But that's different, I doubt birds intentionally carry anything from Africa either.

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.

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.

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.

This is not a valid concept. Vertebrates develop from stem cells the same way insects do.

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.