By Noreen O’Brien, special correspondent to Marine Parts Express
Oölogy, the study of birds’ eggs, was a passion for many people around the world during the 18th and 19th centuries. Some eggs were collected at a furious enough pace that some species were threatened to extinction before the hobby was put to an end. It is easy to see why eggs were collected, given their elegance and variety.
All birds are oviparous—they all bear young in the form of an external egg. Birds must fly to escape predators, so for them, to carry an embryo to development is not such a good idea, given the added weight they would have to carry.
Generally, large birds lay large eggs and small birds lay small eggs. However, the egg size relative to the bird size varies. For instance, the ostrich lays the largest egg, weighing in at nearly three pounds, and is less than two percent of the delivering female’s body weight. The kiwi, a flightless bird found in New Zealand that weighs about four pounds, lays one, and sometimes two eggs, each of which weighs nearly 25 percent of the female’s body weight.
Most birds lay one egg per day, in the morning hours, until the clutch is complete. Larger birds may lay an egg every other day. The time it takes to pass an egg varies from a few moments in smaller birds to an hour or more in larger birds like geese.
Egg shapes, determined by two factors, vary from elliptical to nearly round and everything in between, with oval being the most common shape. The depth of the pelvis of the delivering bird is the first factor. Slender birds, such as swallows, tend to have longish eggs with both ends being similar in shape. Species with a deep pelvis, an owl for instance, generally lay spherical eggs, with the ends being approximately equal.
The environment in which the egg is laid is the second factor in determining its shape. For instance, guillemots, colonial birds nesting on bare ground on rocky cliff ledges along the seacoast, lay eggs that are pointy on one end and larger and rounded on the other. This shape allows the eggs to roll in a tight circle, making it more difficult for them to roll off the ledge.
Most shorebirds have similarly shaped eggs to those of the guillemot, though not as pointy. Shorebirds tend to keep the average of four eggs laid in a uniform circle with the points aimed at the center, thereby making it easier for the birds to incubate the clutch evenly.
The environment also determines egg coloring and patterns. Strongly colored and patterned eggs tend to belong to those birds that nest in an exposed area, often on the open ground—shorebirds, for instance—because they need the camouflage.
As a base color, camouflaged eggs may be white or cream-colored, or dark olive-brown, such as a loon’s, but they have an additional pigment, usually black. These eggs may have streaking, marbling, spotting or blotching—whatever it takes to allow the eggs to blend in to the immediate surroundings.
Most cavity nesting birds, woodpeckers, puffins, and others whose eggs are not exposed to predators, lay eggs that are all white, or of a solid color, most often a shade of blue. The white eggs may have the added benefit of being more easily seen by the parents in the dark of the cavity.
The coloring and markings of birds’ eggs varies widely from species to species, but also in individual birds. Guillemots lay eggs that are highly variable from a dull white to a pale blue-green, boldly marked with dark spots and blotches. The variability here allows each parent bird to pick out its own eggs from those of others.
Because of the wide variability and the similarities of eggs between species and individuals, it is almost impossible to make positive egg identification by color and shape alone. Egg collecting today is against the law in most countries, including the United States. Oölogists now study eggs collected long ago and left in museums.
To learn more about birds’ eggs, look for two books, “A Guide to Nests, Eggs, and Nestlings of North American Birds” by Paul J. Baicich and Colin J.O. Harrison, and “Birds’ Eggs: The Visual Guide to the Eggs of Over 500 Bird Species from Around the World” by Michael Walters.
Till next …
After playing around with, and finally rejecting, bad puns such as “the yolks on us” and how “eggcited we are to help our customers,” all we can say for Marine Parts Express is that we think boating is a more enjoyable hobby than collecting eggs, even if all you have is a little “roe” boat.
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September 21, 2011 / Noreen O'Brien / 0
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