<10,000 years ago
||Hebert PD, Stoeckle MY, Zemlak TS, Francis CM. Identification of Birds through DNA Barcodes. PLoS Biol. 2004 Oct2(10):e312. DOI: 10.1371/journal.pbio.0020312 p.1659 right column bottom paragraphPubMed ID15455034
|| Crochet PA, Lebreton JD, Bonhomme F (2002) Systematics of large white-headed gulls: Patterns of variation in Western European taxa. Auk 119: 603–620. link  Crochet PA, Chen JZ, Pons JM, Lebreton JD, Hebert PD, Bonhomme F. Genetic differentiation at nuclear and mitochondrial loci among large white-headed gulls: sex-biased interspecific gene flow? Evolution. 2003 Dec57(12):2865-78. PubMed ID14761064
||Primary source  abstract: "[Investigators] used mitochondrial DNA control region and cytochrome-b gene sequences in an attempt to resolve some of those uncertainties." Primary source  abstract: "[Investigators] measured genetic differentiation among species of large white-headed gulls using mitochondrial (cytochrome b haplotypes) and nuclear (microsatellites) markers. Additional information was added using a previously published study of allozymes on the same species."
||P.1659 right column bottom paragraph: "The diagnosis of species is particularly difficult when they are young. Moreover, hybridization is often common when the ranges of recently arisen species overlap, further complicating identifications. Such newly emerged species are sometimes referred to as superspecies (Mayr and Short 1970), or species complexes, to indicate their close genetic similarity. For example, the white-headed gulls are thought to have diverged very recently, some less than 10,000 years ago (primary sources), and hybridization is common among many of them. It is thus not surprising that their COI [cytochrome c oxidase I] barcodes and other gene loci are very similar."