Foraging habitat shift in the narcissus flycatcher, Ficedula narcissina, due to the response of herbivorous insects to the strengthening defenses of canopy trees

The indirect effect of tree defense on bird foraging habitat via lepidopterous larvae was studied by focusing on phenological changes in each component. The seasonal changes in foraging habitat of the narcissus flycatcher (Ficedula narcissina), the distribution pattern of arthropod populations, and the leaf characteristics related to the defense system against insect herbivores were studied in a cool-temperate deciduous forest in northern Japan. Narcissus flycatchers foraged in the canopy from late May to mid June, on the ground from late June, with lepidopterous larvae being the most preferred prey (about 80% of total prey items), and again in the canopy in July. The biomass of lepidopterous larvae was much more abundant within 3 weeks after budbreak in the canopy in late May, and thereafter decreased rapidly in mid June. By contrast, the larvae were abundant on the forest floor from early to late June. These were mainly caused by their downward migration in early to mid June. After budbreak, the leaf toughness and tannin content of two deciduous tree species (maple and oak) increased, while nitrogen and water content decreased. The strengthening of defense traits of canopy leaves caused changes in the abundance and distribution of the lepidopterous larvae. The foraging habitat of the flycatcher subsequently shifted in response to the changes in abundance and distribution of the lepidopterous larvae. In this study, the evidence of the indirect effects of trees on bird foraging habitat was presented by focusing on the phenological changes in relationships among each component in this three-trophic system.