Blog #432 Avian Eggs: Shape and Strength

 For Ursula and Orpheus, their three eggs are all important: the eggs are this year’s entire reproductive effort—their raison d'être. These fragile but durable containers encase an embryo and all the supplies necessary for it to grow into a young bird. 


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. The non-round eggs can be either symmetrical ellipses or asymmetrical with one strongly pointed end. Countless explanations have been offered for these variations in shape, but none are conclusive. 


Egg shape appears to correlated with flight patterns. It is thought that aerodynamic forces work to streamline a bird skeleton and musculature to adapt to the bird’s life history. A 2017 study compared the shape of 1,400 bird species and found body shape a direct adaptation for powered flight. Each egg was examined for symmetry (how similar the two ends are) and ellipticity (how oval an object is). The study concluded that the amount and type of flight influences a bird’s body design and determines what size and shape of egg will fit through its oviduct. 


The streamlined bodies of strong fliers and long-distance migrants, such as Osprey, shorebirds, cranes, and gulls, are not designed to lay round eggs, but rather elongated and asymmetric eggs. Ospreys lay pointed, elongated eggs reflecting their strong flight and migratory habit. Birds like limpkins, some owls, and bluebirds, who fly only short distances, have rounder 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. An elongated egg can hold the same amount of nutrients as a round egg albeit with a requisite larger surface area. One theory proposes that birds 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 necessarily the best predictor of egg shape for all birds. Other factors may come into play to make better forecasts. In an extreme example, common murre eggs are believed to be extremely elongated and pointed to keep them from rolling off the narrow ledges where they nest.


The many shapes of eggs, courtesy of the Indian Press.




The Infinite Colors and Patterns Melanin Makes in Osprey Eggs




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


Bird diets are low in calcium, forcing birds to seek supplemental calcium in animal bones, antler sheds, eggshells, shells, and minerals deposits in dirt. Egg-laying females need to consume calcium, the key hardening ingredient in egg shells. When given the opportunity, birds often store calcium. This is most important the day before the eggs are laid. Even after laying eggs, female birds need calcium to replenish their depleted reserves. Ospreys get ample calcium from the fish bones they eat. 


Melanin is a group of dark brown pigments found in most organisms that produce feather, skin, eggshell, hair, eye, and skin pigmentation. The more melanin produced, the darker and stronger the animal’s eyes, hair, and skin. 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 (up to 4+ pounds) during incubation. Unlike calcium, melanin is synthesized in the birds' bodies in special cells called "melanocytes." 


The countdown to Hatch Day begins on May 11!


Eyes to the sky!


Candace


Candace E. Cornell 

Cayuga Lake Osprey Network

Friends of Salt Point, Lansing, NY



EYES TO THE SKY!


WATCH!

Salt Point Osprey Nest Cam


READ!

 On Osprey Time: Ospreys of the Finger Lakes


VISIT!

Cayuga Lake Osprey Trail Nests Driving Tour

Complete Cayuga Lake Osprey Trail



HELP PROTECT OSPREYS:

•Stay 300 feet away from nests during the breeding season. If the Osprey vocalizes, you are too close! Back off Immediately.

•Carry binoculars to view wildlife from afar.

•Help keep local waters clean, healthy, and safe.

•Recycle used fishing lines, twine, and nets, which can kill Osprey.

•Join the Cayuga Osprey Network and volunteer to help monitor Osprey nests: cec222@gmail.com.

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