Birds and Their Eggs

Joe Meche is a member of the board of directors of the North Cascades Audubon Society and is in his eighth year as editor of the chapter newsletter. Joe is also a member of the board of directors of the Washington Brant Foundation. He has been photographing birds and landscapes for over 30 years and has been watching birds for more than 50 years.

The age-old question about the chicken and the egg will be discussed ad infinitum. One thing we do know is that all birds begin their lives inside one of nature’s true marvels—the egg. Last month, we discussed bird nests and the wide range and diversity of sizes and shapes of the nests of each species. As remarkable as the nests are, equally noteworthy are the eggs that contain the next generation of birds.

Ornithologists and even casual observers have long been fascinated with eggs. “I think, that, if required, on pain of death, to name instantly the most perfect thing in the universe, I should risk my fate on a bird’s egg,” said T.W. Higginson in “The Life of Birds,” written in 1862.

Thomas Wentworth Higginson was a poet, naturalist, correspondent of Emily Dickinson and a colonel in the Union army during the Civil War. Like many early ornithologists, Higginson was inspired by birds’ eggs and this widespread interest in eggs influenced the development of ornithology as a comparative science. As with nests, eggs come in a variety of sizes and shapes, colors and textures. The beauty of the avian egg is that it’s a self-contained chamber that nourishes and protects the growth and development of the embryo within.

The evolution of the eggs of reptiles and birds followed similar paths to compensate for their specific needs. Bird eggs, however, evolved further than those of most reptiles and some of the changes allowed birds to utilize a wider range of habitats. The principal nutrients in reptile eggs are protein, while in bird eggs the stored nutrients are fats. The fats yield more water and a higher amount of energy than the protein of reptile eggs. The shells of bird eggs are also more complex that those of reptiles.

This enclosed chamber that we know as the egg allows for the exchange of oxygen, carbon dioxide and water vapor through the shell membrane, but still protects the developing life within. The main factor in the successful growth of the embryo and eventual hatching of the egg lies in the parents’ ability to control the egg’s immediate surroundings and temperature. If all this weren’t enough of a miracle, the things that go on inside the egg are even more remarkable. Layers of specific fluids that surround the embryo provide for respiration, elimination of waste, and blood flow as the development continues.

Eggs come in a variety of sizes and colors, but only four basic shapes. Egg shapes range from almost perfectly round to pointed or pear-shaped and present a compromise between strength, clutch size and egg content. One of the most interesting shapes can be found in the eggs of murres and other cliff-nesting birds like guillemots. Given the space constraints on near-vertical cliffs, these eggs are almost pointed on one end and roll only in a tight arc, alleviating the chances of the egg falling off the nesting ledges.

Egg color varies and, depending on the species and the nest site, can be anything from a dull white to a bright blue. When you go to a paint store, it seems that every manufacturer has a color listed as robin’s egg blue. In reality, a variety of birds lay bright blue eggs and the function of this color is not known. There is speculation, however, that when viewed from above, by a predator perhaps, the blue eggs might appear to look more like a break in the vegetation rather than an easy meal.

The intricate camouflage patterns of some eggs that are laid in the open are meant to protect the eggs from predation. Killdeer eggs, for example, are perfectly camouflaged to blend into the surroundings where a simple scrape serves as a nest. Some birds, such as cavity nesters lay round, whitish eggs since there is no need to conceal the eggs.

It stands to reason that egg size would be in direct proportion to the size of the bird. The smallest bird, the bee hummingbird, does in fact lay the smallest egg and the ostrich stakes claim to the largest. However, the weight of the individual egg is a different story altogether, since smaller birds lay eggs that are heavier in proportion to their respective body weight. A wren will lay an egg that is 13 percent of its body weight while an ostrich egg is only 1.7 percent of the bird’s body weight. Birds with young that are precocial (able to move about soon after hatching) lay relatively larger eggs than those whose young are altricial (born in a mostly helpless condition).

The number of eggs laid by a female during a single nesting cycle is referred to as the clutch, and birds are known to lay between one and 23 eggs in a clutch. The clutch size is determined by a number of factors, such as available food supply, nest site and latitude, and whether the young are precocial or altricial. Ground-nesting birds lay larger clutches since their nests tend to be more vulnerable to predatory mammals. Some birds are considered determinate layers and lay a certain number of eggs and no more. Indeterminate layers, on the other hand, will continue to lay eggs if an egg is removed or lost from the original clutch.

One of the most significant threats to many species of smaller birds, such as warblers, is the existence of brood parasites. Brood parasites are the lazy bums of the avian world. They don’t make their own nests but lay their eggs in the nests of other birds. With some species, the incubating host bird will sit on any egg that’s in the nest and the resulting, often much-larger nestling can overwhelm its smaller nest mates.

Some birds, such as the yellow warbler, recognize the alien eggs and are known to build a new nest and lay an additional clutch right on top of the first. American robins and gray catbirds recognize their own eggs and will expel eggs of parasites from their nests. Brood parasites exist locally in the form of the brown-headed cowbird.

In the 1950s and 1960s, many bird populations were ravaged by the pesticide DDT. One of the primary requirements for a successful egg is the proper shell thickness, and the eggs of many birds, especially the larger birds, became so thin that the adults would crush the eggs when attempting to incubate.

One of natural history’s potentially most devastating catastrophes was averted when DDT was banned in this country. Species that were most affected have started to rebound since the ban was enacted. Birds like ospreys, peregrine falcons and bald eagles are most notable among those species. Rachel Carson’s “Silent Spring” brought this problem to light and is considered to be the beginning of the modern environmental movement.

Modern engineers would be hard-pressed to design and build anything as perfect, or certainly as beautiful, as the basic egg. They are lightweight, yet strong, and provide everything the developing bird needs until it hatches. The life of every bird begins in the egg, where it is nourished and protected throughout its development, from fertilization to hatching. Eggs are some of nature’s finest work, and the egg as an art form began when Czar Alexander III of Russia commissioned Peter Carl Faberge to create the first Faberge egg as an Easter gift for Czarina Maria Fedorovna in 1885—art imitating nature. §