Skip to content

Despite sharing some features with mammals, including warm-bloodedness and a four-chambered heart, birds evolved from a different group of reptiles than did mammals.

Charles L. Ripper, Rock dove (pigeon) skeleton
Charles L. Ripper, Rock dove (pigeon) skeleton, The Cornell Lab of Ornithology's Handbook of Bird Biology, Second Edition, edited by S. Podulka, R. Rohrbaugh, & R. Bonney, Princeton University Press: 2004.
Show a larger view of this picture

Birds are distinguished from other animals by having feathers, which in most adult birds fall out and are replaced at least once a year. In addition to acting as airfoils for flight, feathers afford birds the insulation necessary for maintaining the high body temperatures that power their flight. The wings of migrating birds generally are longer and more pointed than wings of non-migratory species. To overcome the drag on their wings during migration, most small birds employ a 'bounding flight,' with their wings closed for half the cycle. Larger species glide between periods of flapping.

Birds' streamlined shape and lightweight, partially hollow skeletons minimize air resistance in flight. Digital and wrist bones of the forelimb are fused to support the large flight feathers of the wing. Bones in the skull and pelvic girdle are also fused for strength and lightness. The breast bone of most birds is large and features a central ridge known as the carina that supports the large pectoral muscles used for the flapping motion of the wings. The pectoral muscles of migrants are especially efficient at energy production. The muscles and skeleton are attached to a system of air sacs that help keep the bird aloft and overcome friction as it moves forward.

Avian circulatory and respiratory systems are very efficient. The proportionally large and continuously inflated lungs and rapid heartbeat meet the intense metabolic demands of flight. The blood of migratory birds is enhanced by a high concentration of red blood cells and, in some birds, contains two different forms of hemoglobin that allow for quick adaptation to variable oxygen availability at varying altitudes.

Prior to migration, bird bodies gain size and become more efficient in fat production and storage, as appetites and food consumption dramatically increase. Many migrants that ordinarily eat insects switch to a diet of berries and other fruits. Activity rhythms during darkness begin to increase, and birds begin orienting themselves and fluttering their wings in the direction in which they will be traveling, as well as flocking with other birds.

Migration causes reduced mass of both body fat and digestive organs (small intestine, pancreas, liver). At stopover points migrating birds must first rebuild their digestive organs before they can refuel and rebuild body fat, a process that takes several days.

For most long distance migrants, birds are born genetically predisposed to fly in a certain direction for a certain amount of time. The first migration is completely under genetic control. As birds gain experience, they learn and apply acquired information about routes, stopover spots, and breeding sites.

Encompassing artistic interpretation of historical scientific tools and models, MMoCA's exhibition Starry Transit An Installation by Martha Glowacki permits visitors to Washburn Observatory to simulate processes of research and discovery about bird migratory navigation by constellations.