Flights of Fancy?

The report of a Pembrokeshire swan (nicknamed by the press 'Hannibal') being 'hyperaggressive' and attacking (and sometimes killing) other birds raises a number of questions (http://www.bbc.co.uk/news/uk-wales-south-west-wales-11331964 ). Some of the commentators have even speculated that there must be some contamination in the water to 'cause' this 'abnormal' behaviour. Mute swans (Cygnus olor) are often highly territorial and intolerant of other waterbirds (especially their own species). A Google search will confirm that such behaviour is frequently recorded (see 'the ASBO swan and 'Hissing Sid'). Even atypically maintained swans such as those at Abbotsbury in Dorset are not free of 'swanecidal' behaviour. Territoriality is used by many species as a means of getting access to limited resources (in this case the grass and pondweed around a nesting site in order to breed successfully and to rear offspring. The species clearly shows variability in this characteristic which is actually highly beneficial to the individual, its mate and its progeny. It is humans that object to the bird doing these actions (it's not 'nice' for a 'peaceful' bird to behave this way).