Population estimates and the timing of waterfowl censuses

Abstract

The effect of the timing of waterfowl censuses on the estimates ofbreeding population sizes of different species was studied in southeast Finland from 1991 to 1994. Lakes used only for breeding and a local stop-over lake that was used for staging during spring migration were analysed separately . Also, the effect ofthe break-up of ice cover on the order of lake occupation by breeding pairs was studied. A standard waterfowl census was made four times in May at an interval of approximately seven days in each year . In general, the timing of waterfowl censuses within the three-week period considered did not cause serious biases in population estimates, though differences between species and between lake types were observed. The timing of the break-up of the ice cover was critical in the build-up of local breeding populations. However, pairs of many species, especially the Mallard, Teal, and Common Goldeneye occupied breeding lakes even when the lake still was considerably covered by ice. Acomparison of recommended census times between earlier studies and this study suggests that in some species censuses could be started much earlier than previously recommended