In comparison to the mammalian respiratory tract, the dead space volume in a bird is, on average, 4.5 times greater than it is in mammals of the same size. ... Avian lungs do not have alveoli as mammalian lungs do. read more
Although birds have lungs, these are fairly rigid structures, which do not expand and contract as they do in mammals, reptiles and many amphibians. The structures that act as the bellows which ventilate the lungs, are the air sacs distributed throughout much of the birds' bodies. read more