Every spring, millions of birds travel to the Arctic to breed. Geese, ducks, shorebirds, loons: the North Slope of Alaska fills with them. A paper published this week on the cover of Science documents that this same seasonal pattern was already established 73 million years ago, during the Cretaceous, when birds were sharing the landscape with non-avian dinosaurs.
The paper is led by Lauren Wilson, a doctoral student at Princeton University who conducted the research as part of her master’s degree at the University of Alaska Fairbanks, with Pat Druckenmiller of the University of Alaska Museum of the North as senior author. Using more than 50 fossilised bird bones and bone fragments, including bones from hatchlings, collected from Alaska’s North Slope along the Colville River, the team identified multiple bird types breeding in the polar region at a time when no evidence of such behaviour had previously existed.
Before this study, the earliest known evidence of birds reproducing in either polar region came from roughly 47 million years ago, well after the asteroid impact that ended the Cretaceous. This finding pushes that record back by 25 to 30 million years.
What the fossil collection contains
The bones come from the Prince Creek Formation, a site on Alaska’s North Slope that has produced an extensive record of Cretaceous-era life, including dinosaur fossils. The birds identified include diving birds that resemble modern loons, gull-like birds, and several types similar to present-day ducks and geese. The presence of hatchling bones is what makes the breeding claim possible: these are not migrating adults passing through, but animals that hatched there.
Finding bird bones from the Cretaceous at all is unusual. Bird bones are thin and porous, and they do not preserve well. Hatchling bones are smaller still, more fragile, and rarely survive the geological processes that create the fossil record. The collection at the Prince Creek Formation is the product of an excavation methodology that treats small as seriously as large: researchers haul sediment by the tub back to the laboratory and examine it under a microscope, recovering bones and teeth invisible to the naked eye in the field. That approach, Druckenmiller notes, has made the site “one of the best places in the nation for bird fossils from the age of the dinosaurs.”
An open question about modern bird origins
Some of the bones from the Colville River collection have skeletal features found only in Neornithes, the group that includes all living birds. If confirmed, these would be the oldest known Neornithine fossils, predating the current record holder by roughly four million years. The oldest currently confirmed modern bird fossils date to approximately 69 million years ago.
The team is cautious about that claim. “It would take us finding a partial or full skeleton to say for sure,” Druckenmiller said. The bones available are fragmentary. The skeletal features in question are suggestive, not conclusive. What can be said is that the Arctic bird fauna of the Cretaceous included animals that, in their habits and possibly their lineage, resemble the birds that fill the same skies today.
What the timing suggests
“Birds have existed for 150 million years,” Wilson said. “For half of the time they have existed, they have been nesting in the Arctic.”
That framing is useful. The seasonal migration to Arctic breeding grounds is often discussed as though it were a behaviour that emerged after the mass extinction 66 million years ago, when the ecosystem rebuilt itself around surviving lineages. The Prince Creek evidence suggests the instinct predates the extinction, and that whatever drove early birds to breed at high latitudes was already functioning well before that event.
Why breed in the Arctic at all? The Cretaceous Arctic was warmer than it is today, with no permanent ice cap, but it still had polar day and polar night: months of continuous light followed by months of darkness. The seasonal pulse of productivity that draws modern birds north, the explosion of insect life and daylight that makes the short Arctic summer unusually rich for raising young, was presumably operating on similar principles then. The paper does not resolve the evolutionary logic, but it establishes that the behaviour is considerably older than the post-extinction world in which most bird evolution has been studied.
Collaborators on the paper include Daniel Ksepka from the Bruce Museum, John Wilson from Princeton University, Jacob Gardner from the University of Reading, Gregory Erickson from Florida State University, Donald Brinkman and Caleb Brown from the Royal Tyrrell Museum of Palaeontology, Jaelyn Eberle from the University of Colorado Boulder, and Chris Organ from Montana State University.
