In that sentence construction 'this' is the subject, so it could be to jest pies, or indeed to jest kaczka, i.e. still using the neuter not the feminine. At least I think that's it, just starting myself and I could well have misunderstood. For now I'm mostly just accepting it seems a normal way of using 'to' if 'this' (or that/it) are the subject. Hopefully one simply becomes used to norms. If it wasn't using 'this' as a subject, you'd still use ten/ta, so ten pies je, or ta kaczka chodzi for instance