Some ancient Egyptian tombs contain millions of mummified ibises, or hook-billed shorebirds sacrificed in honor of the ibis-headed god Thoth. The origins of these avian mummies have long been unclear, but now, a new genetic survey published in the journal PLoS ONE suggests the vast majority of sacrificial birds came from the wild.

Archaeologists had previously suspected the African sacred ibis (Threskiornis aethiopicus) was domestically bred in order to produce the incredible number of mummies found across Egypt. But while researchers have found facilities where the ancients bred cats, dogs and even crocodiles for the sole purpose of mummification, there is no archaeological evidence of similarly large-scale ibis hatcheries.

National Geographic’s Antoaneta Roussi reports that there is some written evidence of large-scale ibis operations occurring in ancient Egypt. Still, the latest findings suggest these bird farms simply acted as halfway houses for captive wild ibises set to be ritually sacrificed.

To better pinpoint the ibises’ origins, researchers sequenced DNA from 40 mummified birds found in six roughly 2,500-year-old catacombs. The team also analyzed genetic samples from 26 modern sacred ibises collected across Africa. According to a press release, 14 of the mummies yielded complete sequences, and all of the modern samples produced full mitochondrial genomes.

Domestication, even for relatively short periods of time, leaves a mark in animals’ genes, with species becoming less genetically diverse due to inbreeding and restricted choice of mate. Since the DNA used in the study revealed similar levels of genetic diversity among mummified and modern birds, the scientists say it’s unlikely the ancient Egyptians were conducting large-scale farming operations centered on ibises.

“Surprisingly, and in spite of the millions of mummies found, no signs of long-term inbreeding were found inside the ibis DNA,” lead author Sally Wasef of Griffith University in Australia tells Newsweek’s Aristos Georgiou. “Most likely, that suggests that the priests tamed wild populations through food temptations within their natural habitats, such as the lakes or wetlands close the temples.”

As Wasef explains to the Guardian’s Nicola Davis, both Tuna el-Gebel and the Lake of the Pharaohs near Saqqara—sites where archaeologists have unearthed four and 1.75 million ibis mummies, respectively—are situated by swamps that could have been used to attract wild ibises.

Ancient DNA expert Pontus Skoglund of the Francis Crick Institute calls the team’s work impressive but points out that if an ibis hatchery was large enough, it may have been able to support a higher level of genetic diversity over time. He tells Davis another potential explanation for the study’s results is that wild birds occasionally bred with hatchery birds, thereby refreshing the gene pool.

Archaeologist Francisco Bosch-Puche of the University of Oxford has discovered thousands of ibis mummies over the course of his career. Speaking with National Geographic’s Roussi, he says the injuries seen on certain ibis mummies are consistent with those suffered by birds raised in modern factory farms.

“We are still talking millions of animals at different sites all over Egypt,” Bosch-Puche adds, “so relying just on the hunting of wild ones does not convince me.”

Wasef tells Davis that no hatchery structures for raising ibises have ever been found in Egypt. She also points out that in many cases, ibis mummies do not contain complete birds, but are instead wrapped around single feathers or pieces of eggshell. This, she says, is a sign that the birds were sometimes scarce—an unlikely scenario if the ibises were actually being raised en masse.

The African sacred ibis went extinct in Egypt around 1850. But today, the bird’s sister species, the Australian white ibis (classified as the same species until the 1990s), roams freely across the continent. Unlike its revered relation, the Australian species has a rather lackluster reputation: Thanks to its not-so-sacred habit of eating out of garbage cans, the bird is commonly nicknamed “bin chicken.”