The relative clause ὃν ἔρεβος ἐτέκετο presents a striking metrical and syntactic anomaly: the verb ἐτέκετο is in the imperfect middle/passive, yet the subject ἔρεβος (Erebus) is the agent of generation. In standard Attic, the passive of γίγνομαι/τίκτω in this mythological context would typically be aorist passive (ἐτέχθη) or middle (ἐτέκετο with a reflexive nuance, which is rare). Here, the imperfect middle is used with an active sense ('Erebus was giving birth'), a construction attested in early poetic diction but irregular in Aristophanic comedy. This may reflect a deliberate archaizing gesture to evoke Hesiodic theogony (cf. Theogony 116–123), contrasting the sublime cosmogony with the absurdity of the birds' new regime. Alternatively, the form could be a scribal corruption for ἐτέχθη, though no manuscript variant supports this; the reading is consistent across the tradition.