This is book number 3 in the Bone Universe series.
A winged society faces the threat of ultimate extinction in the thrilling finale to Fran Wilde's Bone Universe fantasy series
Unbound Worlds' Best Sci-Fi & Fantasy Books of September 2017
A city of living bone towers crumbles to the ground, and danger abounds. Kirit Densira has lost everything she loved the most - her mother, her home, and the skies above. Nat Brokenwings - once Kirit's brother long before the rebellion tore them apart - is still trying to save his family in the face of catastrophe. They will need to band together once more to ensure not just their own survival but that of their entire community.