POPULATION ECOLOGY AND CONSERVATION OF THE BARN OWL TYTO-ALBA IN FARMLAND HABITATS IN LIEMERS AND ACHTERHOEK (THE NETHERLANDS)

Over the last decades, the Barn Owl population has markedly decreased in range and breeding numbers in The Netherlands as in most western Eu ropean countries. For effective conservation and population management , it is essential to know which factors are responsible for this decli ne. The present study deals with the Barn Owl population in the easter n part of The Netherlands. Population trends and demography (productiv ity, dispersal, mortality) were studied in two different districts (Li emers and Achterhoek) over two consecutive nine-year periods (1967-75 and 1976-84). Trends in population levels and demographic parameters a re analysed in relation to external (environmental) factors, especiall y food supply, winter weather conditions, nest site availability and c hanges in rural landscapes and in farming practices. In Liemers the Ba rn Owl population has decreased markedly since 1960, especially in are as which have been subject to urbanisation and to large-scale land con solidation aimed at agricultural intensification. In contrast, in Acht erhoek the Barn Owl population increased in the period 1965-85; landsc ape diversity is much better preserved in this district. More Barn Owl s breed in small-scale mixed farmland than in large-scale uniform farm land. A significant, positive, correlation was found between the Barn Owl breeding density and the length of hedgerows, lines of trees and w oodland edges. Both in Liemers and Achterhoek, no clear trends over ti me were noticed as regards breeding performance, dispersal patterns an d mortality in adult Barn Owls. However, first-year mortality in Lieme rs in the second period (1976-84) proved to be higher than in the firs t period (1967-84) and in both periods in Achterhoek, In Liemers, prod uctivity was too low to compensate for the high mortality in which roa d deaths took a heavy toll. This district proved to be a 'sink area', where the Barn Owl population persists only due to continuous net impo rts of owls. In contrast, Achterhoek is a 'source area' where producti vity exceeds mortality. The relative importance of the various demogra phic parameters for the population balance is presented in a diagram ( Fig. 25), which also gives a quantitative assessment of the sink (Liem ers) and the source (Achterhoek).The key factors which limit Barn Owl numbers proved to be time- and region-dependent. In the 1980s, a conti nuing 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 farml and of Achterhoek. The loss in nest site availability in the study reg ion (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 t hese nestboxes for breeding. The mean number of young raised in nestbo xes was significantly higher than that of 'free' nest sites. In Liemer s the improved nest site availability could not stop the population de cline. The proximate factors causing this decline are the loss of fora ging 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 availabili ty is the environmental limiting factor in well-preserved landscapes w ith a rich and buffered food supply. The relationships between the mos t important external (environmental) factors and the main internal (de mographic) parameters, as found in this study for the 1980s, are prese nted in a diagram (Fig. 35). A number of recommendations can be made f or the protection of the Barn Owl and its habitat. These are summarize d at the end of this article. Conservation measures should be linked t o a land use strategy which favours not only the Barn Owl, but also br oader conservation interests including historic-cultural values and th e scenery in the wider countryside. Such a strategy will be profitable for other endangered birds and other scarce species associated with f armland. Monitoring of population trends of breeding birds in the stud y region showed that birds from seminatural habitats (open water, mars hland, woodland) are relatively safe if they are non-migratory or wint er in Europe. In contrast, many species breeding in farmland are threa tened, in which Africa-migrants run double risks. There are better pro spects for farmland birds with limited dispersals, such as the Barn Ow l, Conservation measures regarding the local habitat (increasing the d iversity in farmland environments including the supply of appropriate nest sites) certainly offer an opportunity for maintaining and increas ing their breeding populations within a relatively short period of tim e (10-15 years).