The Good Hawk (Shadow Skye, Book One)

Agatha is a good Hawk. She patrols the sea wall with pride, despite those in her clan who question her right to be there, on account of the condition she was born with. But when disaster strikes and her clan is brutally swept away in the longboats of an invading deamhain horde, she must leave the island and team up with the only other survivors - Jaime, a skinny teenage Angler who is afraid of the sea, and Lileas, a young girl from a rival tribe - to find a way of rescuing the only family they have ever known.