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Message: <blockquote data-quote="smiths" data-source="post: 1364385" data-attributes="member: 4346">Simply looking at my collection of photos of 2nd-cycle Caspian Gulls (from Belgium mostly, but also Finland, Poland, Ukraine and Switzerland) in which P10 can be properly judged (26 birds in total), reveals 14 birds with a mirror and 12 without -- 54 and 46% respectively. Of those 12 without, 11 look like typical Caspian in all other respects, while one shows a browner underwing than usual.The point is: it is not unusual to come across a 2nd-cycle gull that looks like a Caspian, but lacks a mirror on P10.In addition, it is well known that some 2nd-cycle argentatus also show a white mirror on P10 already, so I am not sure if the presence or absence of this feature would prove much in the case of a potential hybrid.The Panov & Monzikov paper perhaps could be relativated to some extent. Especially their ideas on 'omissus' have been contradicted by more recent studies (e.g., <a href="http://www.gull-research.org/papers/21crochet.pdf" target="_blank">Crochet et al 2002</a> and  <a href="http://www3.interscience.wiley.com/journal/117989723/abstract" target="_blank">Gay et al 2007</a>). The article by Gay et al (2007) deals with hybridization between Caspian and Herring Gulls. The conclusions are that:a) Hybridization indeed occurs between these two species and, yes, F1 hybrids are fertile;b) The zone of secondary contact between the species is actually quite narrow, and may be limited to Central Poland. As often the case in large gulls though, offspring (including hybrids and backcrosses) may disperse far from their natal colony and start breeding hundreds of miles further;c) Gene flow between the two species may be asymmetric, i.e. male cachinnans seems to be more likely to pair with female argentatus than female cachinnans with male argentatus (though this needs further testing);d) While there appear to be no barriers to neutral gene flow between the two species in the area of secondary contact, there does seem to be selection on the colouration of bare parts, which reduces introgression. More precisely, birds with light-coloured irises and eye-rings (like argentatus) may be reproductively isolated from birds with dark irises and eye-rings (like cachinnans). Again, this needs further testing, but "introgression of these phenotypic traits seems nevertheless slower compared to neutral markers".This suggests that there is a premating barrier to introgression, be it incomplete, and that hybridization might not be as widespread as suggested by Panov & Monzikov.</blockquote>

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