Fourth occurence of slender-billed gull Larus genei in the Baikal region, and its nesting in pair with black-headed gull Larus ridibundus

In a colony of black-headed gull (BG) in Irkutsk, a male slender-billed gull (SG) was found incubating a clutch of 3 eggs with a BG female in 2003. The eggs, being found 3 June as early incubated, were unable to be differed from BG ones in sizes and coloration. Chicks of the mixed pair, hatching 23-24 June, were similar in down plumage to BG ones by rufous coloration, but these were somewhat lighter, and one chick had the same black cap as in several SG chicks, instead a black head spotting as in BG ones. The month-aged feathered chicks differed from usual BG ones by absence of brownish color at a mantle (though, ca 10% BG fledglings in Irkutsk had the similar coloration), and by lighter bill of a more fine color than in BG. It is fourth record of SG around Lake Baikal and generally in the East Siberia, ca 2000 km eastward of its nesting range, but a nesting trial and hybridization SG x BG is registered firstly in the region as well as likely in Russia and worldwide.