Het gebruik van zeetrektellingen bij de analyse van populatieschommelingen (2). Dwergmeeuwen Larus minutus langs de Nederlandse kust

Waterbirds wintering in low concentrations in large bodies of open water are generally difficult to monitor during standardized waterfowl censuses. Little Gulls, a second example of such waterbirds, are abundant passage migrants and fairly scarce winter visitors in Dutch nearshore waters. With the obligation to monitor Little Gulls following the Birds Directive (NEM 1), SOVON was interested in other techniques with which (spatial and temporal) fluctuations in numbers in Dutch waters could be followed. In this paper, seawatching data were evaluated. The Dutch seawatching programme is unique in Europe, as a result of its highly standardized technique and intensity of observations (100,000 hours of observation since the early 1970s). Not only was the technique standardized, but it remained unchanged over nearly 40 years and still is widely used. In an earlier paper, the baseline data is described (spatial, seasonal and diurnal patterns in observer effort; Camphuysen 2009). In that same paper, the seasonal pattern in abundance of (small) divers was described, and changes in the ability or preparedness to specifically identify passing divers over the years are presented and discussed. This contribution deals with Little Gulls. Little Gulls are migrants in autumn (mostly in Oct-Nov), winter visitors in Dec-Jan and spring migration is mostly confined to Apr-May. Recorded numbers are large and in some years at least 30,000 individuals must move through Dutch coastal waters. Median passage in autumn is slightly later (but not significantly) in recent years, but spring passage is significantly advanced and the passage is less peaked than previously. Much higher numbers are recorded in Noord-Holland than in Zuid-Holland in spring, whereas in other seasons the abundance indices are not that different. There are large variations in numbers between years, these fluctuations are not necessarily similar in neighboring regions, suggesting that differences in visibility, for example as a result of weather conditions, may have been responsible for some of these differences. There are numerous uncertainties about the ecological backgrounds of the highly variable numbers in winter. In spring, with the advance in timing of passage and the less peaked nature of the passage, Dutch waters have gained importance for these attractive gulls as a feeding area. Recent ship-based surveys have indeed revealed the presence of substantial offshore feeding concentrations of Little Gulls, often ‘out of reach’ for coastal seawatchers. The database is incomplete, because many recent counts have not yet been digitized to update CvZ files. Internet portals are currently more practical and are increasingly popular to process data and it is recommended to link these new databases with the old files.