The black swans are friendly with people but vicious with other swans.

Two black Australian swans have settled on Brayford Pool, much to the annoyance of around 250 white mute swans living there already.

Birds on both sides have been injured in the ensuing fights.

Although nobody knows for certain where the black swans were living previously, it is thought they may have come from the nearby Fillingham Lake.

President of the Swan Preservation Society Nick May said: "They're very tame, but very vicious with the other swans.

"They'll feed out of your hand, so we don't know if they've come out of captivity somewhere.

"Because Brayford Pool is so overpopulated with swans, they're having to fight for their own territory.

"They will probably learn to live with each other if they do stay - it's a natural thing that's happening, so no authorities can do anything about it."

Alison Townsend from the Weirfield Wildlife Hospital in Lincoln said: "[Swans] do tend to stick to one area. If they're in an area, they will remain in that lake.

"And as long as their food source is good then obviously they will be happy and they will stay in that area."