Een onderzoek naar verschillen in fourageeractiviteit tussen Visdief Sterna hirundo en Noordse Stern Sterna paradisaea op de Engelsmanplaat (NL)

The foraging activity of Common Tern Sterna hirundo and Arctic Tern Sterna paradisaea was studied on the low sandy island of Engelsmanplaat in the Dutch Waddensea area (53°28’N – 06°30’E). The foraging activity was measured by counting birds carrying food to the colony and birds flying towards the colony but not carrying food. Separate counts lasted 5 minutes and were made from various points around a mixed colony in low dunes (these points are called sectors, see fig. 4). The influence of several variables on the foraging activity of both species was tested: progress of the breeding season (period), sector, time of day, tidal rhythm, water level, temperature, cloudiness, precipitation, wind direction, and wind speed. Of these only progress of season, sector, time of day, tidal rhythm, and water level had a demonstrable influence on foraging activity. Common and Arctic Tern were very similar in their reactions on the studied variables: During the incubation period the number of birds carrying food is lower than during the subsequent feeding period, although the maximum activity is about the same. This is caused by a higher activity during less suitable stages of the tidal cycle by terns looking after chicks. The number of terns flying without food is equal in both periods. The Arctic Tern is less strictly limited to certain sectors than the Common Tern. The Common Tern forages almost exclusively in the sectors “West” and “Rif” (see fig. 4, and table 3). Activity is maximal in the early morning and just prior to sunset. These peaks of activity are mainly due to an increase in birds carrying food. In the early afternoon there is also a slight increase in activity (see fig. 7). Common and Arctic Tern are most actively fishing from 130 minutes after high water to low water (Common Tern) or to 1 hour before low water (Arctic Tern). The activity is minimal just before high water. Bird without prey show much less fluctuations with the tidal cycle (see fig. 8). In the Discussions our results are compared with literature references from other study areas.