POPULATION ECOLOGY AND CONSERVATION OF THE BARN OWL Tyto alba IN FARMLAND HABITATS IN LIEMERS AND ACHTERHOEK (THE NETHERLANDS) ONNO DE BRUUN ABSTRACT 5 1. INTRODUCTION 6 1.1 Background 6 1.2 Aims 7 1.3 Outline of the paper 7 2. STUDY AREA 8 2.1 Liemers 8 2.2 Achterhoek 9 2.3 Review of the main landscape types 10 3. GENERAL METHODS AND MATERIALS 11 3.1 Monitoring Bam Owl populations 11 3.2 Studying demographic Bam Owl population parameters 13 3.3 Surveying population trends in scarce and characteristic breeding birds in the study area 14 4. DISTRIBUTION AND BREEDING DENSITIES 15 4.1 Breeding occurrence in Liemers and Achterhoek 15 4.2 Breeding densities and population trends 19 4.3 Population levels compared with other populations 21 5. NEST SITES AND FORAGING HABITATS 22 5.1 Breeding sites and nest sites 23 5.2 Structure of Bam Owl home ranges in various landscape types 27 5.3 Review of the main habitat requirements 28 6. PREY STOCK AND OWL'S DIET 30 6.1 Occurrence of prey species in various landscape types 30 6.2 Review ofthe prey stock 33 6.3 Bam Owl diet in Liemers 34 6.4 Bam Owl diet in Achterhoek 36 7. REPRODUCTION IN RELATION TO FOOD SUPPLY 36 7.1 Non-breeders and the breeding segment in the Bam Owl population 36 7.2 Fledging success in Liemers and Achterhoek 40 7.3 Second broods 41 8. DISPERSAL PATTERNS 43 8.1 Post-fledging dispersal and other movements 43 8.2 The balance between immigration and emigration: sink and source areas 46 9. MORTALITY AND CAUSES OF DEATH 47 9.1 Seasonal recovery pattern 47 9.2 Annual mortality and life-expectation 48 9.3 Causes of death of Bam Owls in the study area 51 Received 1 October 1993, accepted 25 April 1994. ARDEA 82: 1-109 2 ARDEA 82 (1), 1994 10. POPULATION PERFORMANCE AND UNDERLYING ENVIRONMENTAL FACTORS 57 10.1 Review of the population performance in Liemers and Achterhoek in the period 1967-84 57 10.2 Critical productivity in Liemers and Achterhoek 59 10.3 Integration of demographic parameters on population level: the annual balance in numbers 60 lOA External factors affecting Barn Owl populations 61 Direct causes of death 61 Effects of harsh winters 63 Effects of vole abundance 64 Habitat changes on farmland 66 Availability of nest sites (including nestboxes) 73 10.5 Environmental limiting factors in Barn Owl populations of Liemers and Achterhoek 74 11. THE FUTURE OF THE BARN OWL AND OTHER BIRDS IN MIXED FARMLAND 76 11.1 Changes in bird populations in the period 1965-1980 in relation to habitat choice and migratory habits 79 11.2 Conservation measures for the Barn Owl and other scarce bird species in mixed farmland 81 11.3 Further research required for more effective protection of the Barn Owl 81 12. ACKNOWLEDGEMENTS 82 13. SUMMARY 85 14. REFERENCES 88 15. SAMENVATTING 104 APPENDICES Barn Owl (Photo los Korenromp) de Bruijn: ECOLOGY AND CONSERVATION OF THE BARN OWL 3 4 ARDEA 82 (1), 1994 This publication has been supported financially by: Foundation Edwina van Reek (Enschede, The Netherlands); Foundation Twickel (Delden, The Netherlands); Society for the Preservation of Nature in The Netherlands ('Natuurmonumenten'); Dutch Ministry of Agriculture, Nature Management and Fisheries (Department for Nature, Forests, Landscape and Wildlife); BirdLifeNogelbescherming Nederland; The Worldwide Fund for Nature, The Netherlands. The board of the Netherlands Ornithologists' Union wishes to express its sincere gratitude to these organisations for their support. 5 POPULATION ECOLOGY AND CONSERVATION OF THE BARN OWL TYTO ALBA IN FARMLAND HABITATS IN LIEMERS AND ACHTERHOEK (THE NETHERLANDS) ONNO DE BRUIJN1,2,3 ABSTRACT Over the last decades, the Barn Owl population has marked­ ly decreased in range and breeding numbers in The Netherlands as in most western European countries. For effective conservation and popu­ lation 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 para­ meters are analysed in relation to external (environmental) factors, espe­ cially 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 Bam Owls breed in small-scale mixed farmland than in large-scale uniform farmland. A significant, positive, correlation was found between the Bam Owl breeding density and the length ofhedgerows, lines of trees and woodland edges. Both in Liemers and Achterhoek, no clear trends over time were no­ ticed 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 popu­ lation 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 popu­ lation balance is presented in a diagram (Fig. 25), which also gives a quantitative assessment of the sink (Liemers) and the source (Achter­ hoek). 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 Received 1 October 1993, accepted 25 April 1994. ARDEA 82:1-109 6 ARDEA 82 (1), 1994 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 (caus­ ing high road mortality rates). In the small-scale mixed farmland of Achterhoek, however, the Barn Owl population grew in parallel with the increased supply ofnestboxes. This supports evidence that nest site avail­ ability 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 (demo­ graphic) 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 Bam Owl, but also broader conservation interests including historic-cultural values and the scenery in the wider country­ side. 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 semi­ natural 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 dis­ persals, 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). lZoological Laboratory, University of Groningen, P.O. Box 14,9750 AA Haren, The Netherlands, and 2Society for the Preservation of Nature in The Netherlands ('Natuurmonumenten'), Noordereinde 60, 1243 JJ 's-Graveland, The Netherlands; 3author's address for correspondence: Morshoekweg 39, 7552 PE Hengelo (Ov), The Netherlands. 1. INTRODUCTION ing numbers in most countries of western Europe, e.g. in Ireland and Britain (Sharrock 1.1 Background 1976), Denmark (Dybbro 1976), Germany (Bauer In Europe the Bam Owl Tyto alba is widely & Thielcke 1982, Nicolai 1993), The Netherlands distributed in lowland and hilly country where (Braaksma & de Bruijn 1976, Teixeira 1979, there is a sufficient supply of small mammals, SOVON 1987), Belgium (van der Straeten & particularly voles Microtidae, mice Muridae and Asselberg 1973) and France (Yeatman 1976). shrews Soricidae. Within its European range the Probably this concerns long-term downward Bam Owl usually avoids natural or semi-natural trends, which are unconnected with periodic habitats such as dunes, marshes, heaths and population fluctuations as caused by cycles in woodland. Nowadays it inhabits predominantly the abundance of voles, the main prey of the man-made landscapes, generally areas which Bam Owl (e.g.