(Photo by Don Reimer)
(Photo by Don Reimer)
Although eggs are found throughout the animal kingdom, egg laying and terrestrial nest building are essential to avian reproduction. And since birds are aerial creatures, the females can’t simply fly around with weighty numbers of eggs onboard. Can you picture a bomber-like female American robin cruising your neighborhood, loaded with an unhatched clutch of eggs?

Next time you crack open an egg, check some basic design features. Located inside the blunt end of the eggshell, you may notice a thin membrane that serves as a temporary air pocket. This slim air space comes into play at hatch time, providing the struggling hatchling with its first breath of air before launching its active hatching process. The chick then uses its “egg-tooth” (a small hard nub on the tip of the upper mandible) to poke a rough hole through the shell casing and gradually divide the shell into two separate pieces.

The eggs of wild birds show a wide range of individual spherical shapes, but four main shapes are recognized. One type, pear-shaped eggs with pointy ends (pyriform), is typically laid by seabirds that nest on narrow rocky ledges; if somehow dislodged, these eggs are more likely to roll in circles rather than tumble straight off the sea ledge.

While the calcium bicarbonate surface of birds’ eggs is inherently white, eggs come in myriad colors and complex spotted, splotched and scrawled patterns. Birds nesting on exposed open ground, such as Killdeer and Piping Plovers, rely heavily on cryptic egg patterns to minimize detection by predators. Eggs of cavity nesters, like woodpeckers and kingfishers, are a plain white color. Inside a darkened nesting cavity, egg color and pattern are less crucial factors.

Two different egg-laying schemes bring divergent outcomes for the hatchlings. Altricial eggs are laid by most songbirds. These eggs produce pin-feathered chicks that are essentially dependent on parental feeding and care for about three weeks. Many altricial species have two broods during the nesting season. By contrast, precocial eggs yield young that are relatively well developed, feathered and mobile shortly after hatching. Ruffed Grouse and waterfowl chicks are examples of this category.

A majority of species lay a fixed number of eggs in each clutch. Domestic chickens, however, are indeterminate layers. This means they can replace a lost egg by laying another. Female Brown-headed Cowbirds (a parasitic species that deposits eggs in host nests) have extraordinary egg-producing abilities, laying nearly one egg per day for up to 40 to 70 days. Over 140 host species are documented as raising cowbird chicks.

Commercial eggs are promoted as a nutritious component of modern dietary cuisine. This fact also led to widespread harvesting of seabird eggs at summer nesting colonies in the late 19th century. “Eggers” followed a prescribed protocol of breaking all eggs that were initially discovered. This practice ensured the freshness of any subsequent eggs laid and gathered at these sites. Needless to say, the cumulative impact of egg gathering caused the eventual collapse of most coastal breeding populations. In recent decades, the restoration of nesting activity at Eastern Egg Rock in Muscongus Bay is one successful human response to this historical dilemma.

Birds’ eggs are prized by any number of natural predators, including crows and ravens, raccoons, foxes, squirrels and snakes. Red squirrels are especially efficient at foraging bird nests as they roam and explore among bushes and tree branches. It is notable that snakes that specialize in egg predation have greatly reduced venom, implying that the main function of venom is to subdue prey. Of nine species and two sub-species of Maine snakes, none is considered venomous.