Verstoring van Kleine Rietganzen Anser brachyrhynchus in Zuidwest-Friesland

DISTURBANCE OF PINK-FOOTED GEESE ANSER BRACHYRHYNCHUS IN SOUTHWESTFRIESLAND During winter 1979-1980, whole day observations were made of a group of Pink-footed Geese in Southwest-Friesland, in order to measure the reaction of the geese to different kinds of disturbances. Most of the disturbances lasted very short: 76% of them resulted in less than 2.5 minute of flying (figure 1 + 2). Especially disturbances by traffic, Grey Herons, birds of prey and cattle were very short (figure 2C, E and F). Helicopters on the other hand could cause a great panic among the birds (figure 2D). Disturbances by men produce the most flying time: 82% of the total flying time for disturbances (figure 3). Among the unintentional disturbances, which are mostly caused by farmers who are working on the land, there are both very short and longer disturbances (figure 28). The intentional disturbances by chasing away or hunting are mostly longer than 2.5 minutes (figure 2A). The geese spent most of their time on foraging: 6.0, 6.9 and 6.5 hours per day in the days around new moon in October, November and December respectively (figure 4a and 4b). The daily foraging pattern is shown in figure 5. In order to mesure the effect of disturbances on the feeding activities of the geese, a comparison is made between a quiet day and a day with much disturbance. Three of those comparisons are found in figure 6, 7 and 8. During the comparable parts of the days, the disturbed geese always eat more than the undisturbed geese. In this way they not only compensate for the lost time, but they even eat more than on quiet days (tabel 2). To compensate for the lost energy they ate about 1.5-2 minutes extra for each minute flying. If the geese are disturbed by the end of the day, they try to compensate by staying longer on the feeding-grounds. This happened on the quiet day of November the 19th (figure 6) and with the undisturbed group of February the 3rd (figure 8). The reaction to the disturbance itself however was very different. In the first case they flew to another feeding-ground. But in the second case they flew away for only a few hundred metres, were looking and preening for some time at some distance and returned wlthin half an hour. In this way they hardly lost energy by flying and only had to compensate for the lost feeding time. Mostly the geese could compensate on the same day. Only one time they flew to the sleeping place after a disturbance in the late afternoon and compensated for this by leaving the sleeping place next morning 10-15 minutes earlier than normal. From literature Ref., it is known what the energy costs are for flying, rising and eating and the net energy intake during 1.5 minute foraging. This resulted in the energy-balance of figure 9. In December the Pink-footed Geese stay 9 hours on the feeding grounds and spent 6.5 hours on feeding. If they spent 94% of their time (the highest eating-percentage ever found) on feeding, they can eat 8.5 hours. In theory that makes two hours left for disturbance: 50 minutes for flying and 75 minutes for feeding to compensate for the extra energy spent on flying. We never recorded such a long disturbance, but from other places in the Netherlands it is known that even more than 50 minutes of flying is caused by hunting. On such days we presume the geese will not be able to compensate the lost energy and will loose condition. For farmers, this altogether means, that they can better not disturb the geese if it is not really necessary. The best way is, to prevent that geese land on places where they are really unwished, for disturbing them without really chasing them away finally means an extra loss of crop.