You might think that birds and reptiles couldn’t be further different when you compare an alligator lazily sunning itself in Florida against a small songbird feeding in your backyard. Buzzards circle above the deserts while vipers swim through the sand below.
There are several main differences between birds and reptiles. The most noticeable is that birds have feathers and can fly. Birds migrate in groups, only have two limbs, and are warm-blooded.
While there are apparent differences between birds and reptiles, I first looked at some similarities.
You will often hear baby birds referred to as little dinosaurs, especially larger species like herons that lack feathers in the nest and have large feet.
Looking at the cassowary’s feet and the shape of a baby heron in the nest, they look pretty reptilian.
After the mass extinction of the dinosaurs, only one group remained. This group consisted of feathered dinosaurs. These slowly evolved into the birds that we know now.
You can look at massive, flightless species, such as the cassowary and the ostrich, and see comparisons between birds and dinosaurs. The cassowary, in particular, has enormous scaly feet that can disembowel a human when threatened. They look like velociraptors with feathers and an excellent defensive helmet on their head.
When we look at some of the dinosaurs that could fly, some of which were believed to have feathers, we can see where the link between birds and reptiles lies. However, a lot has changed along the evolutionary tree since then. While many birds can look like their ancient ancestors, there are many differences between today’s birds and reptiles.
Ability to fly
Let’s start with the most striking factor – the ability to fly. While not all bird species fly, as some lost the need to and others evolved flippers, most use flight to move around. It is an easy way to get around, and various species have developed strategies to improve their flight for migration, speed, and hunting. Some, like swifts and albatross, can stay on the wing for long periods without landing.
Reptiles, on the other hand, don’t fly. Some have developed the ability to glide from tree to tree, but this isn’t the same process. Instead, they use flaps of skin between their limb to sail through the air like a wingsuit. There are also mammals whose gliding strategies are closer to those of the reptile than the reptiles are to birds.
Many birds will breed in the arctic in summer and fly south for more abundant food in the winter. Others will reproduce in Europe and then head to Africa for the summer. This ability to switch between summer and winter homes is a brilliant survival strategy, primarily when only the strongest survive the trip and pass on their genes to the next generation. Reptiles don’t have the same ability to travel long distances and have smaller territories.
Some birds are born without feathers. As their wings and flight feathers develop, these altricial birds rely on warmth and security in the nest. This is common in songbirds and raptors. But, there are precocial birds that are born with feathers and the ability to run from birth. This is common in waterfowl and waders that need to evade predators on the ground. Either way, they all develop essential feathers for flight, warmth, and display.
Reptiles, on the other hand, have scales. This protective skin is typically green or brown so that it can blend into its surroundings. Birds can also use camouflage with mottled feathers that break up their silhouette.
Some reptiles can change their color to match their surroundings through chromatophores that contract and expand in their skin. This is rare and more common in cephalopods. Birds can’t do this, although some birds of paradise can trick the eye with iridescent feathers that look different in the light.
Warm-blooded vs cold-blooded
This is an essential difference in the defense mechanisms of these species. On one side, you have birds that can regulate their body heat, which is necessary when staying warm in winter and incubating eggs on the nests.
Reptiles need to bask in the sun to warm their blood and get the energy they need. This is why so many reptiles will hibernate in winter while birds find other strategies to get through the season. Birds flock together and often huddle for warmth in trees and may even be seen using street lights and other man-made structures. Reptiles will often bury their eggs so they can take on heat from the ground for incubation.
This is another area where you can see a difference between some birds and reptiles. It is commonly believed that a critical similarity between birds and reptiles is their ability to lay eggs, in which their young develop outside of the creature’s body. This is true for many reptiles, but there are a few viviparous species that give birth to live young.
The idea of flocking together shows another behavioral difference between many birds and reptiles. Reptiles typically go it alone, living and hunting solitarily and only coming together to mate. There is no need to stay in a group if there is no predatory threat or other benefits. Other reptiles of the same species are just competition for food. Predatory birds, like falcon and eagles, also prefer to be alone with individual territories for hunting. But, many bird species thrive in large flocks where they have safety in numbers.
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Predatory tactics are also different between reptiles and birds. You will find a lot more dangerous and venomous reptiles out there. They rely on their bite and the venom’ to incapacitate prey; this is how some creatures like Komodo dragons can take down much larger animals.
For predatory birds, the kill often comes from sharp talons and beaks, as well as the force of their body weight as they land on their prey. However, there are a couple of poisonous bird species. The Hooded Pitohui from Papua New Guinea, for example, has poisonous plumage where the toxin could be hazardous to humans.
Number of limbs
One of the most obvious physical differences is the number of limbs. Birds always have two legs. This is true from the largest raptor down to the smallest hummingbird. Those limbs’ physiology will vary – some birds have toes in different places for a better grip, and others have talons or webbed feet for swimming. But it is always two legs. With reptiles, you have four-legged creatures – like most lizards, tortoises, and crocodiles – or those with no legs, such as legless-lizards and snakes.
Finally, we have to talk about the other striking part of avian physiology that we automatically think of with birds. That is, of course, the beak. Beaks are a strange adaptation that allows birds to pick up seeds, break nuts, tear into flesh, or probe into the mud – it all depends on the shape and strength.
Beaks mean that birds don’t have jaws in the same way as other animals do. Nor do they have teeth. Birds can’t chew their food, but they do have tongues. Again, these are very different in purpose and shape, depending on the bird’s diet and feeding habits. Some are long enough to probe into the wood, while others have barbs to hold fish in place.
Reptiles do have jaws and teeth – often a lot of teeth in the case of crocodiles and alligators. One of the oddities here is the tortoise, which is somewhere between birds and reptiles.
The tortoise has jaws with crude, tough edges that work like teeth, but the mouth has a solid pointed beak-like quality. Some dinosaurs with beak-like structures, such as the Ornithischia family, were shaped like a bird’s beak but with teeth. This highlights the solid evolutionary link between both birds and reptiles and the dinosaurs.
In short, while both birds and reptiles can trace their roots back to some prehistoric ancestors, the branches of the evolutionary tree are so far removed now that there are many key differences.
Birds took to the air with only a few species evolving the need to be flightless. This has led to migratory and hunting behavior unique to the avian world. Then there is the fact that these are warm-blooded animals. But, there are still moments when you can look at beaks, scaly bird’s legs, and the featherless hatchlings and see those ancient ties to reptiles.
Bryan Harding is a member of the American Society of Mammalogists and a member of the American Birding Association. Bryan is especially fond of mammals and has studied and worked with them around the world. Bryan serves as owner, writer, and publisher of North American Nature.