The animals who live and flourish, mainly on land, but come to the trees or its branches for safety and shelter, are called arboreal animals. For example, monkey, flying lizard, flying squirrel, bats, birds, etc. 

The bird and bats are adapted for the aerial mode of life. Main adaptation in aerial animals are as follows: 

(i) The body of birds is streamlined i.e., spindle shaped, for the easy and swift passage through the air. 

(ii) Forelimbs are modified into wings, for flying in. 

(iii) The entire body of birds is covered by light, elastic and waterproof feathers. They also help in maintaining body temperature. 

(iv) Birds have hollow bones, strong flight muscles light body covering, sharp eyesight, modified beaks and tails. Birds do not have teeth and urinary bladder. 

(v) Ovary and oviduct are single, lungs are provided with air sacs, which are air filled and make the body light. 

(vi) The feet of birds are modified, for climbing and perching.