I found this definition in Liddell-Scott today and I began wondering whether the gloss is correct. There is nothing in Aristotle’s Historia animalium which would imply that the egg in question is “rotten or putrid”.

κῠνόσουρος, ον, ᾠά addled eggs, Arist.HA560a5; cf. οὔριος iv.

See for yourself.

If it thunders while a hen-bird is brooding, the eggs get addled.

Wind-eggs that are called by some cynosura and uria are produced chiefly in summer. Wind-eggs are called by some zephyr-eggs, because at spring-time hen-birds are observed to inhale the breezes; they do the same if they be stroked in a peculiar way by hand.

᾿Εὰν δὲ βροντήσῃ ἐπῳαζούσης, διαφθείρεται τὰ ᾠά.

Τὰ δὲ καλούμενα ὑπό τινων κυνόσουρα καὶ οὔρια γίνεται τοῦ θέρους μᾶλλον. Ζεφύρια δὲ καλεῖται τὰ ὑπηνέμια ὑπό τινων, ὅτι ὑπὸ τὴν ἐαρινὴν ὥραν φαίνονται δεχόμεναι τὰ πνεύματα αἱ ὄρνιθες· τοιοῦτον δὲ ποιοῦσι καὶ τῇ χειρί πως ψηλαφώμεναι.