…a story of acid and eggshells

Our mothers and dentists tell us to drink our milk because calcium helps build strong bones and teeth. Of course we are not the only beings for whom calcium is critically important. Rodents often chew cast-off deer or moose antlers and the bones of dead animals to obtain this essential element. Corals build their protective shells of calcium carbonate—the basic component of coral reefs. The shells of mollusks and crustaceans have high proportions of calcium; think of snails, clams, oysters, mussels, shrimp and crabs, for example.

Birds need lots of calcium for building eggshells. The calcium requirements of domestic chickens have been well studied, and commercial diets for laying hens are calcium-enriched. But until recently, virtually nothing was known about the calcium needs of wild birds – and even now there is still much to be learned.

However, it is well known that acidic conditions lower the availability of calcium in water and soil, and by the 1990s the effects of acid rain (from industrial effluents) on forest and lakes in eastern North America and western Europe were clearly evident. Then avian ecologists began to wonder if acidification also affected nesting birds, which have high calcium requirements: ninety-eight percent of the dry weight of eggshells consists of calcium carbonate, some of which is gradually absorbed by the growing embryo.

Some birds, notably species with large bodies such as ptarmigan and geese, are capable of storing extra calcium in their bones, building up their calcium supplies well before the nesting season and then depleting that storehouse during egg formation. But small-bodied birds, such as songbirds, generally can’t do this, so they have to obtain the needed calcium at the time of egg laying and chick rearing. Spiders and sawfly larvae reportedly provide more calcium than flies or butterfly and moth caterpillars, which in turn provide more than aphids and beetles.

But sometimes none of these provide enough calcium for egg-laying females. Nesting birds commonly search far and wide for sources of calcium, sometimes at a considerable distance from their nests. Many small birds consume eggshells, crushed bone, or calcareous grit to supplement the diet. Some food items can provide high levels of calcium; for instance, snails, millipedes, and woodlice (isopods), but their calcium content decreases and they also become less abundant on acidified soils.

Several studies have now shown that low calcium supplies can be limiting to avian reproduction. Negative effects can include smaller clutches, delayed egg-laying, thin eggshells, shell defects, smaller eggs, lower hatching success, and slower chick growth. Evidence of calcium limitation has been found in many kinds of small birds, such as woodpeckers, swallows, chickadees, nuthatches, flycatchers, and dippers. Experimental supplementation of calcium, by providing crushed eggshells or bone in the diet, may eliminate such negative effects. Adding lime to acid soils can lead to more abundant snails and, thus, better reproductive success and even higher densities of nesting birds.

Calcium limitation sometimes can be found in areas without acid rain; for example, the well-known effects of DDT leading to pathologically thin eggshells and poor hatching success was due, in part, to the effect of that poison on calcium metabolism. And tree swallows normally consume low-calcium prey and have improved nesting success when the diet is supplemented.

Calcium availability is even important in determining the amount of spotting or speckling on bird eggs. Historically, spots on eggs have been assumed to provide some sort of camouflage, tending to disguise temporarily untended eggs. However, another kind of explanation of egg speckles is also possible: when calcium is scarce, eggshells tend to have weak spots. Many birds reinforce these weak spots with reddish or brownish pigments (byproducts of blood synthesis), creating the speckled appearance. Evidence for this explanation includes the observation that individuals of some species nesting in areas with limestone bedrock (and therefore good calcium supplies) produce eggs with few or no speckles, while individuals of the same species nesting in nearby non-limestone areas lay very speckled eggs. There may be still other explanations for speckled eggs, but a comprehensive explanation of egg pigments across all bird species awaits future synthesis.

Is all of this relevant to Southeast? Probably, but the exact relationship remains to be determined. Here, our soils tend to be very acidic, with low calcium content. But cedar trees, especially yellow-cedar, accumulate calcium in their tissues; old, senescent foliage falls to the ground, enriching the calcium supply in the soils below these trees and reducing the acidity there, thus creating little hot spots of calcium, so to speak.

These observations raise several questions in my mind. For instance, do nesting birds in Southeast suffer signs of calcium deficiency? Or do they have access to good calcium sources—and if so, what are they? Are bird eggs in Southeast more speckled than eggs of the same species elsewhere? Do nesting birds in forest with cedar trees show fewer signs of calcium limitation that those in forests without cedars? If so, does the decline of cedar forests in Southeast have repercussions for avian nesting success and density?