After decades of mysterious bat deaths at Durham Cathedral, newly-published citizen research has cracked the case. 

A paper in the European Journal of Wildlife Research looked into the deaths of scores of protected bats each year in the cathedral, and concluded that they are caused by overheating in their roosting sites. 

Every summer, thousands of Common Pipistrelle bats gather in the cathedral in a little-understood phenomenon known as "seasonal swarming". Some of these bats roost below the lead roof of the cathedral cloister, which heats up under the sun on clear summer days. 

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On warm afternoons, bats can often be seen flying within the cloister in broad daylight while trying to find cooler places to roost, but if they fail to move in time they begin to suffer from heatstroke and become unable to fly. As a result, they become grounded when they eventually attempt to leave their roosting sites.

Every year, large numbers of bats are grounded between July and September, sometimes running into the hundreds. In the last decade, 870 bats have been grounded, with 316 fatalities, all involving juvenile Common Pipistrelle bats. 

Since the 1980s, Durham Bat Group has tried to rehabilitate these grounded casualties, and in 2012, the group initiated a "citizen science" project to work out a cause of the annual bat die-offs. 

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Over the last decade, over 30 volunteers have been involved in recording the number and condition of bats found in the cloister. Durham-based consultant ecologist and researcher Dr Christopher Paul Bell volunteered to analyse the results collected during the research. 

Dr Bell commented: "For a long time we had no idea why so many bats were dying each year at the cathedral. We even sent a sample of bats to the Animal & Plant Health Agency to carry out post-mortems, but the results were inconclusive.

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"However, it eventually became clear that there was a highly significant correlation between the numbers grounded at the cloister and temperature during the day."

As research has identified a strong correlation between temperature and bat deaths, suggestions have been made on how to prevent future deaths of the protected species of bat.

Dr Bell proposed the installation of solar panels, which has recently been done at Salisbury Cathedral. This would serve the dual purposes of protecting the bat colony and providing solar power.