NAGTEGAAL J & VAN BRUGGEN J (2018) Inland colonisation by Lesser Black-backed Gulls Larus fuscus and Herring Gulls Larus argentatus; an overview up to and including 2015 . LIMOSA 91 (4): 168-180.

Lesser Black-backed Gulls and Herring Gulls are traditionally known as coastal birds in the Netherlands. In the second half of the 19th century Herring Gulls were also found breeding inland, while Lesser Black-backed Gulls started breeding inland from 1989 onwards. Both species were often found breeding in colonies of Black-headed Gulls Chroicocephalus ridibundus. From 2000 onwards, the number of breeding pairs of Lesser Black-backed Gulls and Herring Gulls in the study area increased slowly. Besides several breeding cases and small colonies, the first large colony was found in the province of Overijssel, with peak numbers in 2007 of 245 pairs of Herring Gulls and 27 pairs of Lesser Black-backed Gulls. Between 2010 and 2015 the number of breeding pairs and locations of Lesser Black-backed Gulls increased at a regional level. Several large colonies can be found in Den Bosch, Utrecht and Amerongen, with dozens of breeding pairs of Lesser Black-backed Gulls. In contrast to the regional level, the number of Lesser Black-backed Gull breeding pairs at a national level has levelled out. The opposite is true for Herring Gulls, both on a regional and national level, Herring Gull breeding pairs are declining. What does this mean for the future? Inland breeding Lesser Black-backed Gulls have increased from 254 pairs in 2015, to at least 400 in 2018. It is expected that the number of breeding pairs will continue to increase in the near future, as well as their distribution. This is probably due to disturbance of coastal colonies and a decrease of marine food supply. Although Herring Gulls are faced with the same problems, the future looks less bright for this species. The number of breeding Herring Gulls is not increasing like that of Lesser Black-backed Gulls, rather it seems likely that they are decreasing. However, the numbers fluctuate too much to be certain. Also hybridisation may play an important role, mainly with species like Yellow-legged Gulls Larus michaellis and Caspian Gulls Larus cachinnans.

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