Owls, not people, explain a Roman-era "frog graveyard" near Cambridge, study finds

A shallow ditch dug into the edge of the English Fens near Cambridge has given up an answer to a question that has puzzled archaeologists for roughly a decade: who, or what, left behind more than 9,000 frog bones in a single deposit from the Roman period. A study published this month in the journal Environmental Archaeology concludes that the bones are the discarded remains of owl meals, not evidence of a human feast, a mass die-off or disease, as earlier theories had proposed.

Researchers led by Paul Clarkson, an archaeologist at Bournemouth University, examined more than 1,700 of the roughly 9,135 small bones recovered from a 20-meter gully at Bar Hill, a site excavated between 2016 and 2018 ahead of the A14 Cambridge to Huntingdon road scheme. Almost 86 percent of the frog bones showed the kind of etching and partial dissolution that results from passing through a predator's digestive tract. None of the bones carried cut marks, burn marks or other signs of butchery, which argues against the possibility that people living at the site had cooked and eaten the frogs.

A mystery since 2022

The gully was first excavated by teams from the Museum of London Archaeology (MOLA) and Headland Archaeology as part of one of the largest infrastructure digs ever undertaken in Britain, covering roughly 350 hectares along the road corridor. When MOLA first reported the find in 2022, it described more than 8,000 amphibian bones concentrated in a single feature that excavators had initially mistaken for the floor of an Iron Age roundhouse. At the time, MOLA floated several competing explanations without settling on one: frogs might have fallen into the ditch during a spring breeding migration and been unable to climb out, a harsh winter might have killed off a hibernating population, or a disease such as ranavirus might have swept through. The organization's own account called it "a puzzling and unexpected find."

Reading the bite marks

The new analysis, co-authored by researchers from Bournemouth University, MOLA, Headland Archaeology and the Natural History Museum in London, focused on damage patterns on individual bones rather than the raw count. Digestion-related wear, the team found, pointed away from mammalian predators such as otters and toward a bird of prey. Weighing the fragmentation and corrosion against reference collections, the researchers judged the bones too lightly damaged to match barn owls, which tend to crush prey more thoroughly, and instead attributed the deposit to short-eared owls, a species that still winters on the fens today. A separate measurement of the frogs' humerus bones indicated that most of the animals died young, in their first winter of life, consistent with a single season of owl predation rather than a recurring, long-term pattern.

• 9,135 small bones recovered from the gully; more than 1,700 examined in detail
• About 86 percent of frog bones showed digestion damage, not burning or butchery
• Deposit radiocarbon-dated to roughly 1,700–1,800 years old, in the Roman period
• Bone fragmentation pointed to short-eared owls rather than barn owls or mammals

A landscape remade for hunting

Radiocarbon dating carried out at the University of Vienna placed the deposit at roughly 1,700 to 1,800 years old, in the Roman period rather than the Iron Age occupation of the nearby settlement. Pollen, snail shell and charcoal evidence recovered from the same sediments suggest that the surrounding valley, wooded during the Iron Age, had been cleared for livestock grazing by the time the bones accumulated. That shift to open grassland, the authors argue, would have suited a short-eared owl's hunting habits far better than it suited forest-dwelling owls, while falling floodwater levels in the retreating fen left shallow pools where frogs could breed undisturbed.

"We don't often get an owl's-eye view of what it was like in the past from archaeological sites," said Clarkson, the study's lead author. "An owl's-eye view is one worth considering."

Vicki Ewens, a senior zooarchaeologist at MOLA who worked on the original excavation and is a co-author of the new paper, said she had weighed many competing ideas before the bone-damage evidence pointed to owls. "I had so many theories as to why such a large collection of frog bones were recovered from this small feature," she said.

Why an old puzzle matters

The case illustrates a method increasingly used in zooarchaeology: treating an unusual bone deposit not as waste to be explained away but as an indirect record of the animals, plants and habitats that surrounded a site, assembled by a predator that otherwise left almost no trace of itself. A single winter's worth of owl pellets captured a near-complete snapshot of the rodents, shrews, snakes and amphibians living in the valley at that moment, data that complements the pollen and charcoal evidence used to track the Roman-era clearance of the fen edge.

The frog deposit is one small piece of a far larger excavation. The A14 scheme's archaeology program, funded by National Highways and carried out with Cambridgeshire County Council, recovered roughly 15,000 objects and more than 500 human burials in total, and specialists are still working through material years after excavation ended. Clarkson and his co-authors say further study of the Bar Hill assemblage, including closer comparison with modern owl pellet collections, could help determine whether a single winter accounted for the entire deposit or whether owls returned to the same ditch across several seasons.