Canada's birds have been shrinking since the 1970s as insect populations collapsed by more than 60%, scientists found

If you’ve noticed fewer bugs splattered on your windscreen on your summer road trips, you're probably not imagining it, and birds are probably feeling that loss more than we are.

A new study, ‘Resource declines shape phenological and morphological responses to climate change,’ led by the University of Michigan, found insect numbers at Canada’s Long Point Bird Observatory have declined by over 60% since the 1970s. As a direct result, tree swallows there are now smaller and produce fewer offspring than previous generations. The research, published in the Proceedings of the National Academy of Sciences, is among the first to examine how biodiversity loss and climate change together are reshaping wild birds, and the researchers say it could point to new ways to help them.

Tree swallows aren't struggling alone. The State of Canada's Birds report shows that aerial insectivores, the birds that catch insects in flight, have declined 59 percent across Canada since 1970, the fastest decline of any bird group in the country. The new University of Michigan study sheds light on why. It’s one of the first to detail how a real insect crash in the wild reshapes a bird species’ body size and reproduction over time.


Why tree swallows are the ultimate insect thermometer
Tree swallows are a rapidly declining species that feed almost exclusively on flying insects, so they are a near-perfect indicator of insect abundance. Tree swallow clutch size is tightly linked to the available insects, noted Charlotte Probst, the study’s lead author and a doctoral student at U-M’s School for Environment and Sustainability. Less insects mean smaller birds plus smaller broods.

The researchers made these discoveries using a rare, decades-long dataset: tree swallow records from 1969 to 2024 and insect abundance data from 1977 to 2011, all collected at Long Point, one of the oldest continuously operating bird observatories in North America. Founded in 1960, it has relied on generations of staff and volunteer scientists to keep the records going.

The researchers were able to draw these conclusions thanks to a rare, decades-long dataset: tree swallow records from 1969 to 2024 and insect abundance data from 1977 to 2011, all collected at Long Point, which, according to Birds Canada, runs the longest continuously operating bird migration monitoring program of its kind in the Americas. It's been gathering data since 1960.

This isn't an isolated story either. The landmark study, ‘Decline of the North American avifauna,’ led by the Cornell Lab of Ornithology and published in Science, found that North America has lost nearly 3 billion birds since 1970. Among the hardest hit are aerial insectivores, birds that catch insects on the wing.

In the Science paper, the authors examined long-term population trends since 1970 for 529 North American bird species and compared them to species that were stable or increasing. The authors found that declines were most widespread in grassland, Eastern forest, and aerial insectivore birds, with the most common species suffering the greatest losses, rather than being offset by gains elsewhere.

It's not just the heat, it's what's disappearing alongside it
For years, climate change has taken the lion's share of the blame for messing up birds' breeding schedules. Warmer springs mean insects are emerging earlier, but birds’ internal clocks are slower, creating a “phenological mismatch”. At Long Point, that mismatch has increased by more than three days per decade since 1977.

But here’s the odd thing. As the number of insects crashed, that gap in timing actually became less important. According to Brian Weeks, the study's senior author and an associate professor at U-M, that's because there just aren't enough insects left anymore for an early or late hatch to make much of a difference either way. In other words, you can’t fully understand what climate change is doing to wildlife without also considering how much biodiversity has already been lost.

Weeks, who works on climate and ecological change at Michigan, has argued that the key issue isn’t just a shifted food peak, but a shrinking food base itself. In his recent research on tree swallows, he found that once insect abundance drops far enough, the birds lose the luxury of fine-tuning timing; a mismatch matters less when there is just less to eat.

A likely culprit hiding in plain sight
So what is really killing the insects? The researchers say it doesn’t seem to be a case of warming temperatures. Instead, they point to timing: insect declines began to rise sharply in the 1990s, just when neonicotinoid pesticides were used widely in agriculture. These chemicals are particularly harmful to insects that develop in water, such as the midges and mosquitoes that make up a large part of a tree swallow's diet.

This is in line with other research. When scientists exposed aquatic insect communities to concentrations of neonicotinoids similar to those measured in real-world waterways, they found a sharp drop in abundance and diversity, according to a 2021 study by Barmentlo and colleagues in the Proceedings of the National Academy of Sciences. And that gives credence to the idea that pesticide runoff, not just a warming planet, might be starving birds like tree swallows of food.

Why this actually counts as good news
Pesticide use can be tackled locally and relatively rapidly, unlike climate change, which needs global co-operation and policy changes over decades. This is a problem that could be fixed on a short time scale, without the whole world agreeing on anything first, Probst said.

The bigger takeaway for all of us
The study serves as a reminder that nature doesn’t usually fail for just one reason. Climate change, habitat loss, and chemical pollution are interwoven, and the only way to disentangle them is through the sort of patient, long-term monitoring that produced this research in the first place. Work, Weeks says, is increasingly at risk as public funding for long-term data collection evaporates.

So next time you notice your porch light isn’t swarmed with moths like it once was, know it’s not nostalgia talking. It is a tangible change, and birds like the tree swallow are already paying the price.