Not a twig, but there is a recorded case of a bird making it to Europe after being shot with an African arrow. (Which it involuntarily carried with it.)
"Before migration was understood, people struggled to explain the sudden annual disappearance of birds like the white stork and barn swallow. Besides migration, some theories of the time held that they turned into other kinds of birds, mice, or hibernated underwater during the winter, and such theories were even propagated by zoologists of the time."
Ok, it seems that Aristoteles believed, that birds actually transformed, but zoologist of that time at most believed, that they hibernated underwater", which seems a more solid theory, than transforming.
Shape stability would be an example of being less complex than insects.
And as you can see, we did not in fact know that in the 18th century. Transformations are not so easy to observe directly; they often happen e.g. underground.
"Shape stability would be an example of being less complex than insects."
Not if the shape is stable, because the underlying cells are too complex to merge into something different. Cells were known already. And that insects transform is known, but birds were closely known and there was not observation of them changing drastically. So that Aristoteles had this thinking is understandable given the time, but after enlightenment, I would not expect that from the early scientists.
> Besides migration, some theories of the time held that they turned into other kinds of birds...
Makes me think of loons, which completely change their plumage for the winter in addition to migrating from fresh water lakes to oceanic waters. Without close study, you might not realize they're the same birds.
