The Norfolk site was chosen for an in-depth study of the vole

The Wildlife Conservation Research Unit at Oxford has been studying the size of populations in Norfolk and Wiltshire.

Dr Tom Moorhouse and Professor David Macdonald noticed a 48% decline in females born at the Norfolk site.

The study found that the shift in the sex ratio could be caused by the stress of wearing the collars.

Endangered species

The water vole (Arvicola terrestris) is the UK's most threatened mammal.

According to Dr Moorhouse and Prof Macdonald: "Radio-collars clearly have the potential to cause some stress to water voles, and it is possible that this might stimulate sex-ratio adjustment.

"Our analysis revealed that the most likely cause for the female decline was a shift in the sex ratio of young raised by radio-collared females."

Researchers have long been aware that the techniques they use had the potential to cause unexpected effects, and there have been many studies into the effects of radio-collars.

However, this is thought to be the first study to show an association between radio collars and sex ratio.