Exploring Our World

Similarities and Differences: Reptiles and Birds

Key Changes Needed:

The development of modified forelimbs, feathers, a 4-chambered heart, air sacs, and hollow bones would be needed to allow reptiles to transitionally change into birds.

Birds are usually classified as “aves.”  This means they are able to aviate, or fly.

The shape of a bird’s beak directly corresponds to the type of diet it eats in the wild.  The shape of the feet directly corresponds to the job they need to perform.

To get a better understanding of the unique nature of birds, let’s consider how they differ from the life form they most closely resemble: reptiles.

1. Heart and Circulatory System

Bird Heart – 4 chambers

Reptile Heart – 3 chambers

Reptiles and amphibians generally have three-chambered hearts.  Mammals and birds have four-chambered hearts.  Crocodiles are the only reptile that have a four-chambered heart.

The four chambers of the bird’s heart completely separates the oxygenated blood from the deoxygenated blood. The three chambers of a reptile heart are connected and this allows for the mixing of oxygenated and the deoxygenated blood.

2. Respiratory System

Birds have tube-type lungs, reptiles have sack-type lungs. In reptiles, air taken in enters the lungs through the trachea and then through the same trachea, air is breathed out. The process of breathing in birds is very different from that of reptiles. Air follows a one-way journey all the way to the lungs and after entering the lungs, it follows the lungs channels and leaves the lungs from the other side.

Birds do not have a diaphragm which is present in reptiles. Reptiles on the other hand have well-developed lungs enclosed in a ribcage.

Birds have a much more modified type of respiratory system than all other living creatures.

3. Excretory system

Birds do not have a bladder, and they excrete their waste materials in form of uric acid rather than urine so as to conserve water as uric acid can be produced in concentrated form due to its low toxicity level. Reptiles on the other hand have got a bladder, except snakes and some lizards. Reptiles excrete their nitrogenous waste is in the form of ammonia, urea, uric acid or a combination of these.

4. Reproductive Activity

For mating purposes: birds build nests, dance, and sing. Reptiles do not have distinctive courtship behaviors like birds.

5. Caring for Their Young

Reptiles leave their young ones to feed for themselves; crocodiles being the only exception. Whereas birds take care of their young until they reach the age that they are capable of taking care of themselves.

6. Body Coverings

Birds have feathers on their bodies, while most reptiles have scales.  Feathers help birds stay dry and warm, attract mates, and fly.

Birds are often said to have scales on their legs and feet; however, they actually have what is called “scutes,” not true scales.  There are exceptions.  The snowy owl has mostly feathered feet.  Turtles and crocodiles have scutes instead of scales.

Scales are formed from the epidermis, or outer layer of the skin.  Scutes are formed in the lower vascular layer of the skin and do not over-lap as scales do.  Feathers come from different genes than scales and attach differently to the skin.

7. Metabolism

Birds have the highest metabolic rate on earth, reptiles have a very low metabolic rate.

8. Regulating Body Heat

Birds are endothermic: meaning they have a system to produce their own body heat.  Reptiles are ectothermic: meaning the regulation of their body temperature depends on external sources, such as sunlight or a heated rock surface.  The term “cold-blooded” may be somewhat misleading in regard to reptiles, because when they are in the sun, their blood is warm.

There are some reptiles, such as the leatherback turtle, that can produce body heat but it isn’t as highly modified a system as it is for birds.

9. Responding to Infection

In case of an infection, reptiles tend to lower their body temperatures as bacteria cannot thrive at low temperatures whereas the temperature of birds increases in case of an infection.

10. Growth Limits

Birds reach a growth limit and stop; most reptiles keep growing until they die.

11. Skin

Birds have tender skin; reptiles have tough skin.

12. Bones

Birds have light hollow bones; reptiles have dense heavy bones.  Bird bones are filled with extensions of air sacs that allow increased respiration efficiency.  Their bones also have thin cross supports internally that give added strength, much like the cross supports of many bridges.

Birds are the only animals that have a fused collar bone.  Their neck vertebrate is also more cervical than that of reptiles.  Birds do not have tail vertebrae just a tiny nub.  Most reptiles have normal tail vertebrae, although some geckoes also have little nub tails.

13. Teeth

Birds do not have a true jaw or teeth, with a bill instead. Most, but not all, reptiles have teeth, except for turtles and a few others.

14. Eye Coverings

Birds do have upper and lower eyelids that can close up to protect their eyes and prevent them from drying out.  Reptiles do not have eyelids.

15. Muscles for Flight

Birds have the muscles for flight, reptiles have no muscles for flight.

16. Eggs

Birds have hard shelled eggs, and reptiles have leathery eggs.

17. The Thigh Bone

The thigh bone in birds is fixed and not moveable as in other land animals which keeps their air-sac lung from collapsing when the bird inhales during flight.

Two Worldview Interpretations:

Undirected Process Formation

Life forms share common traits and body parts with other life forms because they had a common ancestor.

Directed Purpose Formation 

Life forms share common traits and body parts with other life forms because of similar purpose and design.