Calcium is Not a Limiting Micronutrient for Egg Production by Green-Backed Tits in a Mosaic Landscape of Mixed-Oak Forest and Cedar Plantation in Subtropical Taiwan

In many passerines, reproductively active females must forage for calcium-rich materials on a daily basis to meet exact calcium demands during egg production. Calcium availability often constrains reproductive output in birds, but is dependent on species-specific foraging traits and local calcium availability, and, thus, land use. Here, we examined whether calcium availability limits the reproductive output of Green-backed Tits Parus monticolus over a four-year period in mid-altitude subtropical forest of Taiwan. We compared soil nutrients and the availability of calcium-rich snails between mixed-oak forest and Japanese cedar Cryptomeria japonica plantation. We also provided supplementary calcium (in the form of snail shells and chicken egg-shells) in the nest boxes of half of all pre-laying pairs breeding in both habitats. The reproductive output of birds across the two treatments and two habitats was recorded. Our results showed that the soil was very acidic in both habitats. Compared to the mixed-oak forest, the litter in the cedar plantation contained marginally significantly more calcium, while the soil had significantly higher pH and exchangeable Ca²⁺. The cedar plantation supported similar, or even higher, snail abundance than the mixed-oak forest. We found no abnormal eggs with defective shells in either habitat. We also found no significant difference in clutch size, egg mass, egg shape index, egg volume, clutch volume, or hatching success across the two calcium-treatment groups or the two habitats. Calcium availability had a weak effect on the reproductive output of birds in the two habitats, possibly because the study area supported a high abundance of snails. Our results, combined with our calculations of the calcium demand of birds, suggest that calcium is not a limiting micronutrient for egg production by Green-backed Tits in the montane forest of subtropical Taiwan at present.