In the rural Midwest, there used to be a specific type of late afternoon when barn swallows would cut low across a field, bank hard at the fence line, and then sweep back up into a pale sky. Once, it was unremarkable. It was the kind of thing that you didn’t realize existed until one summer.
Quiet and easy to ignore, that absence proves to be a major issue. In North America, swallows and other aerial insectivores—birds that capture flying insects in midair—have been in decline for many years. However, the scope of what has transpired is only now fully apparent. Over the past 40 years, the Barn Swallow’s population has decreased by about 95% throughout the continent. Since 2005, the number of Cliff Swallows in some monitored areas has decreased by an estimated 98%. These species are not on the periphery. These were common birds, the kind that spent summer evenings swooping over baseball diamonds and building nests in eaves.
It is necessary to follow two threads simultaneously in order to understand why this is occurring, and neither is straightforward. The first is the decline in insect populations. Flying insect abundance at the location decreased by more than 60% between 1977 and the present, according to research from Canada’s Long Point Bird Observatory, which has been monitoring tree swallows since the 1970s. The decline was almost exactly tracked by the birds. They shrank in size. They did not lay as many eggs. They did not raise as many chicks. Food availability and reproductive success were directly and quantifiably correlated, season by season, rather than being subtle or indirect.
The story becomes awkward in the second thread, which deals with pesticides. The steepest portion of the insect decline appeared to accelerate in the 1990s, coinciding with the widespread use of neonicotinoid pesticides, according to researchers examining the Long Point data.
These substances are incredibly effective at eliminating insects, and even minute amounts that make it to wetlands can wipe out the aquatic larvae that swallows rely on for breeding, such as midges, mosquitoes, and tiny flies. The timing is hard to ignore, but it’s still unclear how much of the overall regional decline can be directly linked to neonicotinoids.
Additionally, there is a mismatch issue that is more difficult to resolve. Swallows use a strategy that made sense when springtime rhythms were reasonably predictable: they time their breeding to correspond with peaks in insect emergence. Insects have emerged earlier due to climate change, but birds’ internal calendars haven’t changed as quickly.
Since 1977, that difference has grown at Long Point by more than three days every ten years. This is especially concerning because, even in cases where birds are able to breed at the appropriate time, the nutritional benefit is much less than it was in the past. The peak is no longer truly a peak. It’s more of a slight increase in a pool that is already empty.
There are unique challenges in the Midwest. A large portion of the hedgerow and wetland habitat that formerly supported dense insect communities close to breeding sites has been lost due to agricultural intensification. As long-distance migrants, swallows travel to Central and South America every winter, so events on one end of their journey may jeopardize their survival on the other. A bird may not return at all if it finds degraded habitat upon arrival or struggles to gain weight prior to migration. Research on identifying the actual chokepoint in the annual cycle is still ongoing.
Some of this might be recoverable. In contrast to climate change, researchers studying the Long Point population have cautiously noted that insect decline might be something that can be addressed locally without requiring broad global coordination. Theoretically, the food collapse causing the bird decline could be reversed by lowering pesticide exposure near wetlands, reestablishing native plant cover, and reducing activities that remove insect habitat from farm edges. It’s another matter entirely whether there is the will to do that on a large scale.
For now, those summer afternoons feel a little strange in ways that are difficult to describe. You can still find swallows because they haven’t entirely vanished. However, the number has significantly decreased, and anyone who spent time outdoors in the rural Midwest thirty years ago would probably tell you the same thing without the need for a scientific study. Sometimes the most crucial information is just what you stopped seeing.
