Black swans are starting to spread in Switzerland, and are spreading discord in their wake.
The swans were imported from Australia in the 1980s by a breeder who was allowed to keep them in a restricted area on Lake Thun, south-east of the capital, Bern.
To prevent them spreading, gamekeepers pierce any eggs found outside the permitted area. Swiss law forbids the release of imported species into the wild.
But in the past few days ornithologists have reported sightings of black swans in two places on Lake Geneva about 100 km away.
Since there are no other black swan populations in Switzerland, they think these birds must have come from Lake Thun.
Although nearly six thousand people signed a petition earlier this year in support of allowing black swans to live freely on the lake, experts are concerned that the spread of the swan could cause considerable problems, as has occurred with other introduced species.