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In my limited understanding, eggs of oviparous creatures other than birds have a flexible outer membrane.

Is there any obvious reason that bird eggs should have fragile shells?


Notes

  1. I'm asking why birds seem to be a special case. If all other oviparians manage perfectly well without shells, what is it about being a bird that requires the extra resources to form a shell?

  2. I believe birds are considered to be modern-day dinosaurs. Didn't dinosaurs have leathery eggs?

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  • $\begingroup$ Well, the birds have to be able to get out of them, don't they? Can a leathery egg withstand an adult bird sitting on it? $\endgroup$
    – swbarnes2
    Mar 12 at 0:00
  • $\begingroup$ @swbarnes2 - (1) All oviparous species have to get out of the egg (2) I don't know if a leathery egg will withstand being sat on. However, birds are not very heavy and I don't think they put all their weight on the egg - they also use their feet and legs to support themselves. $\endgroup$ Mar 12 at 0:07
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    $\begingroup$ Bird egg shells are actually pretty strong. Although they can break from a concentrated blow (cracking an egg) they resist distributed pressure, as from the parent sitting on eggs, quite well. Do any of the other creatures with leathery/flexible egg casings actually sit on their eggs? $\endgroup$
    – jamesqf
    Mar 12 at 3:38
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    $\begingroup$ I don't understand the question. How e.g. frog eggs are more resistant? $\endgroup$
    – Winston
    Mar 12 at 11:34
  • $\begingroup$ @Exocytosis - (1) A table-tennis ball is easily squashed and will not recover its shape. A hollow rubber ball can be squashed and recover its shape. Similarly with eggs. Both types of squashing may or may not damage the embryo, however the hard shell, once broken, will allow the ingress of bacteria and viruses. (2) Frogs survive as a species, so their eggs must have sufficient resistance. $\endgroup$ Mar 12 at 12:13
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Your question is, I think, based upon a false presumption: that avian eggs are more fragile than those of other oviparous animals.

The paper Evolution of the Cleidoic Egg Among Reptilian Antecedents of Birds1 suggests that increased calcification of the avian eggshell is an evolutionary response to protect against predation and microorganisms, particularly in soil. The trade off was less permeability and water absorption. To offset that "cost," birds' eggs evolved the albumen or "egg white" which adds liquid content.

This is discussed more (and referenced) in Frank B. Gill's Ornithology2, which goes on to further explain that the shells additionally protect eggs from the weight of incubating parents.

Other functions, such as that of oxygen exchange, are permitted by the calcium shell due to microscopic pores. Also, crystalline calcite (the main ingredient of the shell) can be used by the embryo to form calcium in development of its bones.

1 Evolution of the Cleidoic Egg Among Reptilian Antecedents of Birds by Gary C Packard and Mary J Packard; American Zoologist, Volume 20, Issue 2, May 1980, Pages 351–362

2 Gill, Frank B.. Ornithology. United Kingdom: W. H. Freeman, 2007.

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