ἐρῳδιὸν
This bird is a night heron. The Venetus A scholia says that it is an auspicious sign for Diomedes and Odysseus as they depart for clandestine activities and appropriate to the marshy area that they are in (see also 10.466–468 on the tamarisk bush for the marshy place of the ambush of Dolon, as well as “The Poetics of Ambush” for the kind of location where ambushes are often set up). The scholia in the Townley (T) manuscript, according to the edition of Maass, likewise call the bird a good sign for ambushers (ἀγαθὸν λίαν τὸ σημεῖον τοῖς ἐνεδρεύουσιν) and appropriate both because of the marshy place and because it hunts at night and is rapacious (νυκτὸς ἀγρεύει καὶ ἁρπακτικόν ἐστιν). The night heron does exhibit ambush-like behavior in its hunting: it waits while standing still for its prey to come into range at night and also plunders the nests of other birds (see e.g. http://www.enature.com/fieldguides/detail.asp?recnum=BD0117 or http://www.sdakotabirds.com/species/black_crowned_night_heron_info.htm). But what we see in these scholia is part of a “system of associated commonplaces” (Black 1962:39 and above at 10.5–9) that the ancient audience would have had for this bird—namely, that the bird itself is an ambusher found in places used by warriors for ambushes—and those associations make it an appropriate omen. Such associations can be common without empirical evidence of the creature’s actual behavior. The fact that the bird is heard but not seen is also appropriate to the poetics of the night, where hearing is the predominant sense.
See Thompson 1895:58–59 for ancient sources on the ἐρωδιός: he notes that the heron is a symbol of Athena on coins. See also Pollard 1977:68–69 for this bird in general. Pollard notes a story from Aelian relating that Diomedes’ men were transformed into herons (Pollard 1977:164), and Vergil alludes to the story that Diomedes’ men were turned into some kind of bird at Aeneid 11.271–274. Other stories show evidence of a connection between herons and horses (Thompson 1895:59, Pollard 1977:167).
For another bird omen at night, compare the bird omen Zeus sends to Priam before he infiltrates the Achaean camp (Iliad 24.315–321).