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When the northern fulmar expanded its northeast Atlantic breeding range from the two known colonies, Grimsey in northern Iceland and St Kilda in the Outer Hebrides, Scotland, about 350 yr ago, the geographical pattern of colonisation – initially the Faroes, then Scotland, followed by Ireland and southern Britain – led James Fisher to propose a sole Icelandic source for the colonists. However, previously-analysed mitochondrial DNA from contemporary samples indicated a St Kildan origin for at least some colonists. If Fisher's hypothesis is correct and Iceland and not St Kilda was the source population for all of the new colonies, the Icelandic signal should be stronger in museum samples collected 100 yr ago when St Kilda was populated by people who harvested large numbers of fulmars. Patterns of genetic, specifically, nucleotide, diversity suggest an Icelandic origin for the pre-1940 samples. St Kilda birds contained a number of closely related haplotypes whereas Grimsey, Iceland, the other putative source population, contained diverse haplotypes. These two patterns are indicative of a younger and older population, respectively. When both nuclear aldolase and mitochondrial control region sequence data from historical samples collected on the newly colonized islands were examined, they contained highly divergent haplotypes characteristic of Grimsey, not St Kilda. Comparison of mitochondrial data from samples collected in the early and late 20th century showed an interesting pattern of haplotype turnover on St Kilda. Prior to 1940 the haplotypes present on St Kilda were genetically similar to one another, yet haplotype sampling in the 1990s showed highly divergent haplotypes on the island. We propose that these new haplotypes are not the result of mutation, but immigration from other colonies in the North Atlantic.