Searching for the causes of decline in the Dutch population of Turtle Doves Streptopelia turtur
European Turtle Doves Streptopelia turtur have experienced a sharp decline in population numbers over past decades. Much uncertainty exists about the main cause or causes. Several pressures have been suggested, but because they affect different stages of the life cycle of the Turtle Dove, it is difficult to compare their contributions to population decline. Here we applied a full life cycle approach to study how different pressures may have resulted in the decline.
This was achieved by combining a review of existing literature on possible threats, pressures, and the vital rates they concerned, with the analysis of an age-structured matrix model. The population model was parameterized using estimates from a mark-recapture analysis and supplemented with vital rate estimates from the literature. Comparison with a Life Table Response Experiment (LTRE) was used to determine whether the Turtle Dove literature focusses on those vital rates in which the most important changes have taken place over time. The population model projected a similar decline to that observed in population counts. The LTRE analysis showed that declines in the number of clutches (halved since the 1960s) and in juvenile survival (relative annual rate of change of -1.33% since the 1950s) contributed most to the decline in the projected population growth rate. Although these vital rates are often reported as possible causes of population decline, the reviewed studies often focused on specific reproductive stages, such as egg survival or nestling survival, which did not show a large temporal change. Thus, there is a partial mismatch between our modelling results and the focus in the literature. Juvenile survival is thought to be affected by hunting, degradation of wintering habitat and infection with Trichomonas gallinae, while loss of foraging habitat seems to affect the number of clutches. The focus of conservation measures should therefore be on these threats and pressures. The first steps have already been taken with completion of the international single species action plan for the conservation of the Turtle Dove and the implementation of the first conservation measures on the breeding grounds.
EHJ de Vries, RPB Foppen, H van der Jeugd, E Jongejans