When big black birds show up in literature, it's generally not a good thing—especially when it's outside your hospital window when you've just given birth to a necromancer. We partially have Edgar Allan Poe to thank for this; after all, he was the first person to write a totally creepy poem about a talking raven and things that go bump in the night. True to its literary roots, the crow Tia encounters in the process of naming Sam is a super bad omen.
Tia isn't totally sure what to make of the giant crow, but it definitely freaks her out. "This particular crow," we're told, "gave her a bit of the willies. It was so big, and it just kept staring" (12.104). Not only that, but a bunch of his friends show up a short while later, and it's probably not Tia's wish for the big moment of naming her son to feel like she's in an Alfred Hitchcock movie.
To combat the potentially evil significance of the crows, Tia gives Sam the middle name Corvus, the genus for crow. Maybe it seems superstitious, but as Tia rationalizes, "Samhain was already starting out at a disadvantage, and he certainly didn't need angry omens on top of that" (12.109). Her thinking seems to be that it's best to own it when it comes to the crows, than hide.
The interesting part is this: When Tia introduces the gang of crows to Sam using his newly adopted full name, all of them immediately flee—except for the giant one that was first to show up. This bad omen truly exposes the depth of the challenge Sam will someday face as a necromancer—the binding may have suppressed his powers, but the potential for his discovery will never entirely go away. He is recognizable to this crow as a kindred spirit, and that means it's only a matter of time before others (ahem, Douglas) sniff him out, too.