By Bryan Stevens
The nesting season is in full swing for most of the bird species that breed in eastern North America. Although they may employ a variety of strategies to ensure nesting success, they all start off their attempts with a clutch of eggs.
It’s the egg that separates birds from most mammals while still linking them to their reptilian kin. Let’s leave the question of which came first, the prototypical chicken or the proverbial egg, to philosophers and instead take a look at the differences birds employ when it comes to the precious life-giving egg in its fragile yet protective shell.
A few birds devote an enormous investment of time to a single egg. For instance, the American flamingo lays only a single egg, which is incubated atop a nest made from a mound of mud for about a month. This flamingo breeds extensively through the islands of the Galápagos, coastal Colombia, Venezuela and nearby islands, Trinidad and Tobago, along the northern coast of the Yucatán Peninsula, Cuba, Hispaniola, the Bahamas, Virgin Islands, and the Turks and Caicos Islands. While other flamingo species are common attractions at zoos, the American flamingo was considered extinct in the United States by 1900. However, recent research indicates that wild birds still make their way into southern Florida. As recently as 2014, a large flock of about 150 wild individuals spent time in the Sunshine State.
While there are more than 300 species of hummingbird distributed throughout the New World, the offspring of these tiny birds all emerge, just like all other birds, from an egg — albeit a very small one. Most hummingbirds are “twins,” hatched with a single sibling that will share their nest and the care of a dutiful mother. The female ruby-throated hummingbird lays two eggs. A female hummingbird gets no assistance from the male and has to find enough food to fuel her own body and help her young in the nest grow and thrive.
Most birds lay multiple eggs, although the number in a clutch may vary dramatically from species to species. A female American robin will usually lay three to five eggs in a nest that builds alone in the fork between tree branches. The size of her clutch of eggs is fairly typical for many songbirds.
Other songbirds are even more prolific. A female house wren, although a rather small bird, may lay as many as eight or nine eggs. Likewise, the golden-crowned kinglet is one of North America’s smallest songbirds, but the female kinglet may lay as many as 11 eggs in a small nest woven of moss, spider’s silk, lichens and strips of bark. By comparison, the female blue-gray gnatcatcher, while similar in size to a kinglet, attempts to lay no more than three to five eggs.
On the other hand, some birds adjust clutch size depending on the resources available to them. In years when their food — a type of caterpillar often injurious to spruce trees — is abundant, the Cape May warbler may lay as many as nine eggs. In years when the aforementioned spruce budworms are scarce, the female warbler may reduce her clutch size to a mere four eggs.
Eggs look fragile, but are actually surprisingly strong. Egg-shaped or “ovoid” objects are considered to be among the strongest shapes in nature. However, strength is no guarantee against a host of hungry predators. Many birds rely on camouflage to protect their eggs. For instance, the female ruffed grouse will usually lay 9 to 14 eggs in a no-fuss nest constructed of leaves in a basin on the forest floor. While incubating her eggs, the grouse hen’s mottled brown plumage makes her almost invisible.
In a similar fashion, the female Eastern whip-poor-will does not build a nest and invariably lays only a pair of eggs, which are placed directly on the forest floor. Her plumage helps her blend with her surroundings, making it extremely difficult to discover a whip-poor-will incubating her eggs.
Many species of ducks are prolific layers of eggs. The wood duck hen may lay as many as 16 eggs in her nest, which may be in a natural tree cavity or a human-made nesting box.
However, not all waterfowl lay a large clutch of eggs. The common loon usually lays only one or two eggs.
Once young loons hatch from their eggs, the parents are devoted caregivers, providing food and protection for the one-month period required for young loons to achieve a degree of independence.
Most of our common birds lay eggs that are significantly smaller than the egg of a chicken. Wild turkey hens lay eggs — as many as 17 eggs in some cases — that are noticeably larger than an average chicken egg.
Who takes the prize for largest egg? That distinction, not surprisingly, goes to the world’s largest bird. Africa’s common ostrich hen, which can weigh as much as 220 pounds, lays the largest known bird egg.
Furthermore, a female common ostrich will usually incubate about 20 of these large eggs, which can reach a diameter of six inches and weigh three pounds.
Whether a bird’s eggs are small or large, these fragile shells — when all goes well — break open to release some amazing miracles. I think anyone who enjoys the hummingbird hovering near a flowering basket on the porch or the flock of American goldfinches at thistle feeders in the backyard will readily agree.