The swan had been living with a cob in the hospital grounds

The pen had been living with a male swan on the pond outside Halton General Hospital's Brooker Centre, in Runcorn.

Staff found it dead on 22 February. It had been shot in the head, but the cob was unharmed.

Paul Webster, of Cheshire Police, said the weapon used to kill the swan appeared to be of a higher grade than an average air-rifle or pistol.

He said he feared that the gunman could strike again and possibly target humans if he was not caught.