I haven't the slightest idea. They don't seem outright malicious in the instances mentioned at least. With what little to sink into, I think I found more references:
Partner. Phroneos used to call him that back in Crete, all the way until that night. A [white bird] flying off under the moonlight. The minotaur aimed his bow and drew the string. It was not too different from plucking his lyre. That last tension before letting go. For Phroneos, for that everlasting friend, but first of all for his brother, intrepid Androgeus, he set the arrow loose. Partner. They were hunting together that night. But by the end of it Phroneos called him something else. And that was the last they saw of each other.
Androgeos. I never asked if you liked the feathers. That night was much like this. I aimed my bow to a [star], as if somehow my arrow could reach it, and set it loose. Three times I fired an arrow, and then [it] fell. Phroneos and I prepared the fire. Our sacrifice to you, feathers of a [holy bird]. As for Phroneos... We had no common blood but we were brothers just the same.
This is probably the night Asterion was talking about. Now, bear with me here, as I try to interpret the second quote. I believe Asterion shot down the bird in three attempts. Star may be referring to a distinct patch of wing feathers, or just a way of saying 'aiming skyward'. The antecedent of 'it' is probably the bird, seeing that it should be plural if referring to the arrows. If that's to be believed, then the holy bird they saw (and shot!?) is a vulture. He did so for Androgeus, who is presumably dead-- sacrifice offered through burning and all. Anyways, that vulture is probably present in/native to Crete, seeing as this is where it happened. Look up 'Crete vulture' and it returns griffon vulture. Now, in a funny way, this legitimately relates back to Luke and his griffon self. Good luck asking him questions though; he's barely helpful given that he's culturally divorced with his Mediterranean roots. Given that Asterion himself refers to it as a holy bird, dared to hunt it, and reacted in wonder upon seeing it, this here is a vote for 'good birdie'. Just to throw it out there, there's some commonality in its appearances. In all the instances, it accompanied some kind of confession, or divulging of relationships.
With the bird business out of the way, I wonder more about this scene. When did this happen? I assume sometime after Asterion was sent to the labyrinth and Androgeus's death, but before Laomedon's arrival. Phroneos was the one who smuggled in Asterion's lyre, and Asterion was frightfully afraid of being lonely come Laomedon's time. He was probably alone by then. What happened here? The "that was the last they saw of each other" is proper ominous. Where was Phroneos when Laomedon and Theseus came? What did Phroneos call Asterion? My inkling is that he said 'brother' to appease Androgeus's death. Thoughts? (Sorry if this is too far removed for the topic at hand)