Colonial breeding in Mute swans (Cygnus olor) associated with an allozyme of lactate dehydrogenase

The Mute swan ( Cygnus olor Gmelin (Anatidae)) is a common water bird of lowland freshwaters and coastal shallows. Its typical breeding system involves lifelong monogamous pairs which vigorously defend large breeding territories, sometimes killing intruding swans that are unable to escape. However, in some unusual circumstances (superabundant food coupled with limited nesting sites) Mute swans may nest colonially. At the only two colonies in southern England a rare allele for lactate dehydrogenase (LDH) was found to be unusually common and colony swans carrying this allele were shown to breed more successfully. This isolated finding could, however, have originated either from a chance 'founder' effect or from human management of the main colony. We now show that this allele is also significantly commoner at two recently formed colonies in Denmark, implying that the association between the allele and colonial breeding may be widespread and longstanding.