Over the last decades, the Barn Owl population has markedly decreased in range and breeding numbers in The Netherlands as in most western European countries. For effective conservation and population management, it is essential to know which factors are responsible for this decline. The present study deals with the Barn Owl population in the eastern part of The Netherlands. Population trends and demography (productivity, dispersal, mortality) were studied in two different districts (Liemers and Achterhoek) over two consecutive nine-year periods (1967-75 and 1976-84). Trends in population levels and demographic parameters are analysed in relation to external (environmental) factors, especially food supply, winter weather conditions, nest site availability and changes in rural landscapes and in farming practices. In Liemers the Barn Owl population has decreased markedly since 1960, especially in areas which have been subject to urbanisation and to large-scale land consolidation aimed at agricultural intensification. In contrast, in Achterhoek the Barn Owl population increased in the period 1965-85; landscape diversity is much better preserved in this district. More Barn Owls breed in small-scale mixed farmland than in large-scale uniform farmland. A significant, positive, correlation was found between the Barn Owl breeding density and the length of hedgerows, lines of trees and woodland edges. Both in Liemers and Achterhoek, no clear trends over time were noticed as regards breeding performance, dispersal patterns and mortality in adult Barn Owls. However, first-year mortality in Liemers in the second period (1976-84) proved to be higher than in the first period (1967-84) and in both periods in Achterhoek, In Liemers, productivity was too low to compensate for the high mortality in which road deaths took a heavy toll. This district proved to be a 'sink area', where the Barn Owl population persists only due to continuous net imports of owls. In contrast, Achterhoek is a 'source area' where productivity exceeds mortality. The relative importance of the various demographic parameters for the population balance is presented in a diagram (Fig. 25), which also gives a quantitative assessment of the sink (Liemers) and the source (Achterhoek). The key factors which limit Barn Owl numbers proved to be time- and region-dependent. In the 1980s, a continuing decline took place in the most devastated landscapes of Liemers, accounted for by progressive agricultural intensification and also by urbanisation and the expansion of the main road network. In contrast, the Barn Owl population increased in the better preserved mixed farmland of Achterhoek. The loss in nest site availability in the study region (which was great in the early years of the study period) has been offset by a major nestbox campaign, which proved to be very successful. Today over 90% of the Barn Owl pairs in Liemers and Achterhoek use these nestboxes for breeding. The mean number of young raised in nestboxes was significantly higher than that of 'free' nest sites. In Liemers the improved nest site availability could not stop the population decline. The proximate factors causing this decline are the loss of foraging habitat (disappearance of vole-rich areas, large-scale reduction of hedgerows) and the sharply increased traffic density (causing high road mortality rates). In the small-scale mixed farmland of Achterhoek, however, the Barn Owl population grew in parallel with the increased supply of nestboxes. This supports evidence that nest site availability is the environmental limiting factor in well-preserved landscapes with a rich and buffered food supply. The relationships between the most important external (environmental) factors and the main internal (demographic) parameters, as found in this study for the 1980s, are presented in a diagram (Fig. 35). A number of recommendations can be made for the protection of the Barn Owl and its habitat. These are summarized at the end of this article. Conservation measures should be linked to a land use strategy which favours not only the Barn Owl, but also broader conservation interests including historic-cultural values and the scenery in the wider countryside. Such a strategy will be profitable for other endangered birds and other scarce species associated with farmland. Monitoring of population trends of breeding birds in the study region showed that birds from seminatural habitats (open water, marshland, woodland) are relatively safe if they are non-migratory or winter in Europe. In contrast, many species breeding in farmland are threatened, in which Africa-migrants run double risks. There are better prospects for farmland birds with limited dispersals, such as the Barn Owl, Conservation measures regarding the local habitat (increasing the diversity in farmland environments including the supply of appropriate nest sites) certainly offer an opportunity for maintaining and increasing their breeding populations within a relatively short period of time (10-15 years).
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