Oh, it’s egg-shaped. But wait—are all eggs the same shape? Apparently not.
Did you grow up learning that eggs are pointy so they don’t roll out of the nest? I did and it made perfect sense to me. Of course, I didn’t realize other birds laid spherical eggs. All I had ever seen was robin eggs and chicken eggs.
Other ideas over the centuries embraced that egg shape was related to the type of nest, clutch size, developmental mode or incubation efficiency within the nest. Scientists hadn’t cracked the mystery of why eggs are shaped the way they are until recently.
What determines egg shape?
Princeton University evolutionary biologist Mary Stoddard conducted research in 2017 and cracked the mystery of egg shape—flight ability.
Yes, egg shape relates to how much the species is designed for powerful flight.
Excellent flyers (such as sandpipers and hummingbirds) have lightweight, streamlined bodies and, therefore, more elongated and asymmetrical eggs.
Why? A streamlined body means a narrower pelvis. In order to fit a baby bird in a narrow eggshell, the egg needs to be elongated.
For birds that spend little time flying or aren’t streamlined for flight (like an owl), their eggs are more spherical. They have a wider pelvis that can accommodate a more spherical egg.
What about the egg membrane?
Have you ever placed a raw egg in vinegar and waited for the shell to dissolve? What shape was the egg membrane? The same as the eggshell. The egg didn’t turn into a limp blob.
Stoddard’s research showed that the egg membrane is also an important factor in egg shape. The pressure within and the thickness makes the membrane a certain shape.
Experiments showed that variations in the membrane’s thickness, composition, stretchiness and differential pressure created variations in egg shape.
Therefore, the pelvis determines whether the egg was wide or narrow and the egg membranes influence the egg shape beyond that—pointed, asymmetrical, elongated.
What shape are eggs?
A family group of birds tends to have the same egg shape, Stoddard determined after examining 1,400 species. Birds can have spherical eggs, pointy eggs, elongated eggs, pointy and elongated eggs, but not short and pointy like a hot air balloon.
Owls tended to have spherical eggs. Osprey and loons have elongated eggs. Crows and ravens have elongated and pointy eggs. Murres have pointy eggs.
Why are murre eggs pointy?
Murre eggs have the pointiest tip of all bird eggs. The idea over the centuries was that the downward-angled tip kept the egg from rolling off the cliff ledge where murres nest.
Tim Birkhead, a British ornithologist who has studied murres for over 44 years, hypothesized two other reasons for the pointy eggs after debunking the first idea.
The circumference of the murre egg is large enough that the arc it would make rolling in a circle is frequently larger than the ledge of the murre’s nest, according to Birkhead. This debunks the first idea.
Instead, Birkhead hypothesized that the egg shape has to do with protecting the egg and cleanliness.
The very pointy egg helps protect the egg from the impact of murres crash-landing on their nest (remember they are on narrow cliff ledges) and being trampled by other murres (space is at a premium and up to 70 murres can be nesting in a square meter).
The pointy shape allows more of the shell to be in contact with the ground than a typical oval egg which would allow the force of impact to be spread across the surface, according to Birkhead. He also discovered the eggshell is thicker where it would touch the ground.
With all those murres nesting in a small area, the area is extremely dirty. Guano and dirt can clog the egg pores which are needed for the chick to breathe. Birkhead discovered that the egg’s blunt end that is raised above the ground by the downward-angled tip has the highest concentration of pores which helps protect the chick from contamination.
Another theory on egg shape
One admirable trait about scientists is they keep asking questions. Another study in 2018, suggested that egg shape is related to breeding sites and incubation posture of the parent.
In penguins and alcids (auks, murres, puffins, murrelets, guillemots), those incubating in an upright posture on flat or sloping ground had eggs of pyriform shape (pointy end). Whereas, those nesting in a cup nest had more spherical eggs.
More research would determine if this is universal across all bird species.
Cracking the mystery of egg shape
Overall, the latest theories are that egg shape is determined by flight ability, egg membranes and incubation posture. However, each species might develop specialized shapes to adapt to their needs such as dirty nests and crash landings.
Some answers in science are straightforward like photosynthesis, but others don’t have a single answer like egg shape. Research is always revealing new ideas. Egg shape is a coalescence of different factors, some which may not be cracked yet.
Resources
- https://www.audubon.org/news/why-are-murre-eggs-so-pointy-new-research-debunks-prevailing-theory
- https://onlinelibrary.wiley.com/doi/abs/10.1111/ibi.12658
- https://vis.sciencemag.org/eggs/
- How eggs got their shape– Harvard School of Engineering
- https://www.science.org/content/article/surprisingly-simple-explanation-shape-bird-eggs
