All bipeds are rational. Swans are bipeds. ∴ Swans are rational. commits the fallacy (hetvabhasa) of :

The middle term 'biped' is present in rational beings but also in non-rational ones like ordinary birds, so it wanders across both.

A valid reason must be present only where the probandum holds and absent everywhere else.

Here being a biped does not invariably carry rationality, so the concomitance fails.

A reason found in both the positive and the negative field is the irregular or wandering middle.

This defect is savyabhicara, the anaikantika hetu.