Albert de Jong
Larus cachinnans hybrid? adult, May 06 2013, Gatovo - Minsk, Belarus. Picture: Dmitry Goncharov.
Cachinnans or cachinnans hybrid forming a pair with one of two present Lesser Black-backed Gull breeding in Belarus in 2012 and 2013. See discussion here.
Below: roof top nesting in Belarus.
This is what local ornithologist Dmitry says:
Last season I found on the roofs of industrial buildings near Minsk (Belarus), several breeding colonies (CG + HG + LBBG). The largest of these was located on the roof size 240х480 m in Gatovo village and included 1,830 nests cachinnans + argentatus + fuscus and part of the mixed pairs. I often have questions accurately identify taxa. Nests are located mainly at the base of rooflights, ventilators, often among the vegetation on the the broken stone-gravel. Most nests contained three eggs (the norm), and some even six (two females - adult + 4 cy). Hatching chicks began in May. Frequently observed bird deaths on various factors: man-made traps, overheating of the roof, falling flying chicks.