RESOURCES:
University of Michigan School for Environment and Sustainability
Resource declines shape phenological and morphological responses to climate change
TRANSCRIPTION:
Caroline MacGregor: You're listening to 89.1 WEMU. I'm Caroline MacGregor. And with me today is Charlotte Probst. She's the lead author of a new study and a doctoral student in the U of M School for Environment and Sustainability, better known as SEAS. And today, Charlotte is going to be discussing what she and others have termed the crashing insect populations, which are resulting in smaller tree swallows that reproduce less. This is fascinating, but it's also concerning. Tell me a little bit about how the study got underway and what you discovered about these tree swallows.
Charlotte Probst: Yeah, absolutely! So, we first were interested in looking at this population in particular is because, a couple of years ago now, my advisor, Brian Weeks at the University of Michigan, found that in a study of migratory birds passing through Chicago across all sorts of different species through time, they were declining in size. And this is a finding that's been replicated in different groups of animals in different areas, so, like birds in the Amazon also seem to be declining in size. And so, there's this pattern, but we weren't entirely sure what is driving it or what the consequences of those size declines are. So, initially, one of the main hypotheses about why these birds might be declining in size is that it could be linked to climate change. So, it seemed like this might be linked to temperature. And one possible idea was that because the environment is getting hotter. Maybe the birds are getting smaller because it might be advantageous to have a larger surface area-to-volume ratio. So, basically, if you're smaller, you have a large surface area. And so, you can dissipate more heat. And so, one possible idea was maybe this has to do with rapid advantageous evolution in response to climate change, which is maybe a good thing. But another possibility that had always been discussed but not tested as much was, well, we know climate change is happening, but we also know that through time, there have been a lot of resource declines, including in insects, which is a primary food source for a lot of different birds. But it's just a lot harder to test that because, unlike temperature data that you can just download from anywhere, there's a lot less long-term monitoring of insect populations. And so, that's what made this study with the tree swallows so special. So, we partnered with the Long Point Bird Observatory, which the sort of flagship operation of Birds Canada, and they've been monitoring tree swallows at three different sites across Long Point, which is a peninsula that sticks out into Lake Erie, for the past almost half-century now. So, they started collecting this data in the 60s. And then, starting in 1977, they started collecting daily insect biomass during breeding season. So, we had this incredible data set of these birds, their size, their reproductive success through time, we could get at their survival because they would band the individuals every year, so we could see if they came back or not. But then, this was paired also with that really detailed insect data, so it was a really incredible effort on the part of a lot of volunteers and staff over the years that made this possible.
Caroline MacGregor: Yeah, it is disturbing. So, I know you focused on the tree swallow, but is this birds across the board here in North America?
Charlotte Probst: Yeah. So, birds across North America seem to be declining in size. But we focused on, yeah, the tree swallows, and part of the reason that makes them an interesting species to study in this context is they are an aerial insectivore, which means they fly around catching insects on the wing. And they are really dependent on particularly insects and also especially aquatic emergent insects. So, think about mayflies, mosquitoes, midges, dragonflies.
Caroline MacGregor: Right, right.
Charlotte Probst: And aerial insectivores are also a group that are in pretty rapid decline across all of North America. They're one of the fastest declining groups of birds. And it seems like that might be tied to potentially declines in insect abundance. So, they were an especially important species, both from a conservation standpoint to study, but also because they were species that we would expect to be particularly impacted by this.
Caroline MacGregor: It is quite alarming to think that their numbers are declining and their sizes are changing. But what are the implications with regard to when birds are smaller and they depend on these insects? When it comes to reproduction, what are we seeing here? Because I'm sure there is some fallout in this area.
Charlotte Probst: Absolutely! So, one of the really interesting findings from our study was we could look to see if tree swallow survival was impacted by changing insect abundance and if tree-swallow reproduction was changing due to decreased insect abundance. And what we found was that they didn't have lower survival through time, so it's not that the birds were dying more, but it's that the birds were reproducing less. So, tree swallow clutch size, the number of eggs that they lay in a given breeding season, is really closely tied to the amount of insects available during that egg formation period.
Caroline MacGregor: While it appears that climate change is behind this, would you agree that it's not a certainty yet with regards to the size of the tree swallows being smaller?
Charlotte Probst: So, climate seems to maybe be linked with declining body sizes, but in our study anyway, what we saw was that, in this population, climate didn't seem to be driving the changes in tree swallow size, and it was basically entirely driven by those declines in insect abundance. And we also saw that it wasn't that climate change was impacting the insect abundance, so the insects weren't declining because of climate change. It's something else that's changing through time that was probably leading to those declines.
Caroline MacGregor: And according to this study, it would appear that birds would want to--or should want to--synchronize their breeding with peaks in insect abundance. So, given the decline in insect populations, clearly, that's not happening the way it should do.
Charlotte Probst: Yeah. So, a main consequence of climate change for birds and other animals as well is something called phenological mismatch, which is exactly what you were describing. So, as springs are getting warmer, then insects are emerging earlier and earlier in the spring. And so, in order to synchronize their breeding with that peak and insect abundance, in order to capture the most energy and the most resources for rearing their young, birds should want to also try to be shifting earlier. And that's the thought. And so, there's a lot of studies showing that those dates are becoming increasingly decoupled, that birds aren't shifting their breeding fast enough in order to keep pace with the earlier emergence of insects. And generally, this is thought to be a problem. But what we were finding was that as insect abundance decreases, essentially the benefit you get by synchronizing breeding with the peak of emergence is going down because there's just fewer resources that you'd be capturing. And also, as you are emerging earlier and earlier in the spring, you're at increased risk of experiencing a cold snap. So, you can imagine we have these in Michigan a lot where we'll have some warm days in maybe March, but then, it'll start snowing, right? And so, you might have insect emergence, but also it's really cold. And so, there's a cost to breeding that early. And so, what we were finding was that actually there are scenarios where it's actually the best of the worst situation, if you want to think about it that way, for a bird to be mismatched and to breed later in the season, even if it misses out on that peak of insect abundance, because it's just not quite worth it anymore. It really underscores that climate change is something that's happening, but also, it's happening at the same time as these massive declines in resources and massive biodiversity losses. And we should expect those things to interact with each other. And we really need to understand both of them if we want to understand how animals are responding to these changes in their environments.
Caroline MacGregor: So, if you were to mathematically measure the mismatch between the timing of tree swallow breeding and peak insect emergence, would this add to weeks, days, as far as a difference since it's been tracked?
Charlotte Probst: Yeah, that's a good question! So, tree swallow breeding is advancing by about one day per decade, but insect emergence was shifting earlier by about six days per decade. And so, because of that, there's this mismatch that's increasingly growing through time.
Caroline MacGregor: I know there was some mention in the study also about pesticide use back in the, I believe, it was the 90s. Tell me about this if you would.
Charlotte Probst: Yeah, absolutely! As I mentioned earlier, we saw that insects are declining at Long Point, so peak insect abundance in general is about 60% less than it used to be. And we saw that it looked like these declines in insect abundance really started to happen around the 90s, which corresponds with the time that pneumonicotinoids, which are commercial pesticide, were released to the market. So, it's just sort of an anecdotal finding at this point, and we want to dig in more to look at if there are specific connections between neonicotinoid use and these declines in insect abundance and then declines in bird size. But it seems like that could be a potential factor, especially because neonicotinoids can run off into waterways where they can be pretty harmful to the aquatic insect larvae that are there, which then results in lower emergencies of these aquatic insects that are really important for tree swallows and other birds.
Caroline MacGregor: In the case of tree swallows, are there any reasons for cautious optimism here?
Charlotte Probst: So, the fact that, this system, it looks like insect declines more than climate change are driving these declines in body size and in reproduction. In some ways, it is sort of scary because solving insect declines is a hard problem, but in many ways, they did leave us feeling optimistic because climate change is a global issue, and what people are doing in the United States or anywhere else around the world is going to affect our birds here in North America. And so, it requires a concerted global effort to solve, and that will take decades. But insect declines are something that can be addressed at a much more local level and also can have turnarounds in sometimes as little as five years. And so, if you're an individual or a city or a state and you want to do something to help the birds that are breeding in your community, addressing things locally, like increasing native plants, improving water quality, reducing pesticide runoff, can have really rather fast and tangible and really important benefits for the bird community.
Caroline MacGregor: All right! I've been speaking with Charlotte Probst. She's the lead author of a new study, and she's also a doctoral student at U of M School for Environment and Sustainability. And we've been discussing a big drop in insect populations that have resulted in smaller tree swallows. Charlotte, this has been a fascinating discussion! Much more to come, and it will be interesting to see where the research goes. But thank you for joining us today!
Charlotte Probst: Absolutely! Thanks, Caroline!
Caroline MacGregor: This is 89.1 WEMU Ypsilanti.
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