Blog # 268 An Egg Is A Promise

 For Ophelia, her three eggs represent an entire year’s reproductive effort. Much goes into these precious little containers, encasing a developing embryo and all the requirements it needs to grow until hatching. Together, they represent the biological imperative to propagate her genes.  

Variety of eggs showing the differences in symmetry and ellipticity, courtesy of ScienceWorld Scholastic. 

Avian egg shapes vary according to species from nearly round to elongate with a  pointed end and no clear explanation why. The non-round eggs can be either  symmetrical ellipse or asymmetrical with a strongly pointed end. Countless explanations  have been offered for this variation, but none are conclusive.  

A 2017 study by Mary Caswell Stoddard comparing the shape of 50,000 eggs of 1,400  bird species found a surprising link between egg shape and flight behavior, specifically as an adaptation for powered flight. Each egg was examined for symmetry (how similar the two ends are) and ellipticity (how oval an object is).  

Aerodynamic forces work to streamline bird skeleton and musculature to suit the bird’s life history. Stoddard surmised that the amount and type of flight influences a bird’s body design and determines what shape of egg will fit through its oviduct.  

Streamlined bodies of strong fliers and long-distance migrants, such as shorebirds,  cranes, and gulls, are not designed to lay round eggs, laying elongated and asymmetric eggs instead. Ospreys lay pointed, elongated eggs reflecting their strong flight and 

migratory habit. In comparison to chickens and other birds that don’t fly much, the bodies of fast or long distance fliers are narrower and more streamlined, which makes laying a round egg difficult. Birds who fly only short distances during the day,  such as a bluebird, have rounder eggs.  

Short-distance flyers like limpkins and owls have symmetrical, spherical eggs. Owls that are strong fliers, such as great horned owls, have slightly elliptical eggs. 

A comparison of symmetry vs. ellipticity in avian eggs, courtesy of ScienceWorld Scholastic. 

This suggests that egg shape has evolved in response to the requirements of flight, but the reason is still unknown. Since an elongated egg can hold the same amount of nutrients as a round egg albeit with a requisite larger surface area. One theory postulates that birds’ bodies have evolved to be light and streamlined for efficient flight,  making a narrower stream-lined format more desirable. While flight patterns are a good predictor of egg shapes, they are not the best predictor of egg shape for all birds. Other factors may come into play to make better forecasters. In an extreme example, common murre eggs are believed to be extremely elongated and pointed keep them from rolling off the narrow nesting ledges. 

Dark Melanin pigments reinforce Osprey eggs, courtesy news.bbc.co.uk.

Egg-laying females need calcium, the key hardening ingredient in egg shells, to form  strong eggshells. Even after laying eggs, female birds need calcium to replenish their  depleted reserves. 

Pileated woodpecker getting calcium chips from a bone, courtesy of Feederwatch. 

Chicks also need calcium for bone development. However, the food of most birds is low in calcium. forcing them to seek out other sources. To gain supplemental calcium, birds will peck at animal bones and shedded deer antlers, eat eggshells of their young and those of other birds, or dig for minerals deposits in dirt. Ospreys get their calcium from the fish bones they eat. When given the opportunity, birds often overtly consume calcium, especially toward the end of the day as they are forming the egg they'll lay the  next morning. 

Melanins, the dark brown pigments in feathers, skin, and eggshells, also add strength to eggshells. The dark spots, lines, blotches, and swirls of melanin in an Osprey egg strengthen the shell wall, as well as camouflage it, so that it can withstand the weight of  their parent (3-4+ pounds) during incubation. Unlike calcium, melanins are synthesized  in the birds' bodies in special cells called "melanocytes."  

The countdown to Hatch Day begins! 

Eyes to the sky! 

Candace

Candace E. Cornell  

Friends of Salt Point  

Lansing, NY 

ALL EYES ON OSPREYS 

WATCH! 

Salt Point Osprey Nest Cam 

READ! 

On Osprey Time 

Ospreys of Salt Point 

VISIT! 

Cayuga Lake Osprey Trail

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