Methodes voor het tellen van zeevogels op zee: een pleidooi voor vergelijkend onderzoek

Tasker et al. (1984) proposed an international standard method for counting seabirds at sea. The major difference with methods previously used is the way in which flying birds crossing a transect band are counted. Traditional methods use continuous counts of birds passing the transect, which results in overestimates of actual density. Tasker et al. (1984) recommend ’snapshot’ counts, a series of instantaneous counts of subsequent parts of the transect, to avoid such bias. Gould et al. 1978 first published this principle of snapshot counts. Many sources of bias make that seabirds counts will only result in “best possible estimates” of densities; the snapshot method however removes one of the major biases.Seabird densities in the Antarctic were studied as a part of the European Polarstern Study (EPOS, Leg 2, Nov.1988-Jan.1989). EPOS investigated marine production in the marginal ice zone of the Weddell Sea. Density estimates for birds were obtained by use of the snapshot method. Continuous counts however, were conducted simultaneously because all published studies of Antarctic seabirds at sea seem to be based on that method. Results of both methods are compared in table 1 giving densities of major Procellariiform species encountered. Many species would be overestimated by about an average factor 2 when continuous counts are used for density estimates (which is done frequently). Ratios differ between species because of variables such as flying speed and time spent on water. Ships’ speed will also influence the differences between methods. By using both methods during EPOS it is possible to make comparisons to results of previous Antarctic studies. It is surprising that no similar comparative data sets have been published. Given a sufficient number of comparative studies, it may be possible to construct correction factors to convert density estimates from old data sets to estimates that are comparable to densities derived from snapshot methods. Many previous studies were based on transects of defined width, the only major difference with the standard method being that flying birds were counted continuously. It is worth trying to investigate whether such old data sets can be used quantitatively. Admittedly, any correction factor should always be used with caution, but comparative studies would still greatly enhance the information that can be retrieved from old data sets. Furthermore, comparative studies are the only way to give quantitative examples of the errors made when counting flying birds continuously. Those who favour the snapshot method as an international standard might do well by combining snapshot methods with old fashioned continuous counts for some time to come.