What can eggshell coloration tell us about female health and male behavior?

Featured Scientist: Kara Hodges, Master of Science 2019, School of Biological Sciences, Illinois State University

Kara wears a face mask and smiles at the camera. She is in a laboratory.

Birthplace: Indianapolis, IN

My Research:  My research explored eggshell coloration to see how coloration was connected to the health of a mother bird. I was also interested in exploring how coloration might impact how male birds cared for their offspring after the birds hatched.

Research Goals:  I currently work for the United State Department of Agriculture (USDA). I study the genetics of animals raised for meat. Moving forward, I would prefer to work on animal behavior and conservation-related questions.

Career Goals: I’m still figuring out what to do next with my life. I’m thinking of going back to school to get a professional degree in a medical field.

Hobbies: I like to run, hike, and garden. I also spend a lot of time caring for my many pets.

Favorite Thing About Science: What I like most about science is that it has layers. One question leads you to another, and soon you’re down a rabbit hole of discovery. 

My Team: My advisors, Dr. Thompson and Dr. Sakaluk, helped me develop my hypothesis, design the experiment, collect the data, analyze the results, and edit my thesis. Fellow graduate students from the Thompson Lab and Sakaluk Lab, and members of the summer field crew, helped me conduct field work and collect data. Dr. Alysia Vrailes-Mortimer and Dr. Nathan Mortimer, and their students, assisted me with the molecular lab work, processing digital photographs, and using machine learning. Dr. Hauber from the University of Illinois helped me extract and measure eggshell pigments. My committee members helped guide my research and provided valuable insight. Last but not least, I received funding to buy materials for my research from the Sigma Xi Scientific Research Honor Society, the American Ornithological Society, the Beta Lambda Phi Sigma Honor Society, the National Institutes of Health, and Illinois State University.

Organism of Study: I studied the house wren (Troglodytes aedon), a wild migratory songbird.

I picture of a plump house wren sitting on a branch.
Photo credit: Andy Witchger, Cornell Lab of Ornithology Macaulay Library

Field of Study: Behavioral Ecology

What is Behavioral Ecology? Ecology is the study of living things and the relationships that they have to each other and to the environment. Behavioral ecology is a branch of ecology that tries to understand why animals do what they do. How does the behavior that we see impact survival and reproduction for the individual or for the species?

Check Out My Original Paper: “Connecting the dots: avian eggshell pigmentation, female condition and paternal provisioning effort”

QR code that links to the original publication.
QR code to the original publication

Citation: Kara E Hodges, Nathan T Mortimer, Alysia D Vrailas-Mortimer, Scott K Sakaluk, Charles F Thompson, Connecting the dots: avian eggshell pigmentation, female condition and paternal provisioning effort, Biological Journal of the Linnean Society, Volume 130, Issue 1, May 2020, Pages 114–127, (Doi: 10.1093/biolinnean/blaa002).

Research at a Glance: The color of an eggshell, or its pigmentation, varies between different species of birds and within species of birds, but it’s not clear what causes this variation. One possible explanation is that the pigments are tied to the health of the female that laid the egg. Another possible explanation is that the pigment acts as a signal to the male, which impacts how the male cares for its young. To explore these explanations, we used the house wren (Troglodytes aedon), a wild migratory songbird that lays brown eggs. House wren eggs are mostly pigmented by protoporphyrin, which makes brown and red colors. Inside the body, this pigment is a harmful waste product that can damage important molecules in the wren. We tested the hypothesis that the color of the eggshell is an indicator of the health of the female. Darker, browner eggs have more protoporphyrin. If an eggshell has more protoporphyrin, then it could mean that the female is producing more of this harmful waste product and is in poorer condition. Alternatively, it could mean that the female is better at removing protoporphyrin from her system, making her healthier. To answer these questions, we had to measure eggshell coloration and the health of the female that laid the eggs. To measure eggshell coloration, we photographed clutches of eggs, or groups of eggs that are laid at the same time and sorted them into “light” or “dark” categories. The sorting was performed by a machine learning algorithm. The algorithm sorted the eggs using characteristics of the shell, such as, the number of spots, or maculations, on the egg, the darkness of those maculations, and the background color of the shell. To investigate the relationship between eggshell coloration and female health, we took blood samples and physical measurements from the females and their nestlings. We found that females laying lighter clutches tended to be older and larger than females laying darker clutches (Figure 1). Additionally, we found that nestlings from lighter clutches were bigger, which strongly predicts survival. This information suggests that females laying lighter eggs are healthier than those laying darker, more pigmented eggs. To see if males change their behavior based on egg pigmentation, we set up a modified cross-fostering experiment (Figure 2). We placed dark eggs in nests that had produced light eggs and light eggs into nests that produced dark eggs. Then, we paired males with the females to see if male behavior changed in response to the color of the eggs, the health of the female, or the nestlings themselves. When the eggs hatched, we recorded the feeding efforts of both parents and compared the results between our groups. We found that male feeding effort was not influenced by eggshell pigmentation

Highlights: Our results indicate that house wrens laying lighter eggs are in better condition than those laying darker eggs. We were able to use data from previous years to estimate the age of the female house wrens used in this experiment. We found that older females tended to lay lighter eggs (Figure 1).

A data figure that shows the scale of pigmentations from light to dark. Zero, in the center of the y-axis, is neutral. Average pigmentation for one year old females is -0.2 toward the dark scale. Average pigmentation for one year old females is 0.6 toward the light scale.
Figure 1. Eggshell pigmentation in the nests of one-year-old and two-year-old house wrens. The y-axis shows the scale of light to dark eggshell pigmentation based on the machine learning algorithm. Figure adapted from Hodges et al. 2020.

Some research has found that the pigmentation of a female’s eggs remains consistent across time. Our results could mean that females with lighter eggs are more likely to survive migration and return for a second breeding season. However, this pattern may not be universal. There are many other factors that could influence eggshell pigmentation, such as, camouflage from predators, shell strength, and UV protection.

We also found that male house wrens don’t appear to react to eggshell pigmentation. This does not mean that they aren’t capable of discerning differences between eggs. It is more likely that, by the time eggs are produced, it is mid-season, and the male would struggle to find an unpaired female. This, along with a short lifespan, creates a situation in which it would be better to stay with a poor partner than to risk seeking a better one.

What My Science Looks Like:

An image os several house wren eggs  lined up next to each other. colors range from light beige to dark brown. None of the eggs are uniform in color and some have dark dots on the shell.
The image above shows eggs laid by house wrens from our study site. Eggshell pigmentation can vary greatly between females. Image adapted from Hodges et al. 2020.

To answer the question about male behavior, we used a modified version of a cross-fostering experiment (Figure 2). Some males were paired with a female that appeared to lay dark eggs (when she actually laid light eggs) and some males were paired with a female that appeared to lay light eggs (when she actually laid dark eggs). This way, the attending male would be exposed to eggs that indicated a level of health that was different than the female laying them. Other males were paired with females that appeared to lay eggs similar to what her actual eggs looked like. Just before hatching, we returned the eggs to their original nests to see if differences in male behavior were due to the offspring or the pigmentation of the eggs.

A two-panel cartoon. The top panel shows a male that sired dark eggs but thinks that he sired light eggs. The nestlings that he raises are from dark eggs. He provides a small food item. The bottom panel shows a male that sired light eggs but thinks he sired dark eggs. The nestlings that he raises are from light eggs. He provides a large food item.
Figure 2. A hypothetical example of the cross-fostering experiment. In this instance, the male in the top panel is exposed to light eggs, but cares for nestlings from dark eggs. The male in the bottom panel is exposed to dark eggs, but cares for nestlings from light eggs. The figure shows that the male exposed to darker eggs works harder to provide for his offspring (he has a larger prey item) than the male exposed to lighter eggs. Figure adapted from Hodges et al. 2020.

The Big Picture: Birds and many other animals choose mates with the highest fitness, or those that are the most likely to create strong offspring that will survive and reproduce. Animals will use certain signals as evidence for higher fitness, but it is not always clear to people what those signals are. Our research evaluates one trait, eggshell pigmentation, as a signal for higher fitness. We found some evidence that eggshell pigmentation is related to female health but did not find evidence that males were using pigmentation as a signal. This research is important because it helps us to understand mate choice and understand which traits provide higher fitness. We can use this type of information to track the health of bird populations over time.

Decoding the Language: 

Clutch: A clutch is the group of eggs that a bird lays at one time in one breeding attempt.

Cross-fostering: Cross fostering is a type of experimental design. Offspring are removed from their biological parents and raised by surrogates. Scientists use this method to see if traits in the offspring (such as adult size) are more similar to their biological parents or their adoptive parents. If the offspring are more similar to their biological parents, this suggests that there is a genetic cause. If the offspring are more similar to their adoptive parents, this suggests that there is an environmental cause or a care-related cause.

Fitness: In biology, fitness refers to the ability of an organism to survive and reproduce. 

Machine learning: Machine learning is a type of artificial intelligence that automates choices. Machine learning can find patterns in large amounts of data. It is faster and less subjective than human decision making.

Maculations: Maculations are the spots or specks on an eggshell. These are usually darker than the background color. You can think of these as freckles on an eggshell. 

Nestling: A nestling is a bird that has recently hatched but is still too young to leave the nest.

Pigmentation: Pigmentation, or a pigment, refers to a compound that produces color.

Protoporphyrin: Protoporphyrin is naturally produced inside the body as a byproduct of blood synthesis. However, it can easily be converted into a form that can cause damage to nucleic acids, proteins, and lipids. Excess amounts of protoporphyrin can cause damage to the body.

Learn More: 

For more information on research with house wrens, check out Dr. Charles Thompson’s Lab webpage

The blood samples collected as part of this study are still being processed to assess gene expression and protein damage. This work could not have been done without the input and assistance of Dr. Alysia Vrailes-Mortimer.

The machine learning techniques used in my study were developed by Dr. Nathan Mortimer

Dr. Mark Hauber is a collaborator at the University of Illinois and runs a lab investigating eggshell pigmentation, as it relates to brood parasitism. You can find information about his work and publications here

There are many articles and books written on eggs and eggshell pigmentation. These papers summarize what is known about avian eggshell pigmentation and the causes of its variation:

Kilner, R. M. (2006). The evolution of egg colour and patterning in birds. Biological Reviews81(3), 383-406.

Reynolds, S. J., Martin, G. R., & Cassey, P. (2009). Is sexual selection blurring the functional significance of eggshell coloration hypotheses? Animal Behaviour78(1), 209-215.

Cherry, M. I., & Gosler, A. G. (2010). Avian eggshell coloration: new perspectives on adaptive explanations. Biological Journal of the Linnean Society100(4), 753-762.

The paper that initially sparked my interest in this research:

Moreno, J., & Osorno, J. L. (2003). Avian egg colour and sexual selection: does eggshell pigmentation reflect female condition and genetic quality? Ecology Letters6(9), 803-806.

Synopsis edited by Aleksandra Majewski, M.S. 2020, Illinois State University, School of Biological Sciences and Emily Kerns, B.S. 2018, University of North Florida, Department of Biology.

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