FOOD-CHAIN CHEMISTRY, REPRODUCTIVE SUCCESS, AND FORAGING BEHAVIOR OF SONGBIRDS IN ACIDIFIED MAPLE FORESTS OF CENTRAL ONTARIO

We examined the breeding success of two insectivorous bird species, th e resident black-capped chickadee (Panus atricapillus) and the Neotrop ical migrant chestnut-sided warbler (Dendroica pensylvanica), in relat ion to food-chain chemistry at a healthy and a declining forest site i n central Ontario, Canada. The health of sugar maples (Acer saccharum) was poorer and the pH of both soil and throughfall was lower at the d eclining site than at the healthy site. The calcium and magnesium conc entrations and the calcium:aluminum ratio in the soil were also lower at the declining site than at the healthy site. The calcium, magnesium , and phosphorus concentrations and the calcium:aluminum ratio in suga r maple foliage were lower at the declining site than at the healthy s ite, indicating extensive acid leaching of the soil and foliage at the declining site. Caterpillars (Geometridae) at the declining site had a lower calcium concentration and beetles had a lower magnesium concen tration than these groups at the healthy site. The calcium:aluminum ra tios in the tissues of insects in both groups were lower at the declin ing site than at the healthy site, paralleling the foliar and soil che mical differences. There were, however, no differences in clutch or br ood sizes, or percent hatch, for either bird species between the healt hy and declining sites. Chickadees consistently foraged lower in the c anopy at the declining site, a possible compensatory behavioural respo nse to canopy dieback and (or) lower food quality in the upper canopy.