A girl that you pick up in a club, bar, street or late night venue at 3am in the morning.
Usually pretty ugly, larger than the average, and very open to invites from anyone who is interested.
John: "How was your night?"
Richard: "Oh met a 3am bird and took her home!"
John: "Unlucky, but sometimes required :-)"
Richard: "True, but waking up next to her was very unlucky"