 In the outer garden is an arboretum , planted in 1977 by Bishop John Bickersteth to commemorate the Silver Jubilee of Elizabeth II . The mute swans on the moat have been trained to ring bells by pulling strings , to beg for food . The first swans were trained by one of the daughters of Bishop Hervey in the 1870s . Two swans which were given to the bishop by Queen Elizabeth II in 2006 , are still able to ring for lunch fed to them by the caretakers who live in the gatehouse . 
