Explanation

Instead of alveoli, birds (aves) have parabronchi.

Parabronchi are the structures in the avian respiratory system

They are the site of gas exchange, where oxygen from the air is absorbed into the bloodstream and carbon dioxide is released from the bloodstream into the air.

Birds have a unique respiratory system that includes air sacs connected to their lungs. 

Parabronchi are the functional units where gas exchange occurs within the avian respiratory system.