Tawny O wl S trix aluco as an indicator of B arn O wl T yto alba breeding biology and the effect of winter severity on B arn O wl reproduction

In the temperate zone, food availability and winter weather place serious constraints on European Barn Owl T yto alba populations. Using data collected over 22 years in a S wiss population, we analysed the influence of early pre‐breeding food conditions and winter severity on between‐year variations in population size and reproductive performance. To estimate pre‐breeding food conditions, we attempted a novel approach based on an index that combines T awny O wl S trix aluco reproductive parameters and the occurrence of wood mice A podemus sp. in their diet. Tawny O wls breed earlier in the season than B arn O wls and are strongly dependent on the abundance of wood mice for breeding. This index was strongly positively associated with the number of breeding pairs and early breeding in the B arn O wl. Winter severity, measured by snow cover and low temperatures, had a pronounced negative influence on the size of the breeding population and clutch size. Food conditions early in the breeding season and winter severity differentially affect the B arn O wl life cycle. We were able to use aspects of the ecology and demography of the T awny O wl as an indicator of the quality of the environment for a related species of similar ecology, in this case the B arn O wl.