Are birds reptiles? Your bird and dinosaur evolution questions answered.

There are an estimated 50 billion birds flying, walking and even swimming on planet Earth according to a study published in the Proceedings of the National Academy of Sciences. That number is divided into roughly 9,700 species. Four species of birds have populations of over a billion on their own – house sparrows, European starlings, ring-billed gulls and barn swallows.

But before they were pigeons and ducks and chickadees, where did the bird family line originate? It's complicated. Here’s everything you need to know about the animal classification of birds and evolution.

Are birds reptiles?

Yes and no: It depends on how you classify animals. There are two main classification systems – the Linnaean system which groups organisms by characteristics and the phylogenetics system, which organizes by ancestry.

Both birds and reptiles are descended from the group Reptilia, which are diapsids, or animals with two openings on each side of the skull. So from a phylogenetic standpoint, birds are reptiles.

But based on Linnaean’s system, birds and reptiles belong to separate groups because of their different physical characteristics. Here, birds alone make up their own separate type of animal. Birds are defined as vertebrates – animals with backbones – that have feathers and wings. They don’t have to fly to be considered a bird, but they all have wings.

Reptiles are a separate group. There are six main kinds of animals:

• Mammals: Warm-blooded vertebrates with hair

• Birds: Warm-blooded vertebrates with feathers

• Reptiles: Cold-blooded vertebrates with scaly or bony dry skin

• Amphibians: Cold-blooded vertebrates without scales who live both on land and in water

• Invertebrates: Animals without a backbone or bony skeleton

• Fish: Vertebrates that live in water and breathe using gills

Both birds and reptiles can be oviparous, meaning they lay eggs. All birds are oviparous, but some reptiles are viviparous, or give birth to live young.

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Are dinosaurs reptiles or birds?

There are both avian and non-avian dinosaurs, so where do they fall in animal classification? Dinosaurs are both reptiles and birds, in a way.

The birds we see flying in the sky or perched on rooftops today actually descended from two-legged dinosaurs called theropods. According to an article in Scientific American, recent discoveries suggest that features like beaks, small skull shapes and beaks evolved long before birds became as we know them today.

So what about dinosaurs’ reptilian roots? Smithsonian Magazine puts it this way: All birds are dinosaurs, but not all dinosaurs are birds. The Dinosauria group – avian and non-avian dinosaurs and eventually birds – is a part of the reptile family tree. So yes, dinosaurs are reptiles.

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This article originally appeared on USA TODAY: Are birds reptiles? Breaking down evolution and animal classification.