Chickens, like all birds, are not reptiles in terms of modern biological classification. This question often arises due to shared evolutionary history and certain superficial similarities that can be misleading. While they belong to different biological classes today, their deep evolutionary roots connect them to a common lineage.
The Ancient Link: Birds and Dinosaurs
Birds, including chickens, are direct descendants of theropod dinosaurs. This places birds within the dinosaur lineage, specifically Maniraptora, a group that includes Deinonychus and Archaeopteryx. This evolutionary connection dates back to the Mesozoic era, with the classic bird body plan gradually forming over millions of years.
The shared ancestry explains why some features might cause confusion. For instance, both birds and many reptiles lay amniotic eggs, which are eggs with protective membranes allowing development on land. Modern birds also possess scales on their legs and feet, a trait inherited from their reptilian ancestors.
Fossil discoveries have provided extensive evidence for this link, with over thirty species of non-avian dinosaurs found with preserved feathers. Early birds, like Archaeopteryx, exhibited a combination of bird-like features, such as feathers, and reptilian traits, including teeth and a long bony tail. The skeletal structures of birds and theropod dinosaurs also show similarities, such as fused collarbones forming a wishbone (furcula) and specific wrist physiology.
Distinguishing Features: Birds vs. Reptiles
Despite their shared ancestry, birds and reptiles are distinct classes of vertebrates, Aves and Reptilia, respectively, each with defining biological characteristics. Feathers are the most obvious distinguishing feature of birds; they are complex structures unique to birds, providing insulation, waterproofing, and enabling flight. While reptiles are covered in scales, these are structurally different from feathers, although both are made of keratin.
Birds are warm-blooded (endothermic), meaning they can internally regulate and maintain a consistent body temperature. This contrasts with most modern reptiles, which are cold-blooded (ectothermic) and rely on external heat sources, like the sun, to regulate their body temperature. This difference in thermoregulation affects their activity levels and habitat range.
Skeletal adaptations in birds, like chickens, are specialized for flight, even in flightless species. Their bones are often hollow and filled with air spaces (pneumatic bones), making the skeleton lightweight yet strong, and many bones are fused for rigidity. This includes a fused clavicle (furcula) and a prominent keel on the sternum (breastbone) for the attachment of powerful flight muscles. In contrast, reptile skeletons generally lack these extensive hollow bones and specialized flight adaptations.
The respiratory system in birds is also efficient, designed for the high oxygen demands of flight. Birds have a unidirectional airflow system through their lungs, aided by a series of air sacs, allowing for continuous oxygen uptake. Reptiles have less efficient lung structures without the complex air sac system.