A method for making whole country waterbird population estimates, applied to annual waterbird census data from Niger

摘要

Regular dry season counts of waterbirds have taken place in Niger since 1992, as a part of the African Waterbird Census. These counts included large sections of the river Niger and its floodplains, as well as up to fifty isolated wetlands. We were involved in these counts in a personal capacity from 1992-1997. In addition to identifying wetlands of international and national importance and Important Bird Areas,the results of the counts can be used to estimate total numbers of birds in Niger for many waterbird species. In this paper we present a method for doing so, which includes (a) separating isolated wetlands from irrigated floodplains and from sections of the river Niger; (b) separating open areas from vegetated areas at each site; (c) estimating the percentage cover that was achieved for each type of habitat at each site; (d) estimating the proportion surveyed of each of these three types of wetland, as part of the total areas present in the country; (e) taking into account relative visibility of each species (e.g. very high for cranes, very low for crakes); (f) using all this information to extrapolate the actual numbers counted to an estimate of total numbers present in Niger at the time of counting, for each species. We also present the resulting information for 97 species of waterbird in Niger, plus three raptors and three kingfisher species largely dependent on wetlands. This information includes where possible a population estimate for the January-February period, comments on population trend, and comments on subspecies concerned. For many species the results are in agreement with the draft Third Waterbird Population Estimates (Delany in lit.), but for others there is a considerable deviation. Species that are possibly considerably more numerous than official estimates indicate include Pholacrocorax africanus, Dendrocygna bicolor, D.viduata, Sarkidiornis melanotos and Himantopus himantopus. Species that are possibly considerably less numerous include several Charadriidae. Further validation of the extrapolation method is desirable and ways of improving it are suggested.