Abby Kimmitt hold a female resident junco at her field site near University of Virginia’s Mountain Lake Biological Station in Pembroke, VA. Photo Credit: Kaitlin Alford.
Abby Kimmitt hold a female resident junco at her field site near University of Virginia’s Mountain Lake Biological Station in Pembroke, VA. Photo Credit: Kaitlin Alford.

Dr. Abigail Kimmitt, a postdoctoral researcher at Texas A&M University, tells us about her paper “Migratory strategy explains differences in timing of female reproductive development in seasonally sympatric songbirds”, as well as her current projects and her journey in becoming an ecologist.

What’s the background of your paper?

It was assumed for a long time that animal migration prevented population divergence: if animals were moving around, populations were more likely in contact with each other, which would encourage gene flow. We know, however, that this idea cannot fully explain divergence in migratory species (or lack there of), because there are countless examples of migratory species complexes that are incredibly diverse.

Seasonal sympatry is a unique population distribution driven by animal migration, which could promote divergence. In seasonal sympatry, populations that differ in migratory strategies (e.g. migrants vs. residents) are found in overlapping distributions for part of the year but are otherwise geographically isolated. In my study system, the dark-eyed junco, migrants and residents are found in overlapping distributions in the winter. Since both populations are seasonal breeders, they must undergo reproductive development every year before they can initiate breeding. Based on observations, we knew that residents initiate breeding in early spring prior to the departure of the migrants for their breeding grounds. If migrants are also reproductive at this time, this could create a potential opportunity for gene flow. But these populations do not appear to be interbreeding. We hypothesized that differences in timing of reproductive development could prevent interbreeding between the populations. Interestingly, these closely related populations might differ in reproductive timing despite experiencing the same environmental cues since the winter solstice.

What is your paper about?

A side-by-side comparison of a migrant (left) and a resident (right) dark-eyed junco. One distinguishable difference between the subspecies is bill coloration: migrants have a pink bill whereas residents have a blue-gray bill. Photo from Adam Fudickar.
A side-by-side comparison of a migrant (left) and a resident (right) dark-eyed junco. One distinguishable difference between the subspecies is bill coloration: migrants have a pink bill whereas residents have a blue-gray bill. Photo from Adam Fudickar.

In this paper, we wanted to study reproductive timing and the underlying mechanisms in migrant and resident populations found in seasonal sympatry in early spring. If populations differed in reproductive timing, this could act as an isolating mechanism that prevented interbreeding between populations. We predicted that residents would be more reproductively developed than migrants in early spring.

Specifically, we were interested in female reproductive timing. Because females lay eggs, females drive breeding phenology and affect reproductive success of both sexes (i.e., males can’t breed if females aren’t laying eggs). Therefore, we thought it was important to understand the hormonal mechanisms that drive female reproductive timing. We collected both ovary and liver tissue, as both of these tissues play a critical role in reproduction, and looked at gene expression of important receptors that bind reproductive hormones.

What are the key messages of your article?

We found that resident females are more reproductively developed than migrant females in early spring. We also found that residents’ ovaries are more sensitive to upstream hormonal signals than migrants’ ovaries, suggesting that responsiveness of the ovary could be one mechanism that drives differences in timing. 

Did you have any problems setting up the experiment/gathering your data?

We first tried to study this question using females in captivity similar to another study that investigated differences in timing in male migrant and resident dark-eyed juncos (Fudickar et al. 2016). We thought that this would be a good method because we could monitor female reproductive development at multiple time points by measuring indicators of reproductive condition in the blood. However, females are less likely to enter reproductive condition in captivity than males and overall exhibit more variability in reproductive development in captivity. Therefore, the measures we were taking from the females in captivity were inconsistent and did not seem to represent what happened in free-living females. That’s when we decided to take the samples in the field and collect tissues so we had multiple measures of reproductive development in females at one time point in early spring.

What is the broader impact of your paper?

Time line of seasonal sympatry between migrant and resident juncos. Migrants are found breeding in the boreal forests of Canada and Alaska (orange range), whereas resident are found year round in the Appalachian Mountains (blue range). In the Appalachian Mountains, migrants and residents can be found in seasonal sympatry in winter and early spring. Figure modified from Fudickar et al 2016, originally designed by Jonathan Atwell.
Time line of seasonal sympatry between migrant and resident juncos. Migrants are found breeding in the boreal forests of Canada and Alaska (orange range), whereas resident are found year round in the Appalachian Mountains (blue range). In the Appalachian Mountains, migrants and residents can be found in seasonal sympatry in winter and early spring. Figure modified from Fudickar et al 2016, originally designed by Jonathan Atwell.

We believe that seasonal sympatry, while not studied until fairly recently, is incredibly common among migratory species. We know that climate change is leading to shifts in timing of migration and reproduction. Changes in timing could lead to secondary contact between young subspecies or species and provide new opportunities for gene flow in diverging populations. Alternatively, climate change could have diverse effects on timing, which could accentuate differences in timing and prevent interbreeding. Either way, diversity could be affected by climate change in systems that exhibit diverse migratory strategies and patterns of seasonal sympatry. It is important to understand the mechanisms that drive differences in timing first in order to predict how these populations could be affected in the face of a changing world. 

About The Author

How did you get involved in ecology?

During a family vacation in the western US when I was 13, my sister explained to me that Steller’s Jays were only found in the west; while similar to the Blue Jays I had grown up with they were a unique species. I became fascinated with diversity specifically in birds and decided right then I was going to pursue my curiosity about the natural world as a career. I received my Bachelor’s degree in Biology and Environmental Science and joined the Ketterson lab as a summer intern after my third year of college. This helped me narrow my interests in ecology and evolution questions. I then joined the Ketterson lab as a graduate student in 2013.

 What are you currently working on?

I started my post-doc in the Delmore lab at Texas A&M just a few weeks ago! I joined the Delmore lab because they are also interested in how animal migration can promote speciation, but with a focus on the genetic basis of migratory behaviour in young species and hybrid zones. Therefore, this experience will expose me to new techniques in genomics and bioinformatics to address the questions that have always driven my research interests.  I will be working on a project studying migratory gene expression across a hybrid zone in the Swainson’s Thrush to better understand how divergence in migratory behaviour might contribute to speciation.

Read the paper here and the plain language summary here. You can find Abigail on twitter @AbbyKimmitt.