The Birds and the Bugs
Did you know that swallows are facing severe population declines around the world? Aerial insectivores, which include swallows, swifts, flycatchers and nighthawks, have declined by 59% in Canada since 1970. Barn Swallows, which are federally listed as threatened, have declined by 76% in Canada. These incredible aerial acrobats have been a focus for A Rocha’s work on Species at Risk since 2014, when the Conservation Science team in BC started monitoring Barn Swallow nest success in the Little Campbell River watershed.
But why the steep declines? The common thread among all aerial insectivores is their food source: flying insects. And the global trend for insect populations is not good. Although most scientific studies of insect declines have come from Europe, the data is worrying. Car ‘splatometer’ tests revealed 50% fewer insects squashed on car license plates in England between 2019 and 2004. And a recent study found an 84% decline in mayflies around Lake Erie from 2015 to 2019.
Many of us may wonder why this is such bad news. After all, insects can be a nuisance when we want to enjoy a summer evening outside! But perhaps lesser known is the massive role insects play in supporting the world’s biodiversity, from critical pollination services (around 65% of the world’s flowering plants depend on insects for pollination) to providing a food source for countless wildlife species. Flying insects are sometimes referred to as “aerial plankton” because of their vital role in the food chain. There is a whole airborne ocean of insects above us that we can’t see and rarely even consider except when we get a mosquito bite. And different birds (and even bats) are exquisitely adapted to exploit different levels and seasonal fluxes of this ocean.
So, what is happening to our aerial plankton? Many insects are declining because they have an aquatic stage that is very sensitive to changes in water quality. Polluted water bodies mean fewer emerginginsects, which in turn limits food supply for insectivores. Climate variation can affect the seasonal timing of insect availability, leading to a “mismatch” between avian predators and insect prey. And of course, pesticides are a well-known culprit of large-scale insect die-offs. Pesticide use is often viewed as necessary for the flourishing of crops and therefore, human food. However, their widespread use means less food for birds (along with a myriad of other problems), which are nature’s inadvertent crop defenders! In fact, a single Barn Swallow can consume a whopping 850 insects per day.
As A Rocha completes its eighth year studying Barn Swallows in the Little Campbell watershed, one looming question is why they seem to thrive in some places yet have completely disappeared from others? An all-too-common story among A Rocha’s neighboring landowners is that Barn Swallows are now absent from sites where they used to breed in large numbers. Even over a relatively short eight years, we have noticed substantial declines in breeding activity at many of the sites we monitor. A logical next question becomes, what is the availability of insects around these sites?
Luckily, we have been able to partner with a much larger study being conducted by professor Peter Dunn from the University of Wisconsin Milwaukee. Prof. Dunn is coordinating the sampling of aerial insects at more than 20 sites across North America with the goal of understanding how food abundance for swallows varies seasonally and geographically.
Sampling involves the use of a Malaise trap, essentially a large mesh tent, which funnels insects up to the top where they become trapped in a jar of ethanol and preserved.
In 2020 and 2021, we set up Malaise traps in two large fields that are known swallow foraging areas surrounding A Rocha’s Brooksdale Environmental Centre. In 2021, we added a third trap in nearby Campbell Valley Park. Insects were sampled three times throughout the summer, roughly corresponding to swallow egg-laying, hatching, and fledging dates. After being collected from the field, each insect specimen is identified, dried, and weighed. The data is sent away at the end of each season to be analyzed along with data from all other sites.
So far, in our neck of the woods we’ve found greater insect biomass at the two Brooksdale sites than at the park. Orders Diptera (flies), Lepidoptera (butterflies and moths), Coleoptera (beetles), and Hymenoptera (ants, bees, and wasps) make up the majority of the insect biomass at all sites. Incidentally, Barn Swallow diets mostly eat flies, though they also consume beetles, wasps, bees, winged ants, and true bugs along with a few moths, damselflies, and grasshoppers. On its own, this data can tell us a bit about what’s available for swallows here in the Little Campbell, but the beauty of large-scale partnerships is that local knowledge can be shared among many scientists, leading to a better understanding of broader biodiversity trends.
Stay tuned as this project reveals new continental-scale insights that could help us better protect the wildlife we coexist with locally, even those pesky flies!