Birds have an extra ordinary respiratory system which plays an important role in keeping the body temperature constant. The abilities of birds to sustain flight and to fly in the thin air of high altitude are striking evolutionary accomplishments. The respiratory system is vital to these strenuous feats and thus most research on the form, function and adaptive significance of the avian lung has rightly on adaptations that enable rapid rates exchange. Morphologically, the avian respiratory system is separated into the lung (the gas exchanging part) and the air sacs (the non respiratory part). Lung tissues of the migratory quail and the farmed quail were subjected to standard processing for transmission electron microscopy. The results reported differences in the number and form of the air capillaries (Acs) and blood capillaries (Bcs). The blood gas-barrier (BGB) was thinner in the migratory quail than that of the farmed quail. The lung of the migratory quail is very efficient because of the presence of an extremely thin blood gas barrier than that of the farmed quail. This would contribute to the remarkable to expend energy during flight, especially at high altitude by the migratory quail which flies for longer distances, and indicate that structural adaptations may occur in the avian lung in response to functional demands.