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Spring in the Arctic brings forth a plethora of peeps and downy hatchlings as millions of birds gather to raise their young.

The same was true 73 million years ago, according to a featured on the cover of Science. The paper documents the earliest-known example of birds nesting in the polar regions.

"Birds have existed for 150 million years," said lead author Lauren Wilson, a doctoral student at Princeton University who earned her master's degree at the University of Alaska Fairbanks. "For half of the time they have existed, they have been nesting in the Arctic."

The paper is the result of Wilson's master's thesis research at UAF. Using dozens of tiny fossilized bones and teeth from an Alaska excavation site, she and her colleagues identified multiple types of birds—diving birds that resembled loons, gull-like birds, and several kinds of birds similar to modern ducks and geese—that were breeding in the Arctic while dinosaurs roamed the same lands.

Prior to this study, the earliest known evidence of birds reproducing in either the Arctic or Antarctic was about 47 million years ago, well after an asteroid killed 75% of the animals on Earth.

"This pushes back the record of birds breeding in the polar regions by 25 to 30 million years," said Pat Druckenmiller, the paper's senior author, director of the University of Alaska Museum of the North and Wilson's advisor for her master's degree work. The bird fossils are part of the museum's collections.

"The Arctic is considered the nursery for modern birds," he said. "It's kind of cool when you go to Creamer's Field [a Fairbanks-area stopover for migrating geese, ducks and cranes], to know that they have been doing this for 73 million years."

The mere existence of the large collection of ancient bird fossils is remarkable, Wilson said, given how delicate bird bones are. That is doubly true for baby bird bones, which are porous and easily destroyed.

"Finding bird bones from the Cretaceous is already a very rare thing," she said. "To find baby bird bones is almost unheard of. That is why these fossils are significant."

The fossils were collected from the Prince Creek Formation, an area along the Colville River on Alaska's North Slope known for its dinosaur fossils. Scientists identified more than 50 bird bones and bone fragments.

"We put Alaska on the map for fossil birds," Druckenmiller said. "It wasn't on anyone's radar."

The collection is a testament to the value of an uncommon excavation and research approach at the Prince Creek Formation. Much of vertebrate paleontology focuses on recovering large bones.

The scientists who work in the Prince Creek Formation make sure to get every bone and tooth they can, from the visible to the microscopic, Druckenmiller said.

The technique, which involves hauling tubs of screened sediment back to the lab for examination under a microscope, has yielded numerous new species and unprecedented insights into the behavior and physiology of the dinosaurs, birds and mammals that lived in the Arctic during the Cretaceous Period.

"We are now one of the best places in the nation for bird fossils from the age of the dinosaurs," Druckenmiller said. "In terms of information content, these little bones and teeth are fascinating and provide an incredible depth of understanding of the animals of this time."

It remains to be seen whether the bones found on the Colville River are the earliest-known members of Neornithes, the group that includes all modern birds. Some of the new bones have skeletal features only found in this group. And, like modern birds, some of these birds had no true teeth.

"If they are part of the modern bird group, they would be the oldest such fossils ever found," Druckenmiller said. Currently, the oldest such fossils are from about 69 million years ago. "But it would take us finding a partial or full skeleton to say for sure."