Differential larval phenology affects nestling condition of Green-backed Tit (Parus monticolus) in broadleaf and coniferous habitats, subtropical Taiwan

Abstract

Trophic interactions between birds and their prey often vary among habitat types, but they are poorly studied in low-latitude regions. We assessed the seasonal effect of larval abundance on the breeding performance of the Green-backed Tit (Parus monticolus), a caterpillar specialist, in broadleaf (mixed-oak forest) and coniferous (conifer plantation) habitats in subtropical Taiwan. We measured the biomass of Lepidoptera and Hymenoptera larvae in five tree species: two evergreen oaks and one deciduous alder in broadleaf habitat and two conifers in coniferous habitat. Alder supported a high larval biomass peak in early spring, dominated by noctuid and sawfly larvae, while the two oaks had later, lower peaks. The diversity of trees in the broadleaf habitat supported a broad food peak, spiking in early spring and gradually declining. In contrast, the coniferous habitat had a comparatively lower larval biomass that increased slightly over the season and included a high percentage of hairy lithosiines. Habitat-specific seasonality in larval abundance affected nestling conditions. Early-brood nestlings in the broadleaf habitat were heavier than those in the coniferous habitat. However, the between-habitat difference disappeared in the late broods because the mass of nestlings in the coniferous habitat increased significantly. We found a linear relationship between nestling condition and the total larval biomass available at demand peaks, but there were no differences in annual breeding density, late-brood frequency, laying date, clutch size, or fledging success between the two habitats. Food availability constrained nestling growth but not survival. Habitat-related trophic interactions are present in these subtropical montane forests.