SignificanceHere, we describe the mechanism for a prominent avian evolutionary adaptation. We find that vertebral fusion in the avian tail is driven by sterile inflammation and resembles bone fracture repair. This finding demonstrates a role for inflammation in developmental skeletogenesis. The immune system is involved in pathological ankylosis, but we find that it is also a crucial contributor to normal vertebral fusion. Evidence of necroptosis indicates that this type of cell death can occur in postnatal development. Also, we document nucleus pulposus structures in tail discs in birds. Lastly, corticosteroid treatment inhibits vertebral fusion, suppressing a Cretaceous avian flight adaptation while also demonstrating an unreported effect of corticosteroids on skeletal maturation.