It's not clear to me these theories were outside scientific enquiry: They had an observation: birds disappear in winter, that they were trying to explain. As

It's not clear to me these theories were outside scientific enquiry: They had an observation: birds disappear in winter, that they were trying to explain. As you say there is precedent with other animals transmuting of which they had observations of. So it's not really unscientific to combine these two pieces of evidence into a theory. Certainly happens often enough today that people do these things, but end up being quite wrong.