Meer of minder stadsvogels: wat weten wij ervan?

What do we know about population trends in birds of (sub-)urban areas? To investigate this, Sovon analysed results of its monitoring projects: PTT (Point Transect Counts for Wintering Birds), BMP (Breeding Bird Monitoring Scheme for Common Birds) and BSP (the same, but for less common birds). Five species were selected: Collared Dove, Swift, Crested Lark, Black Redstart and House Sparrow. At the same time an inquiry into knowledge on distribution, population numbers and trends, was conducted amongst 241 local organizations of birdwatchers, of which 25% responded. Some general results were; (a) bird censuses in urban areas are not popular, with some notable exceptions, (b) population trends in some species can be monitored by Sovon research, but results refer mostly to rural populations and not to urban situations, (c) some species cannot be monitored yet by Sovon, and (d) extra-information in local organizations of birders may be illuminating at times but is often based on very local impressions only. Collared Dove populations have increased until the mid 1980’s. Sovon results indicate a decrease since about 1985 (figure 1), whereas local birders report various trends (figure 2). Information on the large populations in towns is scarcer than that on smaller rural populations. Swifts can not be monitored by Sovon at the moment. Local birders often report opposite trends, perhaps as a result of local differences in building and renewal activities. As half of the local ornithological groups report a decrease, the present national situation looks unfavourable (figure 3). Crested Larks have decreased rapidly In the past decades. Nearly al local birder’s groups reported an alarming decrease or even extinction in their region (figure 4). The species has become that rare that Sovon-monitoring by sample surveys has become difficult because of very small numbers in study plots. Black Redstarts have colonized large parts of the country in the course of the 20th century. As breeding densities are usually rather low and as the preferred habitat is disliked by ornithologists, numbers in the Sovon monitoring projects are 100 small to calculate reliable trends. About half of the local birder’s groups suppose that the species is still increasing (figure 5), but locally opposite trends exist (figure 6, lower part). There are some indications that rural populations are influenced more heavily by harsh winter conditions in South West Europe than are urban populations (figure 6, upper part, South West Drenthe versus Hoogeveen). The population trend in the House Sparrow, the commonest breeding bird, is not exactly known, but Sovon-results (figure 7) as well as local information (figure 8) suggest a recent decrease. However, Sovon-results are mainly based on rural populations and local information is scarce and often not well documented. The main conclusion is, that more attention should be paid to birds of urban areas. In order to monitor these species, it is essential that more long-term breeding bird surveys should be conducted in towns. General surveys of all species are needed as well as species-directed studies (table 1). In a densely populated country with large numbers of birdwatchers it should not be too difficult to document population changes in species that live next-door. It will be one of Sovon’s goals in the near future to achieve this.