Ornis Hungarica. vol.12-13. (2003) p.75-89.
Deriving population estimates for wintering wildfowl in Great Britain
Information on the numbers of individuals in a population represents some of the most basic data that are needed to conserve populations effectively. Over the past decades, many wildfowl populations have undergone rapid changes in numbers as well as changes in distribution in response to the creation of refuges, management of populations, the creation of man-made wetlands and climate change. These continuing changes make it necessary to update population estimates on a regular basis. Data on the numbers of wildfowl wintering on wetland sites in Great Britain come primarily from the Wetland Bird Survey (WeBS), a joint scheme of the British Trust for Ornithology, the Wildfowl & Wetlands Trust, the Royal Society for the Protection of Birds and the Joint Nature Conservation Committee to monitor non-breeding waterbirds in the UK. Coordinated monthly counts by volunteers at wetland sites throughout Great Britain form the basis of the scheme, which focuses mainly on the months September through to March. It is not a simple task to calculate population sizes from extensive, volunteer- based surveys such as WeBS. In particular there are three main problems associated with the derivation of population estimates from WeBS data. Firstly, not all wetlands are covered by the scheme. Secondly, those that are covered do not represent a random selection of wetland sites. Thirdly, on any one count occasion there will be a number of missing counts from individual sites. In this paper we discuss methods for deriving population estimates for wintering wildfowl in Great Britain, by using WeBS data and evaluating past assessments of population sizes. A variety of different methods have been used to generate previous estimates and so it is important to distinguish whether a perceived change in population size is a real biological phenomenon or arises due to differences in the sampling method, the extrapolation method or the formula used to derive the estimate. The results of this analysis would suggest that previous population estimates have tended to underestimate the number of wintering wildfowl, and the resultant implications for conservation are discussed.