Long-term decline of juvenile survival in German Red Kites

Abstract

The Red Kite (Milvus milvus) is a raptor species of conservation concern in Europe and especially in Germany, where about 50% of all breeding pairs are found. Agricultural intensification and deteriorating food availability, but also anthropogenic mortality due to poisoning and collisions, are major threats for the species throughout its strongly restricted distributional range. Despite the great influence of mortality on the population size of the long-lived Red Kite, information on current age-specific survival probabilities and their change over time is lacking. We analyse a long-term dataset of > 29,000 Red Kites marked with metal rings and about 1500 recoveries of dead birds from 1970 to 2015 with a multinomial ring-recovery model. We model age-dependent recovery probability, based on separate datasets of birds marked as nestlings and as adults/immatures, and thereby estimate juvenile, subadult and adult survival probability over nearly 50 years for a major part of the German Red Kite population. The results show a substantial long-term decline in juvenile survival of more than 40% from the 1970s until today. Furthermore, from 1974 to 2014, adult survival probability shows a consistently decreasing trend (− 0.26% year−1). We estimate the recovery probability for dead Red Kites in the first year (as juveniles) to be two times lower than that for birds that reach subadult/adult age. Also, the recovery probabilities of all age classes show a decreasing trend over time except for the last years. The spatial and temporal distributions of juvenile Red Kite recoveries suggest an increase in mortality at the breeding grounds, but in > 60% of cases the cause of death is unknown. The age-structured differences in recovery probability potentially highlight differences in the most important causes of mortality; however, further investigation of the cause of death in all ages classes is required to gain a more accurate picture. Our study provides valuable data for a better understanding of Red Kite demography and ongoing changes in population size.