Question

why are birds light in weight​

Answer 1

Bird in Flight, Strong but Light

The feathers of a bird are, for their weight, among the strongest structures in the world. The bones of this Magnificent Frigatebird weigh less than its feathers! To further reduce weight while maintaining strength, many bird bones are fused. In addition, the pectoral and pelvic girdles and ribs are joined to make a rigid box that supports those long wings, just as the wings support the bird.

Answer 2

birds light in weight​ because  the their bones are hollow, feathers are light, and wingspan is only feathers