The vertebrae in birds are mostly

Tail vertebrae of birds are fused to form

$S$ - Statement: In ostriches,the aortic arch turns to the left.
$R$ - Reason: In kangaroos,the aortic arch turns to the right.

Which characteristic is exclusively found in birds $(Aves)$?

Assertion $(X)$: Birds can consume food very quickly.
Reason $(Y)$: They possess a crop.

What is the primary function of pneumatic bones in birds?