Occasional individuals of some species have nested in their wintering range (through suspending their normal spring migration), leading in some cases to the establishment of new nesting populations. Examples include the North American Barn Swallows Hirundo rustica found nesting in Argentina, within the usual wintering range (Martinez 1983), and Eurasian White Storks Ciconia ciconia, Bee-eaters Merops apiaster and others now nesting in South Africa (Snow 1978, Harrison et al. 1997). Among seabirds, Leach's Storm Petrels Oceanodroma leucorhoa, which breed widely in the North Atlantic and Pacific, have been found breeding on an island off South Africa, and in potential nesting burrows on the Chatham Islands off New Zealand, within the wintering range of northern hemisphere birds (Imber & Lovegrove 1982, Whittington et al. 1999). This phenomenon of 'migration suspension' may have been going on for millions of years, considering the numbers of bird species that have conspecifics, or closely allied forms, breeding in equivalent habitat in the opposite hemisphere (Snow 1978, Newton 2003). It may result from occasional individuals becoming 'time-trapped' in winter quarters, switching under the influence of southern hemisphere daylengths to an annual cycle appropriate to the southern hemisphere (for an example in experimental conditions see Gwinner & Helm 2003).
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