Evaluating citizen-based presence data for bird monitoring

Species monitoring is one key approach to assess human impact on nature. Four main approaches have been applied in bird monitoring: comparing distribution atlases from different times, repeated monitoring programmes, checklist programmes, and species-specific surveys. We evaluate the potential of another type of data for monitoring: presence-only data on species sightings reported voluntarily by the public to open-access internet platforms, here the Species Gateway (SG) in Sweden. We regressed data on the 242 bird species observed 2001-2009 in the Swedish Bird Survey (SBS) against the corresponding SG data. We also investigated the relationship for groups of species utilizing different habitats and displaying a significant inter-annual variation, a coefficient of variation >0.3 according to the SBS. We found support for a positive relationship in inter-annual variation in population level between the two datasets. However, the species subsets displayed variation from positive to negative correlations. Restricting the analysis to species with high inter-annual variation increased the positive correlation between the data sets. The reason for the mismatch between the two datasets may be explained by temporal changes in the willingness to report certain common species to the SG. Another reason may be an imbalance in the coverage of common versus uncommon species among the two datasets. The use of voluntary citizen-based data requires great care and good knowledge of the limitation of the data. If these requirements are fulfilled, we suggest that they may be a complement to standardised programmes, especially for assessing uncommon species of conservation concern.