Birds’ eggs


When thinking about the miracle of birds, the first thing that comes to mind is flight. We earthbound humans are often a little jealous of avian flight – they do it so easily, and we have to pass through long security lines and board airplanes!

But a close second bird miracle, I would argue, is their eggs. They are marvels, with each species producing eggs of different sizes, colors and shapes. One of the most common nature illustrations available in books and online are bird egg identification charts. Just google “bird eggs” and be amazed at all the choices available.

Researchers, both past and contemporary, collect bird eggs and nests to be added to museum specimen collections. Once properly curated, identified and stored, bird eggs last indefinitely. There are millions, if not billions, of bird eggs in museums around the world, and at the time of collection they played a major role in documenting new bird species.

Locally, The Evergreen State College and the University of Puget Sound’s Slater Museum contain thousands of eggs, all neatly labelled with species name and date/location of collection. I know this because over the years I have donated abandoned Western Bluebird nests to these universities.

Typical bluebird nest
Typical bluebird nest

Because each June my bird-watching thoughts turn to nesting, I think about the amazing story of birds’ eggs. Last June I wrote a column titled “Nesting and Eggs.” This column supplements the 2023 information.

Birds are the only animal that produces hard-shelled eggs. Reptile eggs are leathery and flexible, and since dinosaurs are the ancestors of both birds and reptiles, this has led to some debate about the nature of dinosaur eggs. Because chicken eggs are such a common element of our everyday life and diet, you may well think you know quite a bit about birds’ eggs -- and you do.

Birds undergo seasonal changes that might be difficult for humans to identify with. They are relatively small creatures, but yet need to have all the biological necessities of life. Their “secret” is that parts of their biology go into a quiescent phase when not needed. For example, post-breeding many birds’ reproductive organs shrink, greatly reducing their biological needs during migration and wintering.

However, when breeding season comes, this situation reverses; birds’ gonads swell and begin to produce sperm and eggs. A female’s ovary (for most species there is only one) and the male’s testes are each connected to a tube called the cloaca. The cloaca has multiple purposes, functioning as the body’s exit for digestion, urinary and reproductive actions.

These hormone changes also produce behavioral changes, including singing, territorial and breeding displays, and nest building. Many bird species molt into breeding season color patterns and some develop elaborate plumes, such as our resident Great Blue Herons. All these changes likely reinforce the gonadal changes also underway.

Mating occurs when the pair touch cloaca and the male deposits sperm into the female. The process after this act varies with species, sometimes occurring only once. Females can store viable sperm adjacent to her cloaca until needed.

You already know the parts of an egg: There’s the yolk, the white, the shell and a thin membrane inside the shell. In avian reproduction, there are these and a few additional parts of an egg, and each has a vital role.

A bird’s egg starts as a single cell and, under hormonal influence, increases in size somewhat and gains a yolk sac. After fertilization (or even unfertilized as in many domestic chickens) it starts its trip down the oviduct (the upper part of the cloaca). First, egg membranes and albumin (proteins – the white of the egg) are secreted from special glands.

The developing egg is hydrated and molded into shape. Calcium is deposited on the outer membrane, forming the shell. As the egg proceeds down the oviduct, there are color-producing glands that, if activated, will color the outer surface of the egg. Then the female lays the egg into its nest.

Using Western Bluebirds as an example, females weigh about one ounce. They are solely responsible for building the nest. Then they typically lay a clutch of 4-6 eggs, one per day. Then they brood the eggs for about 14 days, warming them under a bare skin patch on their breast.

Following hatching, both females and males feed the young until they fledge at about 20 days of age. The parents continue to feed the fledglings while the female rebuilds the nest for a second clutch of eggs (two nesting cycles are common in good habitat areas).

Female Western Bluebird
Female Western Bluebird

When I was tending bluebird boxes, I was shocked to occasionally find a dead female in the nest box. After reflecting on the difficulties and biological demands of the breeding cycle, I find it surprising that I did not find more fatalities.

The biological challenge I describe here is typical of all the small-bird nesting going on right now. Whether juncos or chickadees or swallows, the burden of breeding is so great for birds that it is almost impossible for us to imagine. It’s another reason to admire the abilities of our avian neighbors.

George Walter is the environmental program manager at the Nisqually Indian Tribe’s natural resources department; he also has a 40+ year interest in bird watching. He may be reached at

Photos for most of my columns are provided by Liam Hutcheson, a 17-year-old Olympia area birder and avid photographer. Liam specializes in taking photos of identification and not nesting activities. For this column, I have relied on photos from other sources.


No comments on this item Please log in to comment by clicking here