Thierry Grandmont: Experimental stress in migration influences decision to breed or not

In this new post, Thierry Grandmont—a Ph.D. candidate at Laval University, Quebec, Canda—discusses with us his recently accepted paper, “Should I breed or should I go? Manipulating individual state during migration influences breeding decisions in a long-lived bird species.” He talks about the link between breeding and migration, challenges (and opportunities!) of conducting research during the pandemic, and how birdwatching was a formative experience.

Une version française de cet article de blog peut être trouvée ici

About the paper

Our paper revisits an experiment carried out more than a decade ago that aimed to experimentally demonstrate that a perturbation (deviation) during migration can have carry-over effects on reproduction. To do so, a team of biologists, including someone who would one day become my supervisor, captured greater snow geese—a long-lived bird species—during their spring migration. They kept a subsample of the females in captivity to mimic an environmental perturbation. While a first paper on the experiment was produced, showing that the captivity indeed decreased reproductive output the following fall, an important part of the available data (and the conclusions that could be drawn from the experiment) slipped through the cracks and remained to be analyzed.

Geese being released from the barn after handling. (credit: Thierry Grandmont)

All of this changed when the pandemic happened. With field work for my project being canceled, it was the perfect opportunity for us to blow the dust off this data and look for answers about the mechanisms underlying this reduced reproductive success. We combined telemetry and field monitoring from the main breeding colony data to contrast two possible mechanisms: a reduction in breeding propensity (the probability to initiate breeding) or a reduction in the current reproductive investment (by delaying arrival date, laying date or by reducing clutch size, hatching success). We showed that time spent in captivity reduced detection at the breeding site, our proxy of breeding propensity, while not affecting other breeding parameters. To our knowledge, this makes our study the first to experimentally demonstrate the role of carry-over effects on breeding propensity.

If this paper reveals the mechanism leading to reduced reproductive success following a perturbation in migration, our study design did not allow us to determine the process by which the decision to skip breeding is made. It remains to be investigated whether it is an acute stress response caused by captivity, inhibiting reproductive hormones, or a phenological delay that led geese to curtail their breeding effort.

About the research

The study of carry-over effects allows us to understand how animals respond to their environment throughout their life-cycle. Our study is one of the few to show that events during migration can not only reduce an individual’s reproductive success, but can possibly alter its decision to breed or not. Skipping the reproductive event is a well-known strategy in long-lived species encountering unfavorable conditions, thereby allowing them to save energy for the following year. However, there is little known about this reproductive parameter. This is partly because non-breeding individuals are often away from the breeding areas, making it difficult to estimate breeding propensity. Our study demonstrates how a perturbation far away from the breeding grounds can lead to this decision. In the current global context—with extreme weather events expected to become more frequent—our experiment gives an insight into long-distance migrants’ response to such events and highlights the importance of learning more about breeding propensity. Understanding the role of carry-over effects on breeding propensity is crucial, especially given the importance of this reproductive parameter on recruitment and demography.

An interesting aspect of our results is the fact that breeding propensity seemed to be higher during years of favorable environmental conditions in the Arctic. While the limited number of years in our study doesn’t allow us to draw conclusions from this, it seems that individuals can overcome the effects of the experiment when conditions are favorable. This indicates that, although the experimental perturbation affects the decision to breed, that decision is taken later on in migration. Telemetry technology in our study could not indicate when the decision to skip breeding is taken and, most importantly, under which circumstances. Hopefully, future studies using GPS technology should be able to complete this task given that GPS devices do not themselves reduce breeding propensity!

About the author

Myself and the first goose I banded. (credit: Thierry Grandmont)

Just like many ecologists, I imagine, I’ve always loved being in the wild and observing the natural world. However, the event that really confirmed my desire to study ecology is when my brother and I started birdwatching ten years ago now while we were still in high school. Since then, we have followed a similar path of studying bird ecology and watching birds in every minute of our spare time. It is this love for birds that pushes me to learn more about them and to try to protect them. This is also why I decided to start a Ph.D. this winter, investigating the impact of recreational activities on the behavior and reproduction of two bird species: the Barred Owl and the Wood Thrush, in Quebec’s provincial parks. Following this project will allow me to do what I love most about being an ecologist: be outside and observe birds, all while sharing my love for them with people I encounter.

If I have learned one thing from my master’s project, it is that working in this field requires a broad array of abilities and one cannot expect to excel in all of them. We all have our own strengths and weaknesses and being aware of those, asking for help when needed, and working with people with different strengths can only make us better ecologists.

Enjoyed the blogpost? Read the research here!

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