The swans of Stratford are pinioned, that is, the outermost bones of one wing (apparently the right wing in this bird) are surgically removed in the first few days of life. This deprives the bird of primaries on that wing and prevents flight. Pinioning of Mute Swans is required in Canada to prevent these aggressive non-native birds from displacing native species. (Clipping primaries annually would be a less mutilating way, though less sure and more expensive, to limit the spread of this species; pinioning has not been practiced in the UK since the 1970s.)