Birds come in all shapes and sizes - from the sprightly hummingbird to the huge, flightless emu. But this what not always the case, according to research by scientists at the University of Chicago and the Field Museum.

Ancient fossils of bird fauna (a set of species that lived at about the same time and in the same habitat) revealed a striking lack of diversity in birds long ago.

In order to learn more about bird evolution and bird species in the past, researchers examined a group of bird fossils dating back to the Cretaceous period - around 125 million years ago. The fossils were collected from an area in China where there was once violent volcanic activity, leading to a plethora of well-preserved fossils.

Despite them being well preserved, understanding the diversity in how the birds behaved, and not just their physical characteristics, required significant scientific legwork. For instance, 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. They were pretty much all between a sparrow and a crow," Jonathan Mitchell, one of the researchers, said in a statement.

That said, a possible confounding factor was the bias potentially introduced by the fossilization process. Some types of birds might become fossilized more often than others, resulting in a diverse population. Yet this seems unlikely.

If this isn't the case, it's possible that early birds were less diverse due to competition with similar groups, such as prehistoric flying reptiles known as pterosaurs. Or, birds simply didn't have enough time to diversify.

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