This forms a more extensive rigid structure than the pelvis of a mammal, fulfilling requirements for flight, locomotion and respiration. Posterior to the synsacrum there are a few free caudal vertebrae, the last of which is the pygostyle to which the long, stiff tail feathers are attached. The central section of the synsacrum is swollen to accommodate the glycogen body, an organ whose function is as yet unclear but which may be associated with balance.
This structure surpasses the pelvic bone of a mammal in rigidity, fulfilling functions for flight, locomotion, and respiration.  The synsacrum's posterior portion contains free caudal vertebrae, culminating in the pygostyle which attaches the long, stiff tail feathers. The central section of the synsacrum is enlarged to accommodate the glycogen body, an organ whose function remains unclear but may be related to balance.