Class Aves: Origin, Adaptations and Key Features

Sometime during the early Jurassic, two groups of reptiles gained the ability to fly and one of these groups later gave rise to the Aves (birds). They developed a range of adaptations that enabled flight such as feathers, anterior limbs transformed into wings, pneumatic bones and horny (corneous) beaks and endothermy i.e. birds are warm blooded animals.

Bird Adaptations for Flight

There are several features of birds that allow them to fly. Firstly, their anterior limbs have transformed into wings attached to a well-developed pectoral musculature. Secondly, they have lightweight bones with internal spaces filled with air. These bones are called pneumatic bones. This feature reduces the density of body of the animal, facilitating flight. Thirdly, they have no colon or bladder to accumulate feces or urine. Their excretion is Uric acid which requires least water and least storage place. Lastly, Birds have an aerodynamic body and lungs with specialized air sacs.

Heart of Birds

The heart of birds is more developed then reptiles and has 2 atria and 2 ventricles. There is no mixture of venous and arterial blood like mammals.

Excretion in Birds

Birds are uricotelic, meaning that, like reptiles, they excrete uric acid. This substance needs less water to be eliminated and it helps to reduce body weight, making flight easier.

Similarities between Birds and Reptiles

In terms of external coverage, birds are similar to reptiles in that they present impermeable keratinized outside. In terms of reproduction, fertilization is internal in both and the embryo develops within a shelled egg. In terms of excretion, both excrete uric acid.

Reproduction in Birds

Birds reproduce sexually. Embryos develop within shelled eggs and embryonic development occurs outside the mother’s body. The eggshell is made of calcium carbonate. The white, or albumen, is composed by albumin, a protein. The yolk is predominantly made up of lipids, but also contains proteins and vitamins.

Body Temperature Control in Birds

While Reptiles are heterothermic, as they do not control their body temperature; birds are the first homoeothermic (endothermic) animals, as they are able to maintain a constant body temperature. This is the reason that many birds live in regions of intense cold. Penguins are an example of birds that live in polar region.

Other important Facts

• Modern Birds don’t have teeth. They possess beak.
• The digestive tract of birds has additional chambers, the crop and gizzard.