This series highlights the fascinating and marvelous ecology of grassland bird nesting, written by Madison Audubon education director Carolyn Byers. Click here for all Into the Nest posts. — *This series highlights the fascinating and marvelous ecology of grassland bird nesting, written by Madison Audubon education director Carolyn Byers.* *Click here* *for all Into the Nest posts.*

““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.”
”

— --T.W. Higginson, 1863

Into the Nest: There is nothing so beautiful as a bird's egg

Last week I told you that bird nests were my favorite things. I lied. Eggs are lovely packages of potential, waiting with bated breath for their moment. The variety of egg sizes, shapes, colors, and textures is staggeringly beautiful. A nest is impressive in and of itself, but when it is softly cradling a clutch of eggs-- the combination is intrinsically perfect.

This post is entirely devoted to eggs. We hope you enjoy!

*A grasshopper sparrow sits tight on her eggs during a nest check. Can you spot her eye? Everything about this nest and bird is working to keep those eggs camouflaged and safe. Photo by Carolyn Byers*

Egg Laying:

Most birds lay their eggs in the early morning. This is likely because carrying a large, fragile egg within the oviduct throughout the day could be risky! Gravid female birds (those carrying eggs) are weighted down by their cargo, and may have trouble avoiding predators. Laying an egg is also a vulnerable time for a bird, and early morning laying may minimize their chance of being disturbed by predators (Gill 2007).

Generally, birds lay one egg each day. Because eggs are so large relative to a bird’s body, there is only room for one in the oviduct at a time (Gill 2007). If a nest is destroyed by predators, bad weather or an accident early on in the season, most species of birds will begin a new nesting attempt immediately.

*A female red-winged black bird emerges from her hidden nest. Photo by Arlene Koziol*

Clutch size:

A “clutch” is the number of eggs produced by a bird during one nesting attempt. Generally, birds will lay as many eggs as they can reasonably expect to raise at a time. Some birds will only raise one nest in each season, while others may try to raise two or three.

Birds that have precocial young will have larger clutch sizes than those with altricial young simply because they are easier to care for. Birds that raise multiple broods of chicks in one breeding season may have fewer eggs per clutch later in the season. This could be because the female’s fitness decreases as the season progresses or because food resources can be scarce in August (Gill 2007).

Egg coloration:

The wide variety of egg colors and patterns is one of the reasons so many people are fascinated with them. They are beautiful -- but the science behind egg coloration is very cool too!

An egg receives its color during the very end of its trip through the oviduct. The shell glands deposit colors while the shell itself is being constructed, producing the base or background color. Then blotches, scrollwork, or speckling is added when the outer cuticle layer is produced (the cuticle is a transparent coating of protein molecules that cover the entire shell; Gill 2007).

Only two pigments are used to produce the wild rainbow that birds’ eggs come in. One is blueish green (biliverdin), the other is reddish brown (protoporphyrin). By mixing these two pigments together in different quantities and working with the white backdrop of an egg, you can achieve blues, greens, yellows, creams, browns, and everything in between.

Check out National Audubon’s excellent article about egg color that also explains how eggs are formed: Cracking the Code on Egg Coloration.

The beautiful pigmentation of bird eggs comes from just two pigments. Photo by Greenleaf & Blueberry — *The beautiful pigmentation of bird eggs comes from just two pigments. Photo by* *Greenleaf & Blueberry*

Grassland bird eggs are stunning. Many have a white or cream background with markings in shades of brown. Henslow’s, grasshopper, savannah, and song sparrows all follow this pattern. Upland sandpiper eggs are buffy or olive in color, with many different shades of brown speckling on each egg. Robins and dickcissels have unobscured blue eggs, while red-winged blackbirds and clay-colored sparrows have bluish eggs with markings on them.

Grassland bird eggs come in a variety of shapes, sizes and colors. (If using a smartphone or tablet, turn your screen to landscape mode to view photo captions.) I captured all of these photos during my thesis work in 2010 and 2011. All of the information about egg size and clutch from Harrison 1978. Click on the images to advance photos.

American Goldfinch

Eggs average 16.2 x 12.2 mm. Typically lay 5 eggs, but clutches can range from 4 to 6 eggs. Photo by Carolyn Byers

American Robin

Eggs average 28.1 x 20.00mm. Typically lay 4 eggs, but clutches can range from 3 to 5 eggs. They may try to raise 2 or 3 broods during a summer. Photo by Carolyn Byers

Bobolink

Eggs average 21.8 x 15.71 mm. Typically lay 5 to 6 eggs, but clutches can range from 4 to 7 eggs. Photo by Carolyn Byers

Clay-colored Sparrow

Eggs average 17.1 x 12.7 mm. Typically lay 4 eggs, but clutches can range from 3 to 5 eggs. Photo by Carolyn Byers

Common Yellowthroat

Eggs average 17.5 x 13.3 mm. Typically lay 4eggs, but clutches can range from 4 to 6 eggs. Usually raise 2 broods each summer. Photo by Carolyn Byers

Dickcissel

Eggs average 20.8 x 15.7 mm. Typically lay 4 eggs, but clutches can range from 3 to 5 eggs. Regularly raise 2 broods during a summer. Photo by Carolyn Byers

Eastern Meadowlark

Eggs average 27.75 x 20.35 mm. Typically lay 3 to 5 eggs, but clutches can range from 2 to 6 eggs. Regularly raise 2 broods during a summer. Photo by Carolyn Byers

Grasshopper Sparrow

Eggs average 18.6 x 14.4 mm. Typically lay 4 to 5 eggs, but clutches can sometimes range from 3 to 6 eggs. They may raise 2 broods during a summer. Photo by Carolyn Byers

Henslow's Sparrow

Eggs average 18.3 x 14.1 mm. Typically lay 3 to 5 eggs, and regularly have 2 broods during a summer. Photo by Carolyn Byers

Horned Lark

Eggs average 21.6 x 15.7 mm. Typically lay 4 eggs, but clutches can range from 3 to 5 eggs. Usually raise 2 broods each summer. Photo by Carolyn Byers

Northern Harrier

Eggs average 46.6 x 36.4 mm. Typically lay 5 eggs, but clutches can range from 4 to 6 eggs-- and sometimes may even have up to 9 eggs! Photo by Carolyn Byers

Red-winged Blackbird

Eggs average 24.8 x 17.55 mm. Typically lay 3-4 eggs, and often raise 2 broods in a summer. Photo by Carolyn Byers

Song Sparrow

Eggs average 19.9 x 15.5mm. Typically lay 3 to 5 eggs, but clutches can sometimes have 6 eggs. They may try to raise 2 or 3 broods during a summer. Photo by Carolyn Byers

Upland Sandpiper

Eggs average 45.0 x 32.5 mm. Typically lay 4 eggs, but clutches rarely have 5 eggs. They raise just 1 brood each summer. Photo by Carolyn Byers

Field guides or descriptions of birds’ eggs typically only depict the most common coloration: the average egg. This is only a small part of the story, though. There is so much individual variation in egg patterning within a species that sometimes the “average” egg is vastly different from the outliers.

Red-winged blackbirds are a great example of this. The background color of their eggs can be grayish to blueish green. The markings on their eggs can be brown to black, and can be small speckling, blotches, or scrollwork- either thickly or sparsely patterned. A gray egg with a few brown blotches on it looks wildly different from a blue egg with black scrollwork - but they both hold red-winged blackbird chicks.

Common yellowthroats are another bird with lots of individual variation in their egg patterning. They can have white or creamy white backgrounds- some are even light pink! The markings on the egg may be gray, brown, or reddish brown. The markings may be just a few dots or a ring of heavy blotches surrounding the wider end of the egg.

*Common yellowthroat egg variation on top, with red-winged blackbird eggs on the bottom. Artwork by Carolyn Byers*

Next time we’ll discuss incubation behavior-- and share our first nest videos with you!

For those of you interested in brushing up on basic egg facts or reading more about egg color, here are a collection of articles and videos for you to peruse:

Cornell All About Birds: The Beauty and Biology of Egg Color
Cornell All About Birds: How Many Eggs does a Bird Lay?
Science: Cracking the Mystery of Egg Shape
Mental Floss: How do baby birds breathe inside their egg?
VIDEO-- FLIGHT: The Genius of Birds: Embryonic Development
VIDEO-- Skunk Bear NPR: How Birds Get Oxygen Inside their Eggs

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*Dickcissels are a beautiful, migratory grassland bird that benefits from the Migratory Bird Treaty Act. Photo by Kelly Colgan Azar*

This summer, to celebrate Year of the Birdand 100 years of bird conservation under the Migratory Bird Treaty Act, we’ll be posting regular articles about grassland bird nesting ecology. My M.S. thesis focused on grassland bird nesting ecology, and I’m excited to share my knowledge—and stories from the field- with you! We’ll go into the nest to learn about chick behavior, adult sleep habits, feeding and fledging. We’ll discuss predation and learn about how adult birds respond to different predators. You’ll get to see beautiful photos of nests, eggs, and chicks, as well as video footage straight from the nest! Best of all, the next time you’re out hiking in your favorite Wisconsin prairie, you’ll feel a bit closer to the birds you love.

If you’re interested in reading more and can’t wait for the next post, you can read more about my thesis work here.

Stay tuned for our next edition of Into the Nest, coming soon!

Written by Carolyn Byers, Madison Audubon education director

Gill, Frank B. 2007. Ornithology, third edition. W. H. Freeman and Company, New York, NY.

Harrison, C. J. O. 1978. A Field Guide to the nests, eggs and nestlings of North American birds. Toronto: Collins.