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Name: Jaime W.
Status: Student
Age: 15
Location: N/A
Country: N/A
Date: June 2002
Question:
Hi How does a bird sit on an egg and not break it? Now when I asked my teacher for some hints he said the shape of the egg is a big factor but I have been unable to find that information anywhere.


Replies:
Birds do not weigh very much. Have you ever tried to break a chicken egg by crushing it in your fist? It is surprisingly difficult. If the egg shell is not cracked, it can withstand quite a bit of force.

Richard E. Barrans Jr., Ph.D.
Assistant Director
PG Research Foundation, Darien, Illinois

I have just written a manuscript entitled: "The Geometry of Avian Eggs" which addresses the very question of the shapes of eggs. I hope to have it published, since very little has been written on the subject, surprisingly. My study was motivated by a Newton BBS question some months back.

For the purpose of your question you can make the following approximate computation. Assume the egg is an oblate oval or revolution, about 5 cm long and 4 cm wide. From the formula for the surface area of an oblate oval or revolution you can calculate the surface area (your teacher will be able to help you find the formula in a handbook, or ask your math teacher). The bird weighs maybe a couple of pounds -- at least for that size egg -- and assume only a single egg (there are probably more, but a single egg will provide an upper limit. Assume 1/2 the weight of the bird is supported by the egg and the other 1/2 is supported by the nest itself. I have not done the calculation but I think you will find that the pressure (lbs/ in^2) on the egg will be quite small, even if you do not take into account that the mother hen may not just "sit" on the egg but supports her own weight partially with her legs.

Should you want a more detailed analysis, you would have to take into account that a rounded shape like an egg, or a suspension bridge across a river distributes the weight in such a way that it is able to support a much larger load than say a bar or a single pillar or column. This is a much more involved calculation.

Vince Calder


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