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Published: Sunday, 3/25/2007

Nature's nest eggs

House sparrow 7/8-inch House sparrow 7/8-inch
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Bird eggs are a wonder, even if we don t consider such cosmic philosophical puzzles as what came first, the bird or the egg and let s not.

They range from the tiny pea-sized wisp of an egg laid by a hummingbird to the ponderous ostrich egg, which weighs about three pounds, is the largest existing single cell known, and contains as much material as two dozen chicken eggs.

In between are all manner of sizes and shapes and colors and patterns, all clues about the bird, or at least the type of bird, that laid them.

Duck eggs, for example, are oily and waterproof. Those of cormorants are rough and chalky.

Most bird eggs are shaped like the familiar chicken egg round at one end and pointy at the other. The shape is the result of the egg being forced through the female bird s oviduct, or egg duct. Muscles contract in sequence behind the egg, pushing it forward.

At this point the egg s wall is still subject to shaping, and the more pointed end develops at the back side. Birds that nest on cliffs have evolved highly conical eggs, these being less likely to roll off a cliff s edge and more likely to just roll around in a tight circle.

Robin 1 1/8 inches Robin 1 1/8 inches
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By contrast, birds that build nests in cavities with holes lay nearly spherical eggs. Eggshells, moreover, are dotted with tiny pores that allow the embryo to breathe. A typical chicken egg has about 7,500 pores.

The base color of bird eggs is white, owing to the calcium carbonate of which the shells mostly are comprised. But some species, especially among the passerines the perching birds or songbirds that make up 60 percent of all species produce colored eggs. This group includes such popular groups as the sparrows and warblers.

Pigments such as biliverdin and a related compound, the zinc chelate of biliverdin, produce a blue or green coloration, and protoporphyrin produces reds and browns.

Though non-passerines often have white eggs, ground nesters have evolved a pattern of spots and speckles. One theory is that such patterns are camouflage against predators. Another theory is that distinctive patterns in each egg help some nesting birds to distinguish among eggs in their clutches to assure that each egg is turned and warmed equally.

It is possible, however, that patterns of spots or speckling in an eggshell may not just be about camouflage or identity cues. Recent research suggests that pigmentation on an eggshell, particularly with the protoporphyrin in songbirds, may reduce brittleness and act like a lubricant, actually strengthening what might otherwise be fairly fragile eggs.

Pigments thus may act like a glue to support thin areas of shells, such as the broader more rounded end, or they may strengthen thin-shelled eggs laid by birds where local soils are poor in calcium. Eggs laid later in a clutch usually have more pigment or glue than earlier ones because the female s calcium supply is running low.

Romantics among us even may contend that the spots and spirals grace bird eggs for beauty s sake. In any event, bird eggs are worth a closer look just mind the nature ethic and avoid disturbing active nests.

Observe from afar.

But each spring it is common to find eggs that have fallen from a nest, or even whole nests that have been abandoned. These allow egg-study up close.

Indeed, if every egg and every nest were a perfect success, we would be overwhelmed by a plague of feathered fiends, not friends.



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