Citizen science in action—Evidence for long-term, region-wide House Sparrow declines in Flanders, Belgium

•We studied the status of the House Sparrow in Flanders.•The data was collected by volunteers during ten years.•Sparrows were less recorded in more densely populated, urban areas.•House Sparrow abundances declined over time.•Results suggest that House Sparrows decreased due to advancing urbanization.

Urban expansion is detrimental for many species. While the House Sparrow (Passer domesticus) initially flourished in the vicinity of men, a decline in House Sparrow numbers has been observed in several European cities during the last decades. A lack of systematic data on the status of this species in the highly urbanized Flanders (Belgium) has been the reason why since 2002, the Flemish population has been called annually to count House Sparrows during the breeding season. Here, we describe the results of the first ten years of sparrow counting. While inhabitants from 99% of the municipalities participated at least once, large differences in numbers of participants were observed among municipalities: the larger the population size, the more people counted sparrows. Results indicated that House Sparrow abundances have been decreasing in Flanders over the past decade. Contrary to several other European regions, the decline appears equally strong in rural and urban areas. However, average numbers of House Sparrows were lower in more densely populated, urban areas, and where less cropland, grassland and parks surrounded the sampling location. House Sparrow abundances also decreased significantly over time at locations where predator pressure increased. These results suggest that the House Sparrow decline in Flanders is due to the ever encroaching urbanization and the reduction of the amount of green space. Furthermore, it shows that data collection by volunteers can be a useful approach to obtain large-scale and long-term data in a relatively easy way, in addition to raising public awareness to the natural environment.