The Recovery of the Barn OwlTyto albain Friesland, Northern Netherlands: Population Growth in Relation to Landscape Features

Abstract

By 1979, the Barn Owl Tyto alba had almost disappeared from the province of Friesland, northern Netherlands. With the placement of nest boxes and by specific forms of landscape restoration, the minimum breeding population of 8 pairs increased to 573 pairs in 2007. In this paper I compare the increase of the breeding Barn Owls across the three landscape types in Friesland, based on soil type sand, peat and clay. Each of these three soil types was divided into 15 categories of openness: open (1–5), partly-open (6–10) and closed landscapes with many trees and bushes (11–15). Each year, a large group of volunteers ensured that all nest boxes (up to 1470 in 2007) were inspected. The increase first became apparent in the sandy soil region with the breeding density exceeding the 1 pair/100 km2 threshold in 1982. This threshold was crossed on peat soils in 1985 and on clay soils in 1990. During the population incline, increasing numbers of nest boxes were made available so that nest sites were never in short supply. The population increase on all soil types stabilized as of 2000. In the peak vole year of 2004 the population density was 25.5 pair/100 km2 on sand, 20.1 pair/100 km2 on peat, and 10.9 pair/100 km2 on clay. To our surprise during the population increase Barn Owls did not show clear preferences for a particular degree of landscape openness on any soil type. To explain the highest densities of pairs that eventually established on sandy soils, the habitat type that is least open, I suggest that food conditions are more constant in the most heterogeneous landscape (sandy soils) which leads to the highest reproductive success. Only in good vole years did reproduction in the clay and peat areas exceeds that of sandy soils.