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Why Are Birds’ Eggs Speckled?

The most common hypothesis why birds' eggs are speckled is that the speckles help camouflage the eggs, making them harder for predators to see.

Four speckled eggs in nest

Photo: Thomas Johnson Photography (flickr)

Bird eggs like these from a Wren are sometimes speckled in order make the shell stronger in weak spots

Have you ever wondered why some birds’ eggs are speckled?

Scientists have long wondered the same thing. The most common hypothesis is that the speckles help camouflage eggs, making them harder for predators to see.

However, some scientists have doubted that dark speckling on a light egg is an effective camouflage. Others point out that although speckled eggs are laid by birds that nest in the open, many speckled eggs are laid in nests in cavities or holes where there is little need for camouflage.

Three scientists at Oxford University had a different idea about the function of speckles. They hypothesized that the spots might increase shell strength, and could be used to compensate for thinner egg shells resulting from calcium deficiencies.

The researchers measured eggshell thickness in four populations of a small songbird that had different levels of dietary calcium. Since the strength of egg shells depends on calcium, birds with low calcium in their diets have thinner and more fragile egg shells. However, the reddish-brown speckles on eggs don’t require calcium at all, they are made mostly from protoporphyrins, by-products of blood synthesis.

The study found that eggs with the thinnest shells had the most speckling overall, and that the darkest spots were specifically located on the parts of the shell that were the thinnest. Also the thinner the shell, the darker the spots.

The chemicals in the pigment might work a bit like glue, supporting weak areas of shell and protecting them from breakage. The scientists now hope to find that speckling serves the same purpose in other species of birds, and perhaps finally answer the long-standing question of why birds’ eggs are speckled.

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