Ardea
Official journal of the Netherlands Ornithologists' Union

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Honer M.R. (1963) Observations on the Barn Owl (Tyto alba guttata) in the Netherlands in relation to its ecology and population fluctuations. ARDEA 51 (2-4): 158-195
A study has been made on the ecology and distribution of the Barn Owl (Tyto alba) in the Netherlands, in connection with the occurrence of mass mortality outbreaks. The following results have emerged: 1. Habitat selection: Tyto alba shows a preference for areas with alternations of elevation and of vegetational types, and for buildings offering easy access. 2. Body weight and starvation: On the basis of literature observations, correlated with field results, a comparative study has been made of the safety margin between normal and starvation weight, fat reserves, life expectation rates and also clutch sizes of this and other species of owls in relation to the phenomenon of mortality bursts in the Netherlands and other European countries. 3. Burst mortality: Recurrent mortality bursts are suggested to be coupled by way of a feedback breeding mechanism to fluctuations in micro-rodent population density, and that this is valid over much of the continent of Europe. Wanderjahre and Sterbejahre are to be seen as facets of the same phenomenon. 4. Disease and parasites: Mortality bursts cannot be caused by coccidiosis on the basis of the latter's clinical picture and epidemiology. Parasites are important only when the owls have become weakened due to other causes. 5. Distribution: The Barn Owl is found in the Netherlands in areas where the following factors coincide: suitable hunting territories (as under 1 above), sufficient prey, which does not have to be at a 'plague level' and the occurrence of anthropogenic structures. These areas are utilised chiefly on clay alluvial soils and in the neighbourhood of water. 6. Population and declines: On the basis of the present study, an estimated breeding population of 3,000 pairs occurs in the Netherlands. There would appear to be a number of local declines in the population, chiefly due to land improvement schemes. There is, however, no reason to expect the total disappearance of the Barn Owl in the Netherlands.


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