The wishbone of the birds is derived from

$A$: 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.

The forelimbs are modified into wings in

Animals of this class lack teeth.

Kingfisher is a bird in which the feet are

What is the function of air sacs in birds?