It usually present in young birds under the age of 6 weeks.

It was during this time that he continued his interest in birds.

In mammals and birds, blood from the body goes to the right side of the heart first.

A similar, though highly expanded system is found in birds.

This walk is a real treat for anyone interested in birds.

The bill is yellow or orange, but turns green in older birds.

A century from now, men will fly through the air in mechanical birds.

Even when I was a child my interest in birds didn't receive much encouragement.

He took a great interest in birds, collecting their eggs.

These are true flight feathers as seen in modern birds.