Evidence of pagan rituals involving swans and other birds in the Cornish countryside in the 17th century has been uncovered by archaeologists.

Since 2003, 35 pits at the site in a valley near Truro in Cornwall have been excavated containing swan pelts, dead magpies, unhatched eggs, quartz pebbles, human hair, fingernails and part of an iron cauldron.

The finds have been dated to the 1640s, a period of turmoil in England when Cromwellian Puritans destroyed any links to pre-Christian pagan England. It was also a period when witchcraft attracted the death sentence.

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Jacqui Wood, leading the excavations, has not traced any written or anecdotal evidence of the rituals, which would have involved a significant number of people over a long period. There are no records of similar practices anywhere else in the world.

Wood, an archaeologist who has advised on the discovery in 1991 of Europe's oldest human mummy, the "Iceman", in an Alpine glacier, has been digging at the site at Saveock Water for the past eight years.

Saveock Water was, in the 17th century, a community of five houses whose occupants worked at a nearby mill.

Human occupation of the site dates to prehistoric times but some of the activity uncovered was more recent. A stone-lined spring that may have been a "holy well" was full of offerings from the 17th century, including 125 strips of cloth from dresses, cherry stones and nail clippings.

There was evidence that the well had been filled and the site destroyed to hide what went on there.

Each of the feather pits, which are about 40 cm square by 17 cm deep (15 by 6in), have been carefully lined with the intact pelt of one swan and contain other bird remains.

The pits where the contents were intact also contained a leaf parcel holding stones that experts have traced to Swanpool beach, 15 miles away, an area famed for its swan population.

"Killing a swan would have been incredibly risky at this time because they are the property of the Crown," said Wood.

There was a particularly macabre discovery in one of the feather pits: fifty-seven unhatched eggs ranging in size from a bantam to a duck. They were flanked by the bodies of two magpies, birds that have long been the subject of superstition in Cornish folklore.

The organic remains survived because they were preserved in the water-logged ground. Although the shells of the eggs had dissolved, the membrane remained, revealing chicks shortly before they were due to hatch.

"A lot of the paganism of the Celts was wiped out by the Romans, but not in Cornwall," said Wood. "Swan feathers had a connection with fertility. It's possible these offerings were being left. Then, if there was a conception, nine months later the person would return to empty the pit."

Wood will deliver a paper on the feather pits at the World Archaeology Conference in Dublin in June.

"Often when secret rituals are abandoned people will talk about 'things that were done in my grandmother's day' but there has been no whisper of this," Wood speculated. "It really makes me wonder whether that is because it is still going on."

Burnt, hanged and drowned

— The pits were created in the 17th century when the law stated "thou shalt not suffer a witch to live"

— Thousands of women, the vast majority innocent, were burnt, hanged or drowned

— The first Witchcraft Act was passed in 1541

— In the mid-16th century, when it was believed that the plague was the work of sorcery, persecution of witches reached a frenzy

— The death penalty for witchcraft ended in 1735

— Last week the Scottish Parliament was asked to approve a pardon for the 4,000 people killed

— The last person to be convicted was Jane Rebecca Yorke, a medium who was fined five pounds in 1944 for claiming to be able to contact dead servicemen