Question

How do the lungs of birds differ from the lungs of mammals?

Answer 1

Respiratory system is a system of organised organs which carries the process of respiration in our body. Respiration is a process during which energy is synthesized. During energy synthesis oxygen is required. Respiratory system helps to bring oxygen rich air inside the body and to remove the waste material and carbon dioxide (produced as by-products during the metabolic reaction) from the body.

Complete answer:
The avian respiratory system is different, physically from the mammalian respiratory system, both in its ability to exchange gas as efficiently as possible and in its structure.

lungs of birds	lungs of mammals
The avian respiratory system facilitates efficient gas exchange through continuous unidirectional airflow	Mammalian lungs as it experiences bidirectional airflow over the gas exchange surfaces.
The avian lung has a flow-through system comprising small air capillaries	The mammalian lung has reciprocating ventilation with alveoli (large terminal air spaces).

The avian lungs do not contract and expand as in mammals lungs. Instead, they have air sacs which can expand and pull air in one-way circuits. Ventilation is much more complex in birds than in mammals due to the presence of air sacs. Air sacs are not the site of gaseous exchange in aves.
Aves respiratory system is much more efficient than a mammals respiratory system because it is able to supply more oxygen to the body per breath. During respiration in birds two respiratory cycles are required to move the air through the entire respiratory system while In mammals, only one respiratory cycle is necessary.

Note:
Salminicola is the only non-breathing animal known on the planet. You wouldn't have much opportunity to turn oxygen into energy if you spent your entire life infecting the dense muscle tissues of fish and underwater worms, as H. salminicola does.