Bird eggs come in a variety of shapes, which has left humans wondering why and how those different egg shapes evolved. For a very long time, scientists had not been able to crack the mystery. However, thanks to a new study conducted by a group of international scientists, we now know that the different shapes of bird eggs is related to each bird’s flight ability. Study findings were published in Science.
In addition to flight ability, researchers found that the membrane of the egg might also play an important role in determining the egg’s shape. Dr. Mary Caswell Stoddard, with Princeton University and the lead author of the study, explained by saying birds that are powerful fliers typically laid elliptical or asymmetric shaped eggs. In addition, she said rather than the shell, the egg membrane is responsible for creating the vast diversity we see in the shapes of different bird eggs.
To reach this conclusion, researchers studied a database of 50,000 egg photos that included 1,400 different bird species. The database was made available by the Museum of Vertebrate Zoology at Berkeley and contained images of bird eggs from around the world that had been collected by naturalists during the 19th and 20th centuries. Using a multi-step multidisciplinary process, researchers applied computer code to quantify each egg’s ellipticity and asymmetry.
Through complex digital image analysis, researchers were able to gather a large amount of data and map the different shapes of bird eggs. For example, they now know that the Least Sandpiper has the most asymmetric egg and the Maleo has the most elliptical egg.
Following the mapping process, researchers developed a biophysical model to explain how the bird’s oviduct might attribute to the egg’s shape. They discovered that when the shell of an egg was dissolved away, the egg membrane retained the egg’s shape. They then looked at altering the membrane by changing both thickness and pressure, and found their model could produce many different egg shapes.
Researchers also looked at the bird family tree and compared the shapes of bird eggs across different lineages. Among the things analyzed were diet, flight ability, nest type, nest location, and clutch size. Dr. Joseph Tobias, with Imperial College London and a study co-author, said the shape and size of bird eggs is related to ecology differences. As birds became more adapted for powerful flight, the study revealed that morphological changes occurred such as a reduction in both body size and the size of the abdominal cavity.
These changes may have created problem for birds. They still needed to pack a large volume into their eggs in order to ensure healthy development of their chicks while working with less space. Researchers believe the birds solved this problem by evolving the shape of their eggs to pointier and more elliptical. This enabled the birds to increase volume while not increasing the egg width.
The results of this study have presented even more questions for researchers. For example, they noted that no other vertebrates lay pointy, asymmetric eggs, except certain theropod dinosaurs. Dr. Stoddard said it is an intriguing find that powerful flight and pointy eggs evolved around the same time. Going forward, she said the researchers are excited about studying how the shapes of eggs changed during the dinosaur-to-bird transition.
By Trixie Dillwood