Environmental Factors Affecting the Reproductive Rate of Urban Northern Goshawks

Abstract

Urbanization has increased with human population growth and the responses from raptor species are gaining more attention from both researchers and the public. Northern Goshawks (Accipiter gentilis) now breed in urban areas in Japan and Europe; however, there are few studies examining the factors that influence their reproductive rate in urban areas. We investigated the reproductive rate (number of fledglings per nest) of the Northern Goshawk population in an urbanized area of Japan from 2014 to 2016, and used a binomial mixture model to examine the relationship between the number of fledglings per nest and environmental factors such as nesting and foraging environments, anthropogenic disturbance, predation risk, and intraspecific competition. The goshawk nesting success rate from 2014 to 2016 was 71.6%, with an average reproductive rate of 1.7 fledglings per occupied nest. The percentage of canopy cover of nesting stands had a significant positive effect on fledgling numbers, and the number of adjacent occupied nests had a significant negative effect on fledgling numbers. The positive effects of canopy coverage may be explained by the protection offered by canopy against direct sunlight, wind, and rain. The negative effect of the adjacent occupied nests may result from an increase in the amount of time and energy goshawks spent in territory defense, and a decrease in available foraging habitat due to intraspecific competition.