Among bluebirds, unhatched or infertile eggs occur in approximately 10 to 15 percent of nests. Typically, eggs don't hatch because one parent is infertile, the eggs are poorly incubated, or the shell is weak and doesn't allow the embryo to develop properly. Most often, inadequate incubation occurs because of inclement weather, disturbances to the nest, or because the female is young and has had little incubation experience. As nest-box monitors, there is nothing we can do about infertile parents. The primary culprits in sulphur and nitrogen emissions are cars and power plants burning fossil fuels such as coal and oil.
Many factors can affect a habitat's ability to combat acid rain.
Most notable is the amount of calcium and limestone in soil, which act as acid neutralizers. Acid rain has more impact on soils that lack calcium and limestone, stripping away essential plant nutrients. The acidic precipitation also causes toxic elements such as aluminum to become more soluble, and thus easier for plants to absorb. Scientists believe that high aluminum levels and low calcium levels interfere with a bird's ability to produce healthy eggshells. Areas such as the Adirondack Mountains of New York, the Appalachians, the Green Mountains of Vermont, and the Canadian Shield are most vulnerable to acid rain because of their high elevations, small watersheds, and naturally acidic soils.