Table of Contents
The avian group, traces its origin back to theropod dinosaurs during the Jurassic Period, evolving further in the Cretaceous period. Termed ‘glorified reptiles’ by T.H. Huxley, birds exhibit reptilian traits such as scales, interclavicle, uricotelism, large-yolked eggs, and the development of extraembryonic membranes. However, birds have evolved beyond reptiles, boasting insulation, homeothermy, heightened metabolic rates, and complete separation of venous and arterial blood, among other features. Notably, feathers, wings, flight muscles, pneumatic bones, endothermy, and sharp vision are key adaptations for flight. J.Z. Young aptly dubbed them ‘the masters of air’. Ornithology is the scientific study of birds. Dr. Salim Abdul Ali (1896-1987), an Indian ornithologist, is widely recognized as the “Birdman of India.”
Morphology
The class Aves comprises creatures, characterized by their feathered bodies and bipedal vertebrates with beaks. With elevated metabolic rates and body temperatures, birds exemplify endothermy. Birds exhibit long-distance migration capabilities.
- Body Divisions
The body of a bird is distinctly divided into a head, a long neck, a trunk, and a short tail. The body is streamlined and boat-shaped, a form that supports efficient flight due to the high surface-to-weight ratio.
- Limbs
Birds are bipedal, meaning they have two legs. Their forelimbs have been modified into wings, enabling flight. Hindlimbs typically have scales and are specialized for various functions such as walking, swimming, or perching on tree branches. The hindlimbs bear the entire weight of the bird’s body.
- Skin
Birds’ skin is dry and lacks glands, except for the oil gland known as the uropygial gland or preen gland. This gland is located at the base of the tail and secretes oil, which the bird uses to preen and condition its feathers. Feathers, scales on legs, claws on toes, and the beak’s horny covering (rhamphotheca) comprise the exoskeleton.
Anatomy
- Endoskeleton
Birds have a fully ossified endoskeleton. Their long bones are pneumatic (air-filled), containing extensions of air sacs and devoid of bone marrow. The skull is monocondylic, while the cervical vertebrae are heterocoelous (saddle-shaped). The last thoracic, lumbar, sacral, and anterior caudal vertebrae fuse to form a structure called the synsacrum. The synsacrum, in turn, fuses with the pelvic girdle, providing support to the hindlimbs. The last 3 or 4 caudal vertebrae fuse to create the pygostyle, which supports the tail feathers.
- Flight muscles
The sternum of birds is characterized by a mid-ventral keel, or carina, which provides attachment points for large flight muscles. Notably, this keel is absent in ratite birds. Ribs are double-headed. The clavicles join with the interclavicle to form a V-shaped furcula, commonly known as the wishbone or merrythought bone. The primary flight muscles are the pectoralis major and pectoralis minor muscles located in the breast area.
- Digestive system
Extant birds lack teeth, relying on their beaks for feeding. The digestive tract of birds includes additional chambers like the crop and gizzard, aiding in food storage, softening, and digestion.
The esophagus, or food pipe, is expanded to form a crop. The crop serves the purpose of storing and softening food. The stomach is divided into two distinct sections: the anterior glandular proventriculus and the posterior muscular gizzard. The muscular gizzard plays a vital role in grinding food, acting like a powerful mill.
The cloaca, the common opening for the digestive, urinary, and reproductive tracts, is further divided into three parts: coprodaeum, urodaeum, and proctodaeum. Each of these sections serves a specific function in waste elimination and reproductive activities.
The bursa of Fabricius, a blind sac containing lymphoid tissue, is connected to the proctodaeum. This unique structure, often referred to as the ‘cloacal thymus,’ has a role in the development of immune cells.
- Respiratory system
Birds have spongy lungs that lack alveoli. To supplement ventilation, they possess nine air sacs connected to their lungs. These air sacs serve to ensure continuous oxygenation of blood. Birds utilise a unique respiratory mechanism called double ventilation, where fresh air flows in during both inhalation and exhalation. The sound-producing organ known as the syrinx is located at the junction of the trachea and bronchi.
- Circulatory system
A bird’s heart is four-chambered, featuring two atria and two ventricles. This arrangement facilitates double circulation, where blood passes through the heart twice during each complete circuit. This efficient system ensures effective oxygenation of the body’s tissues.
- Excretory system
The adult functional kidneys of birds are metanephros and consist of three lobes. Most birds lack a urinary bladder, with the exception of the ostrich. This absence of a urinary bladder serves as an adaptation to reduce the overall body weight of birds, contributing to their flight efficiency. Birds are classified as uricotelic (excrete uric acid), which requires less water compared to urea or ammonia.
- Nervous system
Sense Organs: Birds possess well-developed eyes with keen vision, aiding in various activities such as foraging, detecting prey, identifying landmarks, and finding resting places from the air. A vascular structure known as the pecten, shaped like a comb, projects from the retina into the vitreous humor, serving functions like nourishing the retina and removing metabolic waste. Sclerotic plates in the eye’s sclerotic layer help maintain eye shape.
- Middle ear: The middle ear in birds consists of a single ossicle called the columella auris.
- Olfactory sense: Birds generally have a poorly developed sense of smell, with the exception of species like the kiwi.
- Reproductive system
Sexes: Birds have separate sexes.
Copulatory Organ: In most birds, a copulatory organ is absent except in flightless birds, ducks, geese, and swans.
Development: Fertilisation is internal, and all birds are oviparous. Their eggs are megalecithal, telolecithal, and cleidoic (protected by a calcareous shell). Development is direct, with hatchlings being either altricial (dependent on parents) in flying birds or precocial (not dependent on parents) in flightless birds.
Examples: Corvus (Common Crow), Columba (Pigeon), Psittacula (Parrot), Struthio (Ostrich), Pavo (Peacock), Aptenodytes (Penguin), Neophron (Vulture).
Classification
Archaeopteryx lithographica is a Jurassic fossil bird and is considered a connecting link between reptiles and birds due to its combination of reptilian and avian features.
- Neornithes
This group includes modern birds that are both extinct and extant.
Neornithes is further classified into four superorders: Odontognathae, Impennae, Palaeognathae, and Neognathae.
- Ratitae/Palaeognathae
Modern flightless running birds. They show discontinuous distribution. Reduced wings, sternum without a keel, absence of syrinx, clavicles, and usually preen gland. Males have a penis.
Examples: Struthio camelus (Ostrich), Kiwi, Rhea (American Ostrich), Dromaeus (Emu), Casuarius, Apteryx (Kiwi)
- Neognathae
Modern birds with diverse representatives.
Examples: Columba (Pigeon), Passer (Sparrow), Corvus (Crow), Eudynamys (Koel), Psittacula (Parrot), Bubo (Owl), Gallus (Fowl), Pavo cristatus (Peacock), Coracious bengalensis (Blue Jay), Alcedo (Kingfisher)
- Peacock (Pavo cristatus) is the National Bird of India.
- Blue jay (Coracious bengalensis) is the State Bird of Andhra Pradesh.
- The hummingbird is the smallest bird.
- Alpine swift (Apus melba) is the fastest-flying bird.
Summary
The avian group traces its lineage back to theropod dinosaurs, evolving during the Jurassic and Cretaceous periods. Dubbed ‘glorified reptiles,’ birds exhibit both reptilian and unique avian traits. With remarkable adaptations for flight, they feature streamlined bodies, modified forelimbs as wings, and strong hindlimbs. Their skin lacks glands but hosts a crucial oil gland for feather maintenance. The endoskeleton is fully ossified, supporting flight muscles and intricate skeletal structures. Birds breathe using air sacs, ensuring optimal oxygenation. Their heart’s four-chambered design facilitates efficient double circulation. Birds lack urinary bladders and excrete uric acid. Developed senses, a complex reproductive system, and distinct classifications further characterise this diverse and captivating class.
FAQs on Aves
What is the origin of birds and how do they relate to dinosaurs?
Birds trace their origin back to theropod dinosaurs during the Jurassic Period. They evolved further during the Cretaceous Period. This evolution suggests a close relationship between birds and certain dinosaur groups, leading to the term glorified reptiles.
What are some reptilian features that birds retain?
Birds exhibit traits like scales, interclavicle bones, uricotelism (uric acid excretion), large-yolked eggs, and the development of extraembryonic membranes, reminiscent of reptiles.
What are some key adaptations for flight in birds?
Birds possess feathers, modified wings, flight muscles, pneumatic bones filled with air sacs, endothermy, and sharp vision. These adaptations enable them to be efficient fliers.
How do birds breathe, and what is double ventilation?
Birds have spongy lungs with air sacs that allow continuous oxygenation of blood. Double ventilation is a unique respiratory mechanism where fresh air flows in during both inhalation and exhalation, ensuring a constant oxygen supply.
How is the heart of a bird different from other animals?
A bird's heart has four chambers – two atria and two ventricles. This setup allows for efficient double circulation, where blood passes through the heart twice during each complete circuit.
What is the significance of the pecten and the syrinx in birds?
The pecten is a comb-like structure in the eye that nourishes the retina and removes waste from the vitreous humor. The syrinx, located at the junction of the trachea and bronchi, is the sound-producing organ in birds.
How do birds reproduce, and what are some notable examples?
Birds are oviparous and have internal fertilisation. Their eggs are protected by calcareous shells. Examples include the Common Crow (Corvus), Pigeon (Columba), Parrot (Psittacula), Ostrich (Struthio), Peacock (Pavo), Penguin (Aptenodytes), and Vulture (Neophron).
What are the major classifications of birds?
Birds are classified into various groups, including Archaeopteryx as a connecting link between reptiles and birds. Neornithes includes modern birds, further divided into superorders like Odontognathae, Impennae, Palaeognathae, and Neognathae. Flightless birds are Ratitae or Palaeognathae.
Who was Dr. Salim Abdul Ali and why is he known as the Birdman of India?
Dr. Salim Abdul Ali was an Indian ornithologist recognised for his extensive work on Indian birds. He earned the nickname Birdman of India due to his significant contributions to ornithology and bird conservation in the country.
What are some unique facts about specific bird species?
The Peacock (Pavo cristatus) is India's national bird, and the Blue Jay (Coracious bengalensis) represents Andhra Pradesh. The hummingbird is the smallest bird, and the Alpine Swift (Apus melba) is known for its remarkable flying speed.
