Abstract

Relevant variables characterising factors suspected to affect timing of breeding in birds may be difficult to measure. Then, easily available proxies, i.e. variables that are expected to vary consistently with some affecting factors, may be used instead. In the present study on tawny owls, I used besides individual characteristics of parent birds both detailed measurements of the prey base of territories, local and general indices of vole abundance as well as local and general weather conditions as explanatory variables. Owls tended to breed the earlier the older and the heavier they were. Breeding was the earlier the higher the abundance of water voles and the proportion of field voles in the territorial prey samples. Owls bred earlier when the local abundance of small voles in the preceding autumn was high. Regional vole indices showed no associations with timing of owls' breeding. Single winter weather variables did not show any significant associations neither. In combination with the advancing effect of water vole abundance, however, the delaying effect of the depth of the snow cover in March was significant. Models based on different data sets showed different kinds of associations between the abundance of small voles and timing of breeding in owls. The best models in which both intrinsic and extrinsic explanatory variables (characteristics of parent birds and environmental factors, respectively) were included did not differ considerably from each other. The occurrence of water voles in prey samples governed the best models. The results suggest, that without detailed knowledge on the prey base of territory, misleading results may emerge and the importance of small voles in governing breeding of owls may be overemphasised.

Highlights

  • The onset of breeding in birds is governed by various intrinsic and extrinsic factors, including individual characteristics of parent birds and prevailing environmental conditions, in particular food supply, respectively [1,2,3,4]

  • The present data were derived from a ten-year period of a long-term study on population dynamics of the tawny owl conducted in Uusimaa (60°N, 25°E), near the southern coast of Finland [8]

  • Large body size was associated with early breeding in females but not in males

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Summary

Introduction

The onset of breeding in birds is governed by various intrinsic and extrinsic factors, including individual characteristics of parent birds and prevailing environmental conditions, in particular food supply, respectively [1,2,3,4]. Determining timing of breeding on the basis of direct observations during the egg laying is a relatively easy task if there is no risk of desertion of the clutch. Convenient it is on the basis of age of nestlings if it can be determined by some measurement of growth.

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