Does the Pochard Aythya ferina vary periodically?—an analysis of census data from Angarnsjöängen, Sweden 1978—2003 and Lake Engure, Latvia 1972—1992

Abstract

Annual counts of the number of breeding Pochard Aythya ferina at Angarnsjöängen, Sweden, 1978—2003, and at Engure, Latvia, 1972—1992, suggest that the populations varied, with a period of about 5 years. This variation is superimposed on the dominating change of the population. The temporal trends were analysed using discrete Fourier transform and by analysis of the autocorrelation function. Although not all of these results were statistically significant, the available information suggests that there could be a periodic variation with a period of about 5 years, since signs of periodicity are seen in the analyses and similar signs are seen in census data from two different populations. A tentative explanation, based on existing theoretical studies, is given for why there would be a periodic variation in the Pochard population. It is suggested that random perturbations which lowers the breeding success causes a periodic population dynamic.