On May 29th 2013, Colin Selway witnessed a herd of Mute Swan and a single Black Swan attacking and killing another Mute Swan.
There is a number of documented cases of a single Mute Swan attacking and killing other Mute Swan but I can find no reference to a herd doing this.
BWP states that 3% of all dead ringed Mute Swan in 1969 died due to territorial disputes with another Mute Swan.
A well documented individual can be found here.
A video of a male attacking another and almost killing it can be found below;
© Camelbirder 2013