Forest soils in many areas of the world are becoming increasingly acidified, in part because of atmospheric deposition of strong acids produced by the burning of fossil fuels. Soil acidification and subsequent calcium depletion of forest soils in eastern North America have the potential to affect forest songbird populations negatively by reducing the availability of calcium-rich food items that are critical for reproduction. Nonetheless, little experimental evidence exists for the purported relationship between soil calcium and avian reproduction. Our objective was to use an experimental and observational study to determine how forest songbird-habitat quality is related to soil calcium availability. We experimentally elevated soil calcium using limestone sand and observed a 1.8-fold increase in Ovenbird (Seiurus aurocapilla) territory density, larger clutch sizes and more nests, but no effects on egg characteristics. The observational study of 14 forested sites in central Pennsylvania likewise showed positive relationships between natural soil calcium levels and Ovenbird territory density, clutch size, and nest density. Again, no relationships existed between soil calcium and eggshell characteristics. Snails are a critical calcium source for many breeding birds, and we suspect that snails are the link between soils and birds because snail abundance increased with liming and was positively correlated with soil calcium. We conclude that Ovenbird habitat quality is related to soil calcium, that birds on our sites were calcium-limited, and that reduced soil calcium could play an important role in bird declines in acidified forests.