Many residents will have noticed the Black Swan that has been on the Thames in recent weeks. Black Swans were first brought to England in 1791 as ornamental birds (just like peacocks and golden pheasants) but like many other captive species they occasionally escape and breed in the wild. The first recorded breeding in the UK was in 1902 and there is a well-established feral population on the Norfolk Broads. Black Swans bred at Great Meadow Pond in the Great Park in 2004, though it wasn’t until three years later that they successfully fledged. According to ‘The Birds of Berkshire’ (2013), they are occasionally found at local gravel pits and as many as ten have been seen around the junction of the Thames with the Kennet in Reading in recent years.
In fact, the iconic Black Swan is native to the wetlands of south-western and south-eastern Australia and Tasmania. It is the state bird of Western Australia and features on its flag and coat of arms. However, this species has now naturalized in New Zealand where it was deliberately introduced (in 1865) to clear Watercress, itself an alien species. In New Zealand, Black Swans soon spread in the absence of any competition and has numbered 200,000 in recent years, becoming a considerable agricultural pest in the process. Evidence from New Zealand populations is that breeding success becomes greater as the sizes of flocks increase and so it is entirely possible that, in time, Black Swans could compete with, and displace, UK native species.
The spread of species around the world, often aided by man, and not always with any consideration of the consequences, is a familiar tale. Our own Mute Swan has founded wild populations in countries as far apart as USA and Japan. Keeping an eye out for changes in the ranges of species, natural and otherwise, will become ever more necessary in a world of changing climates.