Starfish have also washed ashore on England's beaches, along with the velvet swimming crabs. (shown here, Nayland Rock, Margate)

Piles of crabs have been washing up on England beaches along the rocky Thanet Coast in recent days, a phenomenon that has puzzled scientists for the past few years.

"It's been a phenomenon for probably a third year in a row," Tony Child, Thanet Coast project manager, told LiveScience. He estimated about 25,000 of the dead velvet swimming crabs (Necora puber) were in piles this year, where birds are now ferociously feeding on their carcasses.

Last year, about 40,000 of the crabs washed ashore on the Thanet Coast, which is a long coastline of chalk reefs in Kent, England. This year, more starfish also washed ashore.

The velvet swimming crab has bright red eyes, with a coat of fine hair on its shell giving it a velvety texture. The crabs come closer to shore at this time of year, Child said, where they feed on the seaweed.

In the past, environmental scientists ran tests to check for disease or other physiological problems with the crabs, coming up empty-handed. But Child said every year the die-offs have occurred after there was snow on the beaches. The meltwater causes temperatures near shore to drop, and Child said the deaths must be linked to hypothermia.

"I don't really know the cause but it seems to be cold-related," Child said.

Reports of the crabs washing ashore began around Christmas, but it wasn't until the recent few days that they've been piling ashore. The crabs will remain on the beach, where they'll be feasted on by birds and otherwise decay and enter back into the system's life cycle.

The crab deaths come on the heels of other bizarre animal deaths, including thousands of blackbirds that reportedly fell out of the skies in Arkansas on New Year's Eve, leading to speculation that booming fireworks disoriented them and caused them to collide with buildings.

Then on Tuesday, about 500 birds were found dead in Louisiana. Then more dead birds were reported in Sweden, and masses of dead fish have washed ashore in several countries.

You can follow LiveScience Managing Editor Jeanna Bryner on Twitter @jeannabryner.