THE BIRDS FEATHER FLIGHT & ENDOTHERMY
Birds are traditionally classified as class Aves. Birds have adaptations for flight. The major characteristics of this class are:
1. Their appendages are modified to wings.
2. They body is covered by leathers.
3. Birds are endotherms.
4. They have high metabolic rate.5
5. Their vertebral column is modified or flight.
6. Flues are lightened bv numerous air spaces.
7. Modern birds possess a horny bill and they lack teeth.
Evolution of birds from reptiles
There are close similarities between birds and reptiles. That is why, birds are called as glorified reptiles. Followings are the possible ancestor of birds:
1. Archosaurs ancestor: The ancestor of the birds is ancient archosaurs. Other flying reptiles in this evolutionary lineage are pterosaurs and pterodactyls. But these are not regarded as ancestor of the modern birds due to:
(a) These reptiles lost the clavicles long before the appearance of birds. Clavicle is an important avian characteristic. Clavicle is used or the attachment of flight muscles in birds. Thus pterosaurs and pterodactyls were not strong living animals like birds.
(b) They did not have feather wings. Their flight surface was composed of membranous fold of skin.
2. Saurischian ancestor: Therefore, these flying reptiles are not taken as ancestor. Thus modern birds were derived from the Saurischian lineage of dinosaurs. This lineage also includes bipedal carnivorous dinosaurs like Tryannosaurus. According to cladistic interpretations the birds are modern dinosaurs.
ANCIENT BIRDS AND THE EVOLUTION OF FLIGHT
An important vertebrate fossil was found in Bavaria, Germany in 1861. It is named as Archaeopteryx (archaios, ancient + pteron wing). It lived during the Jurassic period about 150 million Years ago. It has following characteristics:
1. It was pigeon-sized animal.
2. It had a long reptilian tail.
3. It has clawed fingers.
4. The complete head of this specimen was not preserved. The imprints feathers on the tail and on short were found. It has rounded wings.
These characters are the main evidence that this was the fossil of an ancient bird. A complete fossil archaeopteryx was discoverered sixteen years later. It has teeth in beaklike jaws. Later four other fossils of Archaeopteryx were discovered. These fossils supported the ideas of reptilian ancestry for birds.
There are two hypotheses about the origin of flight in birds:
1. Evolution of Gliding
According to this hypothesis gliding was most primitive than flight. Most zoologists consider Archaeopteryx as the oldest bird fossil. It is very close to the main line of evolution between the reptiles and birds. The lifestyle of Archaeopteryx supports the hypotheses on the origin of flight. The clavicles (wish bones) of Archaeopteryx ere well developed. Clavicle provides points of attachment for wing muscles. The sternum, wing, bones and other sites for night muscle attachment were less developed than in modern birds. These observations indicate that Archaeopteryx as a glider. It used flapping flight for short distances.
Some zoologists think that the clawed digits of the wings were used to climb trees and cling to branches. The ancestor of birds jumps from branch to branch, or branch to ground. Then it started gliding. Later, weak flapping started. It supplemented gliding. Finally, winng-powered flight evolved.
2. Evolution of flight
According to this hypothesis flight was more primitive than gliding. Some zoologists studied the hind limb structure of the earliest birds. It suggests that they were bipedal.They run and hop on the ground. They used their wings for capturing flying insects from air and ground. Their teeth and claws resembled talons (claw of eagle) of modern predatory birds. They used these teeth and claw for grasping prey. They also started flight over short distances. This flight led to wing-powered flight. Two fossil evidences support this hypothesis:
(i) Fossil of Sinornis
A second ancient bird Sinornis was discovered in China. It supported the view that archacoptervx was closely related to ancestral bird stocks. Sinorirs fossils are 135 million Years old. It has only 15 million years younger than Archaeopteryx. Sinornis has some primitive dinosaur like characteristics. It also had characteristics similar to modern birds. These characteristics are:
1. It had shortened body and tail
2. Its sternum has a large surface area for flight muscles.
3. The claws were reduced.
4. The forelimbs were modified for folding of wings at rest.
These characteristics indicate that powered flight was well developed in birds 135 million years ago.
(ii) Fossil of Eoalulavis
Another fossil of bird, Eoalulavis was discovered from early Cretaceous deposits in Spain. It was found in 115 million year old rocks.It provided additional information about the evolution of flight. It had a wing span of 17 cm. The fossil of Eoalulavis had a wing structure called an Alula. Alula is also present in many modern birds. It is used in slow hovering flight. Its presence in this fossil indicates that complex flight was slow, hovering. This flight evolved at least 115 million years ago.
DIVERSITY OF MODERN BIRDS
1. Archaeopteryx, Sinornis and Eoalulovis are transition between reptiles and birds. But zoologists do not know about the direct ancestor of modern birds.
2. Many fossils of birds have been found between the period of 100 million and 70 million years ago. Some of these birds were large flightless birds. Others were adapted for swimming and diving. Some were fliers. Most birds like Archaeopteryx, had reptile like teeth. Most of the lineages of these fossils have become extinct. ‘Ilk. dinosaurs were also belonged to this lineage. They became extinct at the end of the Mesozoic era.
3. Some of the few birds survived. These birds enter into the Tertiary period. They were the ancestors of modern toothless birds. The phvlogeny of modern birds is controversial. The adaptive radiation has produced about 9,100 species of living birds. They are divided into about 27 orders. This classification is based or characteristic behaviors, songs, anatomical differences and ecological niches.