Research is being carried out on why blue tits have been nesting in cigarette butt bins and if it could have any long-term impact on the birds.

Douglas Russell, a senior curator who is based at the Natural History Museum (NHM) in Tring, Hertfordshire, is carrying out the study after more tits were reported to be nesting in the bins.

The phenomenon has been noted anecdotally for some 20 years, said Mr Russell, who recently recovered two bins from Old Buckenham, Norfolk, and Harrogate, Yorkshire, where the birds had fledged.

Tests would be carried out on the boxes and nests to find out more about any toxicity passed onto the birds, and to establish if the bins need redesigning to prevent tits from nesting in them.