Birds come in all shapes and sizes. There's everything from the speedy, tiny hummingbird to the flightless and massive ostrich. Yet this wasn't always the case; it turns out that prehistoric birds show a stunning lack of diversity.

In order to learn a little bit more about bird evolution and bird species in the past, the scientists examined a group of bird fossils dating back to the Cretaceous period, which occurred about 125 million years ago. These fossils were collected from an area in China where there was once violent volcanic activity that led to a plethora of well-preserved fossils.

Even though these fossils were well-preserved, though, the scientists had to delve deeper to figure out the diversity in how the birds behaved. They used modern-day birds to build a statistical technique that could relate the physical characteristics of a bird to its diet, behavior and habitat. For example, long legs might be associated with birds that wade through water while the shape of a bird's beak could be related to what it ate.

"There were no swans, no swallows, no herons, nothing like that," said Jonathan Mitchell, one of the researchers, in a news release. "They were pretty much all between a sparrow and a crow. They were all pretty much the same. They were ground-dwelling or forest-dwelling little birds, mostly eating insects and seeds."

That said, there could be compounding factors that led to these results. It's possible that there was a potential bias introduced by the fossilization process. Some types of birds might become fossilized more often than others, which could have reduced the diversity that the researchers discovered. Yet this seems somewhat unlikely.

It would make sense that the fossils would be biased toward larger birds and birds that lived in water; yet this wasn't the case with the fossils studied. Instead, the researchers found a wealth of fossilized small, land-dwellers.

So why was there this lack of diversity? It's possible that early birds were less diverse due to competition with similar groups, such as prehistoric flying reptiles known as pterosaurs. In addition, the birds simply may have no had enough time to diversify.

The findings reveal a little bit more about bird evolution and how and when birds originated.

The findings are published in the journal Proceedings of the Royal Society B.