Eggs Rule

Nature WatchJune 18, 2008
By Susan BensonCNHM Director of Education

Eggs Rule!

What is an egg rule? Is it the number of eggs we should have in the morning for breakfast? Maybe not, but there is an ecological concept called an egg rule that expresses that the average clutch size laid by songbirds and birds in several other orders that tends to increase as one moves north in latitude.

There are few natural objects more interesting than bird eggs. Birds lay hard-shelled eggs made mostly of calcium carbonate. The hard shell keeps an egg from dehydrating and allows parents to sit on the eggs during incubation. Although bird eggs are hard-shelled, they possess microscopic pores, which allow oxygen to pass into and carbon dioxide to exit the shell.

Eggs come in a variety of colors and patterns. Colored and speckled eggs are laid in areas where they need to be camouflaged. Blue or green eggs are laid by birds that nest in shady places such as trees or shrubs (robins). This makes them less visible in the dappled sunlight. White eggs are laid by birds nesting in cavities (owls, wood ducks,) thought to happen because the eggs are usually hidden from natural predators. Birds that lay white eggs in open nests hide the eggs because they begin to incubate with the laying of the first egg. Others such as ducks, geese and grebes cover their eggs with plants when leaving the nest. Patterned eggs blend in with grass or small stones and are laid by birds that nest on the ground (gulls, sandpipers).

The shape of the egg depends on where the bird nests. The most common shape for eggs is oval. Birds that lay their eggs on ledges need eggs with a pointed end so they will not roll off the ledge (vulture). Round eggs are generally laid by birds nesting in a protected area, such as a cavity (owl). Birds that lay many eggs typically have eggs that are pointed. This allows incubation of several eggs in a small area (bobwhite). The number of eggs laid varies by species from as few as one for a seabird to nearly 30 for the bobwhite. The bobwhite needs about 3-10 minutes to lay an egg, but the goose is reported to take 1-2 hours.

This time of year the Museum staff hears stories from visitors about bird eggs and their nests. I was startled myself this week to have scared my resident phoebe out of her nest. She had set up in the same spot under the eaves that she has been using for many years. Enjoy the nesting birds in your own back yard!

Nature Watch is brought to you by the Cable Natural History Museum. For 40 years, the Museum has served as a guide and mentor to generations of visitors and residents interested in learning to better appreciate and care for the extraordinary natural resources of the region. The Museum invites you to visit its facility in Cable at 43570 Kavanaugh Street or on the web at www.cablemuseum.org to learn more about exhibits and programs.