Notes on reproduction in a declining population of the Ortolan BuntingEmberiza hortulana

Abstract

In this study a Swedish material of 167 Ortolan Bunting clutches ringed during the years 1915–1990 is used to test the hypothesis that the longterm population decline since the early 1950s is caused by an impaired breeding success. It is assumed that an impaired breeding success is accompanied by an increased frequency of replacement broods and that replacement broods are smaller and later than first broods. It was found that the number of youngs in the nest decreased significantly with calendar date and that the distribution of the clutches over the season was significantly different in northern Sweden compared with the rest of the country. However, no significant change over the years was found in the number of youngs per nest or in the seasonal pattern of nestlings. The results indicate that those Ortolan Buntings breeding in Sweden after the 1950s are as successful breeders as those during earlier decades, measured in number of youngs per nest. In a brief discussion of possible reasons for the population decline it is argued that more information on losses incurred along the migration routes and in the winter quarters should be of greatest interest.