Breeding ecology of Northern Goshawk at hilly terrain area in central Japan

Abstract

The breeding ecology of the Northern Goshawk was investigated from 1994 to 2003 in a study area of about 400 km(2) in an area of low hills 50 km north of Tokyo. We studied the breeding density, nesting environment, breeding success, and other breeding variables of the Goshawk. Breeding density averaged 12.8-14.0 pairs per 100 km(2) in 1996-2003. Nearest nest distances averaged 1.74 km. Nests were built in both coniferous and deciduous trees: of 214 nests, 54% were in Japanese cedar, 30% in red pine, and 13% in fir trees. Nests were built in trees that averaged 13.6 m tall, at a height that averaged 69% of the height of the nest tree. Nest trees averaged of 40.6 cm (diameter at breast height). Nests were located 68 m on average from forest edge, 155 to from a house, and 100 m from a road. Reproductive success (successful nests/active nests) averaged 72%, and annual reproductive success was between 53-87%. The average number of fledglings per active nest was 1.49, and the number of fledglings per successful nest was 2.06. Sixtyone percent of nests were reused for breeding in two consecutive years. The longest period over which a nest was reused was nine years. When breeding was successful, the reuse rate was 65%, but only 50% when breeding failed. This difference was statistically significant regardless of nesting success, there was no significant difference in the distance between replaced nests. The cause of breeding failure was mostly unknown. Out of 26 failed nests, we specified the causes of failure as: poaching of nestlings (3), persecution (4), fall of breeding nest (4), predation by crows (5), eggs failed to hatch (4), death of parent (4), and, other reasons (2). A significant difference was found in the distance between successive nests depending on whether failure was natural or by human intervention. When human intervention caused failure, the replacement nest was built further away in the following year.