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Bird eggs and shells

By Ranger Steve Mueller

 

Reminder Note: The Rouge River Butterfly Count is July 7 at 9 a.m. If you missed the nature niche article, e-mail me for details. Participants will receive Mo Nielsen’s Michigan Butterflies and Skippers Field Guide. 

Bird eggs taste good to us and other creatures like birds, snakes, and mammals. Successful egg hatching is a difficult challenge. 

This year, an American Robin built a nest in a precarious location at Ody Brook and by the time its second egg was laid, I saw the nest on the ground. The first egg lay unbroken. A second egg was laid on the remaining nest platform base but was abandoned. 

Size of eggs correlates with bird size but variation occurs. Precocial birds whose young develop adequately to leave the nest promptly like ducks, have larger eggs with more nutritional content. That allows the chick to grow more inside the egg before hatching. It readies the young for rapid nest departure.

Altricial birds like chickadees that hatch small, naked and featherless develop in smaller eggs. The parent feeds the helpless babies and needs to continue incubating to keep young from dying of hypothermia. 

Egg shape is important for survival in nature niches. Round eggs can role like a marble. Oblong eggs that are narrow at one end and wide at the other will roll in circles. Cliff dwelling birds use little nest material and the oblong egg shape prevents them from rolling off the cliff. They roll in circles. Screech owls have nearly round eggs placed in a cavity nest where they cannot roll away. 

More tapered eggs that are somewhat pointed are characteristic for birds with cup nests like those of thrushes and sparrows. It allows eggs to fit better for incubation. The adult bird forms a brood patch on her breast that is featherless and filled with blood vessels during breeding season. She cools her breast by sitting on cool eggs. The heat is transferred to the developing embryos in the eggs. When the egg warms against her breast, she rolls the egg to a cooler portion of the shell. The activity evenly heats the eggs. 

Eggs exposed in view would be targets for predators if they were white against the modeled gray brown ground location like where killdeer and many other shorebirds have nests. Eggs shells become colored as the shell is developed in the oviduct. They frequently have spots that develop when the egg is temporarily stationary and are streaked with movement. A ring may be present at the wide end as the egg is pushed along the oviduct. 

Birds tend to lay one egg a day until they complete their clutch. During the 24-hour egg development process, the egg is fertilized by a male and the embryo moves through the oviduct where the shell encases the lifegiving material for chick development. 

The embryo is small with a large yolk attached to nourish development. As the chick grows, the yolk becomes smaller as nutrition is transferred to the young bird. Egg white acts as a shock absorber, provides protein, water, and insulation. Under the shell surface are two membranes that protect from bacterial infection. 

Between the two membranes an air sac is located at the large end of the egg. The air pocket facilitates movement of oxygen and carbon dioxide in and out of the egg. The shell might appear solid and impervious but the embryo would suffocate without constant diffusion of vital gases through the shell.

Eggs are laid wide end first. As the egg shell is developing, muscles in the oviduct contract to push the egg along. The pressure narrows the end where contractions force movement. Calcium is added along the way making the egg ridged by the time it is laid. When preparing your next egg meal, examine the shell, membranes, egg white and yolk. Then enjoy good nutrition and taste. 

Natural history questions or topic suggestions can be directed to Ranger Steve (Mueller) at odybrook@chartermi.net – Ody Brook Nature Sanctuary, 13010 Northland Dr. Cedar Springs, MI 49319 or call 616-696-1753.

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