Golden Eagles Aquila chrysaetos typically inhabit environments with few trees, such as steppes or mountainous areas above the tree line. The Japanese Golden Eagle Aquila chrysaetos japonica, is an exception to this rule as it occurs in montane deciduous broadleaf forest. Seasonal changes in the spatial structure of deciduous broad-leaved trees may exert a considerable influence on the prey selection of the Japanese Golden Eagle, which may in turn influence its breeding success. In order to evaluate the effects of leafing on changes in feeding patterns and nestling growth of the Japanese Golden Eagle, we assessed the proportion of prey items taken, the delivery frequency and delivery amount to the nest by means of a video recording system. During spring and summer, Japanese Golden Eagles feed principally on Japanese Hares Lepus brachyurus and various species of snakes (Rhabdophis and Elaphe spp.); the total proportion of both prey items attained 98.3 percent (in frequency). Before leafing was terminated hares were delivered more frequently to the nest than snakes, but after leafing, more snakes than hares were delivered. The total amount of food tended to fall rapidly as this switch was made from hares to snakes. In nests where this change in feeding was noted, the total body length of chicks was smaller than in the nests of pairs that did not switch prey during the nestling period. When Japanese Golden Eagles change to specialized feeding on snakes as the leafing period advances, it seems that the reduction in food quantity affects the growth of their chicks.