To filter or not to filter: assessing the exclusion of hunting and persecution data in ringing recovery studies

Abstract

In recent years, ringing recovery records in Europe have been identified as a potential important source of data for assessing the impact of climate change and other long-term changes on wintering population distributions of migrating birds. As with any voluntarily- gathered data, however, there are clear sources of bias that might impact results if researchers are not selective.Conversely,methods that exclude data should not be universally applied without proper assessment of their impact on final results.We examine the specific and seemingly conventional method of excluding data collected from hunted or intentionally killed individuals in studies of winter distributions of migrating European birds, to evaluate whether the exclusion or inclusion of this data does indeed affect the outcome of the analysis. We find that the exclusion of these data is warranted for many species, however may impact the power of statistical analyses unjustifiably for others. Our findings showed that wintering range sizes of species are similar when using data of killed or non-killed birds, although theremay be latitudinal differences. Furthermore, we did not find evidence that wintering ranges of investigated 37 species would havemoved northwards in general as would have been expected due to climate change. We suggest that rather than a universally-applied method of exclusion, these contributed data should be analyzed in context of the hunting history of the species in question and the time frame being considered, and give guidelines on how to most efficiently utilize this important data resource.