Another general need of a flyer is a rigid body,
that can stand up to gravity and maintain a constant aerodynamic shape,
without sagging or flopping about with every wing beat.
A good way to hold the skeleton rigid without the need for
constant muscle flexing is to fuse multiple bones together.
Nowhere on the bird is this concept more apparent than at the hips.
The sacrum is a feature common to most terrestrial vertebrates.
It is a typically short series of vertebrae that are all fused together, and
are each fused to the left and right ilia, or the upper hip bones.
But birds take this to the extreme.
They've evolved what is called a synsacrum.
And the synsacrum is essentially an extraordinarily long sacrum.
It incorporates multiple additional vertebrae from the abdomen and
tail, essentially turning the back half of a bird's skeleton
into one solid bone structure.
The front half of a bird is also held together more rigidly
than other vertebrates.
Have a look at the rib cage.
See these backwards projecting processes on each rib?
These are called uncinated processes.
Notice the uncinate process of every rib, overlaps the next rib behind it.
And this helps lock the entire rib cage together.
Behind the synsacrum there's another fused structure.
This is a pygostyle, and
pygostyles are formed by a series of fused vertebrae at the tip of the tail.
This stiffens the tail tip and makes it a good anchor for
rectrices, that is a fan of tail feathers.