Hollow bones are characteristic of

$A$: All birds,except the one like Koel (cuckoo),build nests for retiring and taking rest during night time.
$R$: Koel lays eggs in the nests of tailor bird.

Carpo-metacarpus and tarso-metatarsus are present in

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

Which one of the following is an oviparous animal?

What is present in birds for grinding and crushing food?