Quiz 27: Birds
The Archaeopteryx fossils helped in understanding the ancestry of birds. As the fossil record of birds was very obscure, the discovery of Archaeopteryx was quite significant. Archaeopteryx possesses morphological characters that look like a combination of reptilian and avian characters. It has teeth like reptiles and a beak like birds. The body is covered with feathers as in birds. Its skeleton is similar to reptiles with the presence of a bony tail. So Archaeopteryx is a connecting link between birds and reptiles and demonstrates that birds are grouped phylogenetically with dinosaurs.
The special adaptations of birds which contribute to the two essentials of flight which is more power and less weight are: • Feathers: They are light weight objects. Feathers have high tensile strength and also great toughness. Hence it contributes to the two essentials of flight-more power and less weight. • Skeleton: The skeleton in birds is made up of pneumatized bones that are hollow and light but they are also strong. So this gives more power and less weight for flight. • Muscle distribution: The flight muscles of birds are arranged to keep the centre of gravity low in the body. Muscles are positioned on the lower side of the body which improves the aerodynamic stability. This distribution of muscle in birds allows great agility which fulfills the two conditions for flight. • Digestive system: The birds complete their digestion very quickly. So their digestive tracts do not have to remain filled with food for long time. The almost empty digestive tract is light weight which aids in flying. • Circulatory system: There is complete separation of respiratory and systemic circulation which helps to achieve high metabolic rate. This in turn provides more power for flight. • Respiratory system: The respiratory system of birds is such that it takes two respiratory cycles for a single breath of air to pass through the respiratory system. This allows continuous one way flow through the parabronchi which form the lungs in birds. The advantage of such a system is that a continuous stream of oxygenated air passes through the richly vascularized parabronchi. As the oxygen supply is continuous and fast, it helps in meeting high metabolic demands of flight. Therefore respiratory system contributes to more power adaptation for flight. • Excretory system: The birds are uricotelic that is their excretory product is uric acid. Uric acid excretion requires minimum amount of water. Therefore birds do not have to maintain high water level for excretion. This helps them to keep their body light. Therefore excretory system contributes to 'less weight' adaptation. • Reproductive system: In males, testes are as bean shaped bodies and enlarge up to 300 times only during breeding season. In females only left ovary and oviduct develop while the right ovary and oviduct are vestigial. Thus reproductive system in both males and females are adjusted to contribute to less weight adaptation for flight.
Marine birds have salt glands to get rid of excess salt. They have one located above each eye. These glands excrete highly concentrated solutions of sodium chloride, up to twice the concentration of sea water. The salt solution flows out the internal or external nostrils. This also gives a runny nose appearance to these birds like gulls, petrels etc.