FIJN R, POOT M, ARTS F, VAN BEMMELEN R, HORNMAN M, DE JONG J, SCHEKKERMAN H, SOLDAAT L & TROOST G (2022) Dutch waters form an internationally important stopover for the Little Gull Hydrocoloeus minutus. LIMOSA 95 (4): 160-177.

Little Gull, the smallest gull species on Earth, is a rare breeding bird in the Netherlands, but the numbers of wintering and in particular migrating birds are much higher. In this paper we give an overview of the importance of Dutch waters for Little Gulls and present some of the knowledge gaps that exist for this elegant species. Over the past years a maximum population estimate of 100 000 Little Gulls over the Dutch North Sea were recorded during the annual aerial survey monitoring (MWTL). These birds are not only found in the coastal zone, but also far offshore. Such numbers suggest that up to 100% of the European population of the species is present in Dutch waters during a short time frame in spring. Remarkable, and different from historic records, is that these birds not only fly past but also show foraging behaviour suggesting true stopover behaviour. Apart from the Dutch North Sea, substantial numbers of the Little Gull are also seen in large freshwater lakes with a maximum of 40 000 in Lake IJsselmeer. No significant population trend is visible, but numbers increased slowly between 1975 and 2010, followed by a substantial decrease. Maximum counts were always recorded during the peak of spring migration in the first half of April, two weeks earlier than the historic migration peak. Adults migrate earlier than immatures in spring. Most ringed birds recorded in the Netherlands come from breeding colonies in Finland, with smaller numbers from Latvia and Poland. The remarkable difference in peak numbers in migration of the Little Gull between spring and autumn is likely due to differences in foraging opportunities with fish larvae and insects available only in spring. The importance of Dutch waters (both marine and inland) for European Little Gulls has important implications for the international responsibility for the Netherlands by maintaining a safe environment for the species. Spatial and temporal management as well as an appropriate assessment of anthropogenic activities in the Netherlands are crucial for successful conservation of suitable habitat for migrating and wintering Little Gulls. The current monitoring schemes are suitable to estimate the distribution and abundance of the species, although structural monitoring of Lake IJsselmeer and other large lakes is lacking. The analysis of migration counts provides us with reliable trend figures. The biggest knowledge gaps currently lie in the absence of species-specific estimates of reproduction and survival, and quantitative data on diet and migration. These data are all essential to be able to adequately explain population changes or assess the impact of anthropogenic pressures, and to answer the central question what the Netherlands should do to keep our waters suitable as stopover site for the vast majority of European Little Gulls.

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