Hawksong
From the book jacket...
"Danica Shardae is an avian shapeshifter, and the golden hawk's form in which she takes to the sky is as natural to her as the human one that graces

Now Danica must convince her people that Zane is as committed to peace as she is -- though she can't help fearing that, despite his word, he will strike as swiftly and lethally as the cobra that is his second form. Among the serpiente, she'll have to pretend to be in love, though when they're alone, Danica's reserve threatens to keep her and Zane worlds apart. And in their midst are dissenters who will do whatever it takes to destroy this union.
Trust is all Zane asks of Danica -- and all they as of their people -- but it may be more than she can give."