The same was true 73 million years ago, according to a paper 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.