Millions of years ago, the birds that roamed the Earth would’ve towered over modern humans, and weighed over a thousand pounds.
Recently, scientists have discovered fossils of birds that we wouldn’t expect to be so large today—prehistoric parrots that stood one meter tall, penguins the size of people, and other, more mysterious birds. The knowledge that the ancient world was home to overgrown bird species isn’t exactly new. But this recent batch of oversized bird fossils begs us to revisit the question: why were these birds so freakin' huge and where did they go?
Though using just a small fossil fragment to peer millions of years into the past is tricky business, paleontologists attribute the success of many of these extremely large birds to an absence of competition or predators.
New Zealand, home to both the recently-discovered giant parrot and penguin, was a hotbed for oversized birds. The island once was home to many now-extinct avian megafauna, including the flightless, 12-foot-tall Moa (which looked kind of like a giant emu, but is actually most closely related to the modern tinamou).
“The island was free of larger mammalian predators for a long time and it is a well known fact that many bird groups tend to lose their flight capabilities and become large, if there is no or only little predation pressure,” Gerald Mayr, one of the researchers who analyzed the giant penguin bones, said in an email.
Giant penguins—found in Antarctica as well as New Zealand—evolved a few million years after the dinosaurs went extinct along with large marine reptiles. Without competition, they penguins, like other island-bound birds, could become huge and flightless without fear of becoming lunch.
The exact advantage of being so big is hard to determine. According to Mayr, being a large vertebrate could help with thermoregulation and heat loss. According to Trevor Worthy, one of the researchers who found evidence of the giant parrot, it also allowed these birds to fill the ecological niche that mammals usually fill, including eating low-energy food in bulk.
A problem for these island birds, though, was that they were likely a bit lazy. “I think many larger birds on islands were just big, not fast or ferocious,” Worthy said in an email.
Because their main evolutionary adaptation was their size, they quickly faltered when other, fiercer competitors came on the scene. For many island birds, that competitor was humans, who both hunted birds directly and began capitalizing on their resources. Faster birds, like the emu and the ostrich, were able to survive despite newfound competition.
In the case of the giant penguins, which sought food in the water, toothed whales and pinnipeds arrived on the scene millions of years later and likely drove them to extinction by outcompeting them.
The case is slightly different for the Pachystruthio dmanisensis, a giant bird discovered in Crimea and the largest ever found on the European continent. It coexisted with large mammals unlike island birds, but researchers think it was likely fast, an added evolutionary advantage.
Despite these ancient birds' impressive girth, whole skeletons of person-sized penguins aren't exactly littering the landscape. Most of these discoveries have been based on small fossil fragments such as a bone or a body part. Worthy attributed these discoveries to dedicated paleontologists, and a bit of luck.
“It is just a matter of serendipity and opportunity,” Worthy said. “In the last 100 years no one looked anywhere near as hard and regularly. Effort is rewarded.”
This article originally appeared on VICE US.