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About a Bird’s Egg

bird egg shapes

Which came first, the chicken or the egg?  Scientists now say that since birds evolved from dinosaurs the egg came first.  In either case bird eggs are an amazing invention.

Bird eggs begin essentially as a food sac or yolk. The fertilized egg cell is part of the yolk and rests on its surface. It is the fertilized egg cell that grows into the embryo.

The female bird’s reproductive tract works like an assembly line. As the yolk moves along it is coated in albumen, or egg white, for protection. Then, shell membranes, the egg shell and the shell covering (cuticle) are added. The whole process moves along at a rapid pace, about a day in many species.

bird egg shapesEgg shape
Eggs are not all the same shape as the chicken eggs that we eat. Some birds eggs are more pointed, or pyriform and, at the other extreme, some are more rounded or spherical.

Egg shape is partially determined by the internal structure of the female. Her oviduct, distribution of internal organs and shape of her pelvic bones all affect egg shape. For example, the Mallard’s egg is subelliptical (not quite a sphere). The Great Horned Owl’s egg is spherical and the Red-tailed Hawk’s egg is elliptical. Other eggs are more pointed.

The shape of an egg’s shell does affect its physical properties. The general spherical shape of an egg maximizes shell strength. The more rounded an egg is the eggs the volume it has for the amount of egg shell produced.

In some species the shape of the egg has evolved over time to fit into the nesting environment.

Shorebirds usually lay four eggs that are somewhat pointed.  The adults orient the eggs so the pointed end points to the center of the nest.  This approach minimizes the amount of space needed for the nest.

Cliff-nesting birds also lay pointed eggs.  For example, Common Murres lay one egg on a bare nesting ledge.  If the egg is bumped it will tend to roll in a circle instead of rolling off the edge.  

Common Murre
Commen Murre

murre eggs

Commn Murre Eggs:  By Didier Descouens – Own work, CC BY-SA 4.0,

Egg collecting
Bird egg collecting, known as Oology, was once a major source in the decline of many different birds.  Fortunately it is now illegal to collect the eggs of all native North American birds. Egg collecting is banned in many countries but is still practiced illegally.  

The British seem to be especially active in the now illegal hobby.  Brit Colin Watson was one of the world’s most notorious egg collectors and specialized in the eggs of rare and endangered birds.  He fell to his death when climbing a tree in 2006. Supposedly he maintained a list of 300 fellow egg collectors.

In 1997 two Brits were fined 90,000 pounds each for collecting the eggs of protected species.  

in June of 2016 another Brit was also arrested and fined for collecting the eggs of the Arctic Tern and Great Skua.

Bird egg images
The Royal Alberta Museum maintains one of the largest online bird egg databases.

About the Author

Sam Crowe

Sam is the founder of He has been birding for over 30 years and has a world list of over 2000 species. He has served as treasurer of the Texas Ornithological Society, Sanctuary Chair of Dallas Audubon, Editor of the Cornell Lab of Ornithology's "All About Birds" web site and as a contributing editor for Birding Business magazine. Many of his photographs and videos can be found on the site.

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Ashley S. Boone

Tuesday 26th of March 2024

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