Finding out edge-cases in complex behavior by accident - while trying to fix bug A, indroducing bug B which then leads to discovering unrelated unexpected behavior C with then explains old mystery bug D.
- Antoine de Saint-Exupéry
What a Freudian story, looking for male genitals. I wonder what would Freud say about that...
I'm guessing/hoping this was more folklore and not a serious theory :)
That said, the idea of animals transmuting into a different species is at least backed by apparent precedent (caterpillar-butterfly, tadpole-frog), and non-trivial to disprove. That maggots spontaneously generate from rotting meat, or some of the 'cryptids' described by Pliny (https://medium.com/exploring-history/meet-six-of-the-beasts-...), err less so.
I look forward to seeing what we're completely, laughably wrong about today, if I live long enough. I hope it's one of the things I am right about --- but I'm not counting on it.
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.