What is the significance of the immobility of the spine of birds?

The spine of birds, except for the cervical and part of the tail, is motionless. The vertebrae of the lumbar, sacral and part of the caudal regions merged into a single bone – a complex sacrum. Together with the bones of the pelvis, it serves as a solid support for the lower limbs on which the bird walks. In addition, the motionless spine does not sag during flight, which contributes to maintaining the streamlined shape of the bird’s body.