A streamlined body shape and a lightweight skeleton composed of hollow bones minimize air resistance and reduce the amount of energy necessary to become and remain airborne. Birds have a large, four-chambered heart which proportionately weighs 6 times more than a human heart. Unlike mammalian or reptilian lungs, the lungs of birds remain inflated at all times, with the air sacs acting as bellows to provide the lungs with a constant supply of fresh air. In addition to these general avian characteristics, migratory birds exhibit a suite of specialized traits. Migrants generally have longer, more pointed wings than non-migratory species, a feature which further minimizes air resistance.
Many migrants face the additional challenge of flying at high altitudes.
Accounting for their ability to withstand the low levels of oxygen available at such altitudes, the blood of migratory birds is characterized by two specialized adaptations. The oxygen-carrying capacity of the blood is enhanced by a high concentration of red blood cells. Secondly, instead of one form of hemoglobin in the red blood cells as is typical in non-migrants and other classes of vertebrates, some migratory birds possess two forms of hemoglobin which differ in their oxygen carrying and releasing capacities. This guarantees an adequate oxygen supply over a wide range of altitudes and allows birds to adapt rapidly to varying levels of oxygen availability. When it comes to fueling migration, fat is where it's at.