Eggshell characteristics and calcium demands of a migratory songbird breeding in two New England forests

Calcium has been reported to be a limiting nutrient for eggshell production in birds living in areas of northern Europe suffering from heavy acid depo sition. To investigate whether calcium might be limiting for birds in north eastern North America, a region also experiencing high and persistent acid precipitation, we analyzed eggshell characteristics and assessed calcium ne eds for eggshell production of a Neotropical migrant songbird, the Black-th roated Blue Warbler (Dendroica caerulescens), in two forests in New England that differed in calcium content of their soils. We found no significant d ifferences between the two regions in warbler eggshell mass, thickness, or in the concentration or the amount of calcium in eggshells. Moreover, calcu lations show that a diet of larval Lepidoptera, a major food source, is not a sufficient source of calcium for this species during egg laying, but tha t ingestion of eight average-sized (60 mg dry mass) snails during the egg-l aying period would supply sufficient calcium for eggshell formation for a 4 -egg clutch. Although current densities of snails suggest that they are not a limiting resource for birds at these sites, recent findings of declining calcium availability in New England forest soils suggest that calcium coul d in the future become a limiting factor for birds in northern temperate fo rests.