Bird eggs are an astounding evolutionary phenomenon. They come in a wide array of sizes, shapes, and colors; in fact, coloration is one of their most unique features. No other amniote–animals that produce eggs with shells and/or membranes–produces colored eggs. And between bird species, coloration varies–sometimes a little, sometimes a lot. But why do birds produce the only colored eggs in the animal kingdom? And where did this trait come from?
A new study in the journal Nature attempts to look at these questions–particularly the evolution of where egg pigmentation. They evaluated eggshells from both living and extinct birds, along with other members of the archosaur crown group (because, y’know, BIRDS ARE DINOSAURS and dinosaurs are archosaurs). From Yale University, Jasmina Wiemann et al. wanted to see where and how often this egg feature showed up in the archosaur lineage. Their preliminary results could be revolutionary to how we understand both birds and their nonavian ancestors.
The Basics of Bird Eggs
There are four basic structures that make up bird eggs:
- Yolk
- Germinal disc
- Albumen
- Shell
The yolk is nutrient-packed and serves as an embryo’s main source of nutrition. They are infamously known for their high cholesterol and fat content, and this is intentional on the bird’s part; this structure of the egg will help sustain the growing embryo over the incubation period. On the surface of the yolk sits the germinal disc. This cytoplasmic disc is very small and contains the genes necessary to create bird offspring.
Surrounding the yolk is the albumen. It is protein-dense and contains globulins that provide immune support. The embryo will feed off the albumen protein in addition to the yolk, and also use it as a source of water. When a fledgling hatches, whatever albumen still remains acts as a lubricant to help them maneuver out of the egg.
The calcium that comprises the eggshell comes from the medullary bone of the female bird. It has three parts, each with different purposes. Pores on the outside correlate with metabolic demands and serve as sites of gas exchange and water transport. The thin and waxy cuticle on the outermost part of the shell protects eggs from water evaporation and microbial invasion. The testa–the largest portion of the eggshell–provides calcium to the growing embryo and is critical to formation of the shell.
How It’s Made
The process for making a bird egg has several steps. First, ovum develop in the female bird’s ovary, and are then released into the oviduct. As a protein-packed yolk in the oviduct, the ovum wait for fertilization to happen. Albumen is then added and plumped with water, before being wrapped in soft, stretchy membranes.
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After that, the shell is built. Calcium carbonate is deposited onto the membranes in layers. Special cells lining the shell gland of the bird’s reproductive system squirt the mineral onto the developing egg.
Pigmentation happens in the final stages of reproduction. Glands in the female bird’s reproductive system fire at certain times to develop the signature background color, patterning, and spotting on the eggshell. After that, a protein coating is added, and the egg is ready to be laid for incubation.
That Pigmentation Tho
Birds are the only living amniotes with colored eggs. Depending on the bird’s incubation strategy, local climate, and specific mating behaviors, these colors can vary greatly between species. Yet this is all done with just two pigments: protoporphyrin (reddish-brown) and biliverdin (blue-green). But how?
The exact reasoning for this is still not entirely clear. Two pigments might be all that birds need. This seems reasonable; by controlling the levels of pigment applied and what regions of the egg receive it, birds are able to produce a wide spectrum of patterns and colorations that can vary greatly between species.
With pigmentation comes a number of benefits. The first, unsurprisingly, is camouflage. Ground-nesting birds lay speckled and/or streaked eggs that blend in with their surroundings; shorebirds do something similar. Pigmentation also helps in anti-microbial defense as well as protection from solar radiation. In cases of non-pigmentation–such as owls–this lack of color is often a result of cavity nesting.
The other main advantage is recognition. Particularly for bird species vulnerable to brood parasitism–where another bird species lays its egg in a host’s nest–being able to recognize which eggs are yours is advantageous. In cases of brood parasitism, an evolutionary arms can occur between host and parasite. The host will evolve changes in egg patterning/coloration to distinguish them more clearly, and the parasite will change its eggs to mimic the host.
Recognition might also be advantageous as an inverse to camouflage. Certain bird species will lay vibrant, colorful eggs that are far more conspicuous. And this might be on purpose; “blackmail theory” argues more vivid pigmentation encourages males to protect the nest more. Because predators are able to easily identify conspicuous eggs, more participation and protection from both partners becomes vital.
So, What Do We See in Dinosaur Eggs?
In their study, Wiemann et al. used Raman spectroscopy to look at the eggshells of 19 species in the archosaur lineage. This included representatives of:
- Crocodilians
- Sauropods
- Ornithischians
- Eumaniraptorans (troodontids and dromaeosaurids)
- Ratites
- Galliformes
- Enantiornithines (Mesozoic “birds” that retained clawed fingers and teeth)
- Extinct relatives of ostriches
- Oviraptorids
The researchers looked at eggshells from each of these lineages to identify and map out their pigments. As well, they looked to characterize the color patterns and deposition of pigment in fossil eggs.
Preservation of biliverdin and protoporphyrin in fossils can occur in trace amounts. However, finding them can be a challenge. The process of diagenesis–changes to sediment occurring between deposition and final conversion to rock–can produce chemical compounds that appear similar to preserved pigments. Once the researchers were able to distinguish between these compounds and actual pigmentation, they were able to map the pigments across an egg’s surface.
The results of the Wiemann et al. study are incredible. In all the eumaniraptorans studied, egg pigmentation was preserved. As well, the researchers observed that nonavian eumaniraptoran eggs were speckled and spotted. Meanwhile, the Raman spectroscopy showed sauropod and ornithischian eggs lacked color, as well as crocodilians.
With their findings, Wiemann et al. concluded birds were not the the first amniotes to produce colored eggs. Rather, egg pigmentation arose somewhere along the maniraptoran lineage and had a single evolutionary origin.
Thinking Through the Implications
This new study has so many implications for our understanding of dinosaurs. First, it gives us greater insight into dinosaur reproduction and how it was likely more complex than we currently understand. But perhaps the most important area of dinosaur research the Wiemann et al. study impacts is our understanding of parental care.
Currently, we know that parenting strategies range between groups of dinosaurs and between species. For example, it’s thought there was age partitioning in sauropod herds, meaning parental care didn’t last long after incubation and hatching. Meanwhile, ornithischians like Psittacosaurus and Maiasaura likely provided greater care for their young. There was a clear diversity of parenting strategies across the major dinosaur clades, and the absence of coloration from certain clades is therefore important.
In this context, egg pigmentation seems to have an important connection to parental behavior. It certainly impacts how birds care for their eggs and their young, and it’s possible to infer this could be an ancestral condition. However, just because a Deinonychus egg might be patterned like those of modern, ground-nesting birds does not mean they exhibited similar behaviors. How far back changes in parental care occurred in birds and their ancestors–in relation to egg pigmentation–still needs greater exploration.
So where does this study leave us? These findings aren’t set in stone, so more work needs to be done to support them. However, they do raise a number of interesting questions worth exploring. For example, when did brood parasitism evolve in birds–earlier in their dinosaur lineage or closer to our modern time? Likewise, could the “blackmail theory” apply to some dinosaur species? Whatever the answers to these questions are, they’re certainly fascinating.
References/Further Reading
Birchard, G. F., M. Ruta, and D. C. Deeming. “Evolution of Parental Incubation Behaviour in Dinosaurs Cannot Be Inferred from Clutch Mass in Birds.” Biology Letters 9, no. 4 (August 23, 2013). Accessed December 7, 2018. doi:10.1098/rsbl.2013.0036.
British Trust for Ornithology. “Incubation.” Accessed December 07, 2018. https://www.bto.org/volunteer-surveys/gbw/gardens-wildlife/garden-birds/behaviour/incubation.
Hamilton, Bill. “Audubon Blog: Bird Eggs.” Audubon Society of Western PA. Accessed December 07, 2018. http://www.aswp.org/pages/audubon-blog-bird-eggs.
Leonard, Pat. “The Beauty and Biology of Egg Color.” All About Birds. June 12, 2017. Accessed December 07, 2018. https://www.allaboutbirds.org/the-beauty-and-biology-of-egg-color/.
Myers, Timothy S., and Anthony R. Fiorillo. “Evidence for Gregarious Behavior and Age Segregation in Sauropod Dinosaurs.” Palaeogeography, Palaeoclimatology, Palaeoecology 274, no. 1-2 (April 1, 2009): 96-104. Accessed December 7, 2018. doi:10.1016/j.palaeo.2009.01.002.
Newbern, Elizabeth. “Incubating Bird Eggs Is More Complex Than You Think.” Audubon. June 11, 2014. Accessed December 07, 2018. https://www.audubon.org/news/incubating-bird-eggs-more-complex-you-think.
Nuwer, Rachel. “Cracking the Code on Egg Coloration.” Audubon. June 15, 2015. Accessed December 07, 2018. https://www.audubon.org/news/cracking-code-egg-coloration.
Pollock, Christal. “Understanding the Avian Egg: From Outside to In.” March 23, 2013. Accessed December 07, 2018. https://lafeber.com/vet/understanding-the-avian-egg-from-outside-to-in/.
Ruxton, Graeme D., Geoffrey F. Birchard, and D. Charles Deeming. “Incubation Time as an Important Influence on Egg Production and Distribution into Clutches for Sauropod Dinosaurs.” Paleobiology 40, no. 03 (2014): 323-30. Accessed December 7, 2018. doi:10.1666/13028.
Strauss, Bob. “How Dinosaurs Raised Hatchlings and Juveniles.” ThoughtCo. June 9, 2018. Accessed December 07, 2018. https://www.thoughtco.com/were-dinosaurs-good-parents-1091906.
Wiemann, Jasmina, Tzu-Ruei Yang, and Mark A. Norell. “Dinosaur Egg Colour Had a Single Evolutionary Origin.” Nature 563, no. 7732 (November 22, 2018): 555-58. Accessed December 7, 2018. doi:10.1038/s41586-018-0646-5.
Yin, Steph. “Why Do Bird Eggs Have Different Shapes? Look to the Wings.” Trilobites. June 22, 2017. Accessed December 07, 2018. https://www.nytimes.com/2017/06/22/science/bird-eggs-shapes-flight.html.