Since the 1970s, the amount of insects at Canada's Long Point Bird Observatory has dropped by more than 60%, according to a new study led by the University of Michigan. Because of this, today's birds are smaller and facing greater challenges to their breeding success compared with previous generations.
The researchers focused on tree swallows, a rapidly declining bird species that feeds on flying insects.
"Tree swallow clutch size is really tightly tied to insect availability," said Charlotte Probst , lead author of the new study and a doctoral student in the U-M School for Environment and Sustainability, or SEAS . "When there's fewer insects available, the birds are smaller and the birds also produce fewer young."
The study also integrates climate data, making it one of the first to consider the role of resource availability alongside climate change in understanding how these pressures are reshaping bird biology. The results show that climate impacts cannot be fully understood without considering biodiversity loss. In the case of tree swallows, finding ways to combat that loss outside of solving climate change could be reasons for cautious optimism, the team said.
"I think this work highlights the significance of biodiversity loss and the complexity of natural systems. We can often lose sight of that and get focused on single processes like climate change," said Brian Weeks , an associate professor at SEAS and senior author of the new study.
"It's clearly important to contextualize climate change within a broader understanding of how humans are altering the environment. In this case, climate change is one thing, and it's shifting the system. But it's not the biggest thing and its consequences cannot be understood without also understanding biodiversity loss."
The study, supported by the David and Lucile Packard Foundation and the Doug and Nancy Schrank Family Fund, was published in the Proceedings of the National Academy of Sciences.
Phenological mismatch
The populations of flying insectivores, including tree swallows, are declining across North America. One factor that's suspected to be limiting their reproductive success is a mismatched timing between their breeding and when insects emerge for the season.
"Breeding is a really energetically intensive part of the annual cycle for birds. It takes a lot of energy to make eggs and it takes a lot of energy to rear young," Probst said. "In general, birds should want to synchronize their breeding with peaks in insect abundance."
When that timing is off, it becomes what's known as a phenological mismatch and these mismatches are one of the major outcomes of climate change. At Long Point, peak insect abundance happens in May, not long after the weather becomes warm enough for the insects to launch into their active lifecycles. But as winters trend warmer, insects are emerging earlier at a rate that's faster than the birds are tracking it.
"People have known about mismatches in important events for decades and they are very worried about it, understandably so," Weeks said. "Our study shows that you really can't understand the consequences of that without also understanding the context from a biodiversity decline perspective."
The team found that the mismatch between the timing of tree swallow breeding and peak insect emergence has been increasing by more than 3 days per decade since 1977 at Long Point. But, because of the plummeting insect abundance, the costs of this mismatch are actually declining through time.
To understand how disappearing insects could dampen the mismatch impacts, consider a year when peak insect abundance occurs particularly early. This also increases the likelihood of the birds experiencing a cold snap that could spell disaster for their nests, Probst said. Were things the way they were in 1977, the risk might be worth it for the nutritional reward, but that reward is much, much smaller now.
"Now, when insects emerge, it's half as many insects and they emerge in a much flatter process, so you don't get a huge pulse at the beginning," Weeks said. "So the fact that those dates are not coinciding as closely anymore, it doesn't really matter as much."
Fully characterizing what's behind the insect population collapse will require further study, but the decline does not appear to be the result of rising temperatures, the researchers said. They did note, however, that the declines accelerated in the 1990s, which coincides with a surge in the use of neonicotinoid pesticides.
These pesticides are incredibly effective and even small amounts that make it into wetlands could be devastating to the aquatic larvae of insects in the tree swallow diet, including midges and mosquitos, Probst said.
"Although insect decline is also a really complicated problem to solve, in our system, it appears to be something that can be addressed at a very local level," she said. "This is a problem that we can fix on a short timescale and without needing to have the entire global community come together to do something like climate change."
Big data for small birds
The team's findings are based on analysis and modeling using data that exists thanks to the foresight of Long Point Bird Observatory, the oldest continuously operating bird observatory in North America. Under the leadership of the late David Hussel, who helped found the observatory after earning his doctorate at U-M, its staff and volunteers started recording data for the birds and the insects decades ago. The team had access set to tree swallow data sets ranging from 1969 to 2024, while the insect data covered 1977 to 2011.
"Basically, the people at Long Point did an incredible amount of work and we're really lucky to have been able to collaborate with them," Probst said.
It was essential to have robust data covering such a long timespan to understand the relationships between climate, food abundance and what's happening to a single bird species at a particular location, Weeks said.
"Another main takeaway of this is the value of long-term, sustained data collection in an era when there's a rapid reduction in public investment in efforts like this," he said. "When you're trying to model complex relationships, you need a whole bunch of data and it typically takes an effort beyond what an individual person or lab can do."
The team's U-M cohort also included SEAS professor Inés Ibáñez, postdoctoral researcher Scott Yanco, master's student Isaiah Clark and undergraduate researcher Mark Ziebell. Co-authors Matthew Fuirst and Stuart Mackenzie from the Long Point Bird Observatory rounded out the full collaboration.
