Breeding Success of the Grey-faced Buzzard Eagle Butastur indicus

Abstract

1) The breeding of the Grey-faced Buzzard Eagle Butastur indices was studied in a mountainious region of central Japan during 1977-1980.2) Mean laying and hatching dates of the first egg were 29 April and 31 May, respectively. The clutch size averaged 2.7 with a range of 2-4. Hatching success was 83%.3) The initial and final brood sizes averaged 2.5 and 2.4, respectively. The average number of fledglings per active nest was 1.9. Mean fledging date of the first young was 5 July. The length of the nestling period averaged 36 days. Fledging success was 78%.4) Of 66 nests examined, at least 64 (97%) contained eggs, 60 (91%) contained hatchlings, and 52 (79%) produced fledglings.5) 21% of nests failed to produce fledglings. Predation and nest collapse caused by breakage of dead branches supporting the nest were the most important factors.6) There were no apparent differences in laying date, clutch and brood sizes, and nest success between years. This suggests a stable food supply during the study period. The rare occurrence of unhatched eggs and of nestling mortality through sibling aggression may indicate a favourable food condition in the study area.