A second grade classroom at my school incubated chicken eggs until they hatched. When their feathers had dried, the fluffy chicks looked too big to have been enclosed within the broken shells still lying in the incubator.
One of the eggs did not hatch. For some reason, the chick inside had gone lifeless and would never break out into the wide world. Mr. Watts, the teacher who had run the whole project, handed it to me. “Feel this,” he said. “It seems heavier than it was in the beginning.”
I gently shook the egg from side to side, feeling the dead weight shift within the shell. Truly, it felt heavier than the eggs I buy for omelets. Was it because this egg had been fertilized and transformed into a nearly-finished young bird? Is it possible for an egg to gain weight while the chick grows inside?
My first thought was that it could not have gained any mass, because it had not eaten anything from outside the shell. Everything inside an egg until it hatches was in there since the moment it was laid. The chick forms from a small part of the material, and absorbs most of the rest as it grows. Since it does not eat or absorb anything from outside the egg, it seemed unlikely that it would actually weigh more at the end of incubation than it had at the beginning.
Since I am not an expert in birds or their eggs, I decided to do some research. The best article I found was on the Poultry Club of Great Britain website, which you can read here if you like. According to the article, a healthy chicken egg loses about 13% of its mass during incubation. Otherwise, the chick does not have room to move around enough to break the shell with its egg tooth, and dies inside the egg as a result. Maybe that is what happened to the egg that inspired this article.
There was one other thing I was unsure of. A chicken breathes oxygen and wastes carbon dioxide, like all animals on Earth. Would it not have to breathe through the shell? If so, there might be gases coming into the egg from outside, which might add mass.
It turns out that there is an exchange of oxygen and carbon dioxide through the shell, but since carbon dioxide is heavier than oxygen, this would reduce, rather than increase, the egg’s mass; indeed, this is one of the reasons – humidity being the other – that the egg loses weight.
Why did the egg feel heavy? Probably because an unfertilized egg – like the ones I get at the grocery store for my omelets – is all liquid inside. The egg with the unhatched chick had much less liquid, a larger amount of air, and a solid mass shifting around, which gave an illusion of added weight. I am reminded of when my children were small. When they would fall asleep and I carried them to bed, they felt heavier than when I would give them a piggyback ride. Their limp and sleeping bodies felt heavier, even though their weight had not changed.
My sons are much too big and heavy for me to carry anymore. But getting old has nothing to do with the weight of chicken eggs, so I suppose this post is done.