Large-scale climatic phenomena and timing of breeding in a local population of the Osprey Pandion haliaetus in southern Finland

Abstract

The effects of large-scale climatic phenomena on the timing of breeding in the Osprey Pandion haliaetus were studied during 1981–2006 in a small local population of about 20 pairs near the southern coast of Finland. The timing of breeding was estimated on the basis of the wing length of the largest young within the brood. The wing lengths were plotted against the date of the measurement to get a linear regression model characterizing the average developmental stage of broods in relation to date. This model was used when converting the measurements to correspond to the median date of all the measurements. Large-scale climatic phenomena were characterized by the winter and monthly indices of the North Atlantic oscillation (NAO). A significant positive relationship between the winter NAO index and timing of breeding in the Osprey, as indicated by the wing length of the largest young in the brood, was found. The relationship was even stronger when only the monthly value of January was considered. These relationships seemed to be linked with consequent earlier ice melting after mild winters. The results indicate prominent effects of large-scale climatic phenomena and that they can be monitored with relatively modest effort.