BRANTA — Ivaylo Danchev Angelov
Population number, age structure and diet of the Golden Eagle (Aquila chrysaetos Linnaeus, 1758) in Bulgaria.
Institution: National Museum of Natural History, Bulgarian Academy of Sciences
Supervisors: Zlatozar Boev
Details: PhD, 2024
Email
Subject Keywords:status, mapping, decline, adult sex ratio
Species Keywords: Golden Eagle (Aquila chrysaetos)
Abstract
SINCE TIMES IMMEMORIAL, THE GOLDEN EAGLE HAS BEEN ONE OF THE MOST CHARISMATIC SPECIES FOR THE HUMANKIND. ITS BREEDING POPULATION NUMBER IN BULGARIA HAS BEEN ESTIMATED BY DIFFERENT STUDIES AT BETWEEN 100 TO 213 PAIRS, BUT THERE HAS NEVER BEEN A SYSTEMATIC CENSUS.
IN THE LAST DECADE, ANECDOTAL OBSERVATIONS HAVE SHOWN SIGNS OF DECLINE AND AN INCREASED SHARE OF IMMATURE BREEDERS IN TRADITIONAL BREEDING TERRITORIES. BY CONDUCTING FIELD OBSERVATIONS POSSIBLY IN ALL KNOWN HISTORICAL TERRITORIES, WE AIMED TO MAKE A NATIONAL POPULATION CENSUS OF THE PAIRS, AND AT THE SAME TIME IDENTIFY THE AGE OF A MAXIMAL NUMBER OF THE TERRITORIAL BIRDS. AN ADDITIONAL AIM WAS TO CONTRIBUTE WITH NEW KNOWLEDGE ON ASPECTS OF THE DIET, NEST BUILDING, COURTSHIP AND INTERSPECIFIC RELATIONSHIPS WITH OTHER SPECIES.
THE BREEDING SURVEY CONDUCTED OVER FOUR YEARS FOUND THAT THE GOLDEN EAGLE IS IN DECLINE, ESPECIALLY IN THE SOUTHWEST OF THE COUNTRY. CONTRARY TO THE LAST ESTIMATION FOR 213 PAIRS, ONLY 91 TERRITORIES WERE FOUND OCCUPIED BY PAIRS, TWO BY SINGLE TERRITORIAL BIRDS AND 81 WERE VACANT. ADDITIONALLY, WE CHECKED 62 POTENTIAL BREEDING TERRITORIES, WITHOUT PREVIOUS DATA FOR BREEDING AND 11 OF ALL RECORDED PAIRS WERE FOUND IN THESE AREAS.
THE AGE AND SEX OF 175 TERRITORIAL EAGLES WERE IDENTIFIED (87 MALES AND 88 FEMALES), OF THEM 120 ADULTS, AND 55 (31.4%) NON-ADULTS (35 IN 4TH AND 5TH CAL. YEAR AND 20 IN 2ND AND 3RD CAL. YEAR). FOR SEVEN OTHER EAGLES ONLY THE AGE WAS IDENTIFIED, BUT NOT THE SEX. THE TERRITORIAL MALES CONSISTED OF 68 ADULTS AND 19 IMMATURES, WHILE THE TERRITORIAL FEMALES OF 52 ADULTS AND 36 IMMATURES. FULLY ADULT WERE THE BIRDS IN 44 TERRITORIES, WHILE THE MALES OR FEMALES WERE OLDER RESPECTIVELY IN 26 AND 7 TERRITORIES, AND 9 TERRITORIES CONSISTED OF NON-ADULTS OF EQUAL AGES. THE ADULT SEX RATIO WAS BIASED TOWARDS MALES (1.31 MALES PER FEMALE), WITH THE MALES OLDER IN 26 OUT OF 33 TERRITORIES, WHERE THE BIRDS DIFFERED IN THEIR AGE.
WE OBSERVED AND PHOTOGRAPHED 172 GOLDEN EAGLE NESTS. THE MOST RECENTLY USED NESTS IN 69 OF THE TERRITORIES WERE IDENTIFIED. IN 25 OF THE UNOCCUPIED TERRITORIES, 32 OLD GOLDEN EAGLE CLIFF NESTS WERE STILL PRESENT IN VARIOUS STAGES OF DISINTEGRATION.
THE OBTAINED RESULTS FOR THE POPULATION STATUS COULD BE EXPLAINED BY VERY HIGH UNNATURAL MORTALITY OF TERRITORIAL BIRDS. ILLEGAL POISONING TARGETING MAMMALIAN CARNIVORES PROBABLY CONTINUES TO BE THE MAIN REASON FOR THE MORTALITY OF PAIRED EAGLES AND TERRITORY ABANDONMENT.
IN ADDITION TO THE MAIN SURVEY, WE PRESENTED NEW DATA (IN TWO PUBLICATIONS AND ONE POSTER) ON SOME LITTLE-KNOWN ASPECTS OF THE BREEDING BIOLOGY AND ECOLOGY OF THE GOLDEN EAGLE: 1) FIRST DESCRIPTION OF BUILDING A NEW NEST BY A PAIR; 2) FIRST DESCRIPTION OF RECIPROCAL PENDULINE FLIGHT AS PART OF THE BREEDING DISPLAY; 3) CONFIRMATION THAT THE MOCK ATTACKS WITHIN THE PAIR ARE PART OF THE COURTSHIP; 4) FIRST DESCRIPTION OF THE INTERSPECIFIC RELATIONSHIPS WITH THE SAKER FALCON DURING CLOSE BREEDING; 5) QUANTITATIVE DATA ON THE BRINGING OF FRESH GREEN BRANCHES IN THE NEST AND NEST CLEANING BY THE FEMALE, GATHERED BY PLACEMENT OF A TRAP CAMERA IN AN ACTIVE NEST.
Published Papers
Angelov I.D. 2024. Exceptionally Low Proportion of Adult Pairs and Male-Skewed Adult Sex Ratio in a Declining Population of the Golden Eagle Aquila chrysaetos (L., 1758) (Accipitriformes: Accipitridae). Acta Zoologica Bulgarica, 76: 241-248.
Angelov I.D. 2023. A first detailed description of building a new nest and new data on the courtship behaviour of golden eagle. Raptor Journal, 17: 29-34. doi: 10.2478/srj-2023-0004
Angelov I.D., MaMing, R. & Dixon, A. 2023. Breeding Dispersion of Raptors in the Junggar Basin, Xinjiang, China, Including a Case of Close Nesting of Saker Falcons and Golden Eagles. Journal of Raptor Research, 57: 676–679. doi: 10.3356/JRR-22-00086