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

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.