The Common Gull (Larus canus) is a typical breeding bird species of the Baltic Sea coast in Schleswig-Holstein, northern Germany, where it has declined substantially in recent decades. Since the mid-1990s, Common Gulls have started to colonize flat roofs of buildings, consisting mostly of gravel and small stones. The total number of roof-breeding Common Gulls in Schleswig-Holstein is estimated to be at least 400-450 pairs for the period 2000-2005, with further increasing trends. Minimum reproductive performance at two colonies in 2000 was 0.9 and 1.8 fledged chicks per nest. The diet at two colonies studied in July 2001 consisted mainly of terrestrial food items. The most important prey categories were cherries, lumbricids, insects and bivalves. It is concluded that Common Gulls show a flexible response to environmental conditions by successfully exploiting a new breeding habitat, by reproducing successfully there and by taking benefit of their wide food spectrum to exploit man-made sites.