless, although it usually seems to be.
Why not write and ask him? I'm sure that a letter via his publisher would get there quickly enough, and in my experience, is always very generous with his time and knowledge, and I'm sure would answer you.
G