Precarious future in the long-term for the breeding population of Golden Eagles (Aquila chrysaetos) monitored in Southeast Bulgaria

Long-lived and late-maturing top predators, such as the Golden Eagle (Aquila chrysaetos), require long-term studies to collect meaningful data on trends in breeding density and productivity and on the effectiveness of conservation actions under large changes in food supplies and human pressure. The study of a remote population of a maximum of 11 pairs in SE Bulgaria detected changes in breeding density and productivity over three periods between 1988 and 2024. Breeding density gradually decreased from 5.3 to 3.4 pairs/1000 km² corresponding to a 36.4% drop in the number of breeding pairs. The population’s reproductive performances declined most between 1993 and 2005 and improved between 2012 and 2024 but without reaching levels seen in 1988–1992. These changes corresponded to the loss of previous food sources in the 1990s: a steep decline in livestock carrion during the winter months and a reduction in the main prey, tortoises, during the warmer half of the year leading a major dietary shift in prey sources. The analyzed characteristics of nesting mountain slopes and nest trees can inform management and conservation of the Golden Eagles. The declining population has an uncertain future due to the high mortality of adult eagles and their replacement by subadult mates in 57% of the pairs after 2020. The replacement of subadult mates with new subadults every 2.2 years on average was most likely caused by illegal shooting and poisoning.