The BBC estimates that between 800 BC and the Roman times, Egyptians mummified as many as 70 million animals for religious offerings.

"Animal mummies were votive gifts. Today you'd have a candle in a cathedral; in Egyptian times you would have an animal mummy,” said Dr. Campbell Price, curator of Egypt and Sudan, at Manchester Museum. In many ways, the mummies were just another business—something almost everyone would need to purchase at one point or another.

"You would go to a special site, buy an animal mummy, using a system of barter. You'd then give it to a priest, who would collect a group of animal mummies and bury them,” Price explained. But despite a dedicated program of breeding and killing animals specifically for mummification, it was hard to keep up with the demand.

So the mummifiers started cutting corners. Or so it seems, based on new research out of Manchester Museum and the University of Manchester.

Over 800 specimens—ranging from cats and birds to crocodiles—were run through X-rays and CT scans for a project followed by the BBC's Horizon program. And what they found wasn’t quite what they were expecting.

"There have been some surprises,” said Dr. Lidija McKnight, an Egyptologist from the University of Manchester. About a third of the 800 contained remarkably well-preserved animal mummies, just as the wrappings would have one believe. Another third contained partial remains. And the final third? Nothing much.

"Basically, organic material such as mud, sticks and reeds, that would have been lying around the embalmers' workshops, and also things like eggshells and feathers, which were associated with the animals, but aren't the animals themselves,” McKnight said.

Dr. Price doesn't think that this necessarily indicates an ancient scam in which the embalmers passed off piles of junk for spiritually-significant tokens. Rather, he speculates that they were turning to "materials associated with the animals during their lifetime" when stocks ran low and that this was a compromise the mourning customers would have been aware of. Think of it as the Ancient Egyptian equivalent of buying a knock-off designer purse. 

[h/t io9]