See West ad loc. Also cf.:

486. ἦμος: looks forward to τῆμος (488), cf. on 582, above (Appendix 1).

κόκκυξ κοκκύζει: the noun κόκκυξ was derived by onomatopoeic reduplication, like τέττιξ (see on 582, above). Κοκκύζω is used of cuckoos but often of cocks crowing also: see Ussher 1973. 77 on Ecclesiazusae 31. The bird’s cry, “κόκκυ” in Greek (Aristophanes Birds 505, 507), apparently was not regarded as pleasant: anon. A.P. 9.380.3.

δρυὸς ἐν πετάλοισιν: the singular πέταλον is infrequent; presumably Hesiod’s cuckoo is perched high up, for the δρῦς is commonly tall, or ὑψικάρηνος (Iliad 12.132) and ὑψίκομος (14.398, etc.).

487. τὸ πρῶτον: the cry is first heard in April in Greece: Pollard, p. 43.

τέρπει δὲ βροτούς: the bird’s call, though perhaps unappealing to the ear, was yet welcome because it indicated the end of winter and the return of spring: Aristotle History of animals 633a11 ἀφανίζεται δ’ ὑπὸ κύνα, φανερὸς δὲ γίνεται ἀπὸ τοῦ ἔαρος ἀρξάμενος μέχρι κυνὸς ἐπιτολῆς; Dionysius On birds 1.13 πρῶτος τῶν λοιπῶν πτηνῶν ἡμῖν τὸ ἔαρ ἀγγέλλων (cited in Thompson, p. 152).

ἐπ ’ ἀπείρονα γαῖαν: ἐπί + accusative denotes omnipresence: cf. Verdenius, p. 17 on WD 11.

489. μήτ ’ ἄρ ‘ ὑπερβάλλων βοὸς ὁπλήν: on the meaning of ἄρα cf. Verdenius, p. 44 on WD 49.

ὑπερβάλλω + accusative, ‘overflow,’ of rivers (Herodotus 2.111); probable sense here = ‘cover.’

μήτ ’ ἀπολείπων: the object is understood to be αὐτῆς (sc. ὁπλῆς). Ἀπολείπω + genitive, ‘fall short of/ frequently of numbers or units of measure (LSJ, s.v. IV. 2).

490. οὕτω: temporal sense (cf. LSJ. s.v. 1.7); refers to τῆμος (498).