The Mysteries of Eggs

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A Variety of Bird Eggs

I’ve always known that birds’ eggs are pretty amazing structures. Encased in a shell with everything it needs to produce a new bird, an egg does so in 10 to 60 days, depending on the species. You can get an overview of the development of an egg here or a much more detailed discussion of the development of a chicken egg, with graphics, here.

One of the more remarkable attributes of the eggs is the strength of its shell. Occasionally I demonstrate this with a member of the audience of one of my lectures. I place a chicken egg lengthwise in the volunteer’s palm, and assuring the fingers bear no rings, instruct the person to try to crush the egg. They do so hesitatingly at first and then squeeze harder. In 40 years, only one person managed to crack the egg. Amazing, right?

I thought I knew a lot about eggs and always figured they were an elegantly simple evolutionary apparatus for reproduction. But I have recently learned that they are far more complex.

Which end of an egg, the narrow or wide end, comes out first? It varies among bird species, but in the chicken the wide end emerges first. Surprisingly, however, the shelled egg comes down the oviduct narrow end first and just before laying, turns 1800 and then emerges wide end first. On the other hand, guillemots lay their eggs narrow end first. Why the difference? Well, nobody really knows or maybe the answer is that it is just not that important.

Another mystery is the coloration of eggs. It was been assumed for many years that birds that nest in cavities such as holes in trees or tunnels in riverbanks such as kingfishers or woodpeckers, have white eggs because predators would never see them. But then there are pigeons and nighthawks and the Short-eared Owl that nest in the open but have white eggs. Lots of shorebirds, nesting on the ground, have cryptically-colored eggs to blend in with the environment. But what about the blue eggs of the American Robin or the European Song Thrush? Must be something else going on here that affects coloration. Could it be that bright blue eggs indicate to the male that the female is especially fit and that it would be beneficial to the male to help her incubate the eggs and/or care for the young?

A study of canaries found that each successive egg laid had higher levels of testosterone. Speculation is that the chicks hatching from the later eggs were more aggressive and thus better at competing for food as nestlings.

You would think ornithologists had bird eggs all figured out but there are still lots of mysteries surrounding these fantastic products of evolution. There is no way I can possibly explain in these short blogs the complexity of eggs, so what I am leading up to is suggesting a read of Tim Birkhead’s latest book, The Most Perfect Thing, all about eggs. He examines everything inside and outside of birds’ eggs, but clear explanations seem hard to come by in many cases. Much of what is known about eggs is truly amazing; but there are still considerable mysteries.


5 thoughts on “The Mysteries of Eggs”

  1. Hi! May I have permission to post this article on the Bluebird Recovery Facebook page, please?

    I’m one of the administrators and the State Assistant Pro Director for the California Bluebird Recovery Program .

    Thank you!

  2. Deborah Berger

    I hope you can answer a question that I’ve had since I saw a program with David Attenborough, called “The Egg: Life’s Perfect Invention.” He told of a German scientist who, in 1896, got it into his head to learn which end of a chicken’s egg comes first. The scientist got some “very tame chickens,” as Dr. Attenborough described them, stuck a pencil up each hen, and made a mark on the egg. And that’s how he saw that chickens push their eggs out big end first.
    I’m fascinated by this German scientist. Could you tell me where I might find out more about him?

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