Het gebruik van zeetrektellingen bij de analyse van populatieschommelingen van duikers Gaviidae langs de kust

Waterbirds wintering in low concentrations in large bodies of open water are generally difficult to monitor during standardized waterfowl censuses. Divers, waterbirds wintering mostly in nearshore waters off the Dutch coast, are a prime example of a group of birds that tends to be underestimated during mid-winter waterbird counts. Divers are difficult to count from ships, because they are easily flushed and may take off kilometers ahead of approaching vessels, but also from aircraft, because they cannot be identified with certainty from the air. With the obligation to monitor divers following the Birds Directive (NEM 1), SOVON was interested in other techniques with which (spatial and temporal) fluctuations in wintering numbers 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 this paper, the baseline data is described (spatial, seasonal and diurnal patterns in observer effort), the seasonal pattern of (small) divers is described, and changes in the ability or preparedness to specifically identify passing divers over the years are presented and discussed. Divers arrive as wintering birds in autumn and depart in May, with some southbound passage early in autumn (towards more southerly wintering areas) and northbound movements in April-May (back to the breeding grounds). Most recorded movements from November to March are probably compensation flights of resident (wintering) birds that have drifted away from preferred sites rather than directed long-distance movements. As such, high numbers (frequent movements) are thought to reflect high numbers wintering off the coast and the index used, number of divers per hour of observation, could be a valuable estimate of changes in numbers through the season, from year to year, or between areas. Estimates of total numbers wintering offshore cannot be made with this material. Since 1972, spring passage has advanced with nearly three quarters of a month (median 12 April 20 March) in both areas of intensive observations (Noord-Holland and Zuid-Holland). Rather few divers were specifically identified, but the wintering population is thought to comprise for 90% Red-throated Divers. The wintering index should thus not be used for Black-throated Divers. A spring passage of adult Black-throated Divers (in summer plumage) was highlighted, and annual fluctuations in numbers in spring may be used to indicate differences in the use of Dutch coastal waters between successive seasons for that migratory species. The database is currently 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.