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Farm-dwelling tree swallows are feeding their chicks pesticide-laced prey

On agricultural landscapes, pesticides travel through food webs and can negatively affect local wildlife populations. However, few studies have considered the long-term, cumulative effect of multiple pesticides on avian species, and most are carried out with treated food in a laboratory setting, not in the living landscape. Other studies have analysed pesticides in the soil and water, but not in the food web itself.

A team of researchers at the Université de Sherbrooke set out to change that with a massive study, published this week in the Ecological Society of America’s journal Ecological Applications, of wild swallows living in an agrarian landscape in Québec.

To understand how pesticides get into food webs, the team spent six years studying tree swallows on 40 farms and analysing the insects that tree swallows bring to their nesting young. The study area spanned the spectrum of agricultural practices and landscapes across 10,000 square kilometres (3,861 square miles). Each farm was home to 10 tree swallow nest boxes. This information gave the team insight into the effect of pesticides exposure on bird populations in the living world. They found evidence of at least one pesticide in almost half of the samples they tested.

Fanie Pelletier, one of the researchers explained: “We studied tree swallows across a very large gradient; this is one of the largest long-term studies on pesticide presence,” Pelletier said. “Every farm was selected to represent the diversity of agriculture. Some of them are in very intensive agricultural areas – just a desert of corn and soy. And others are on smaller pastures with cattle.”

Rather than sampling the ground, the water, or other insects found on the farms, the team wanted to test the actual insects the swallows were feeding their young. They kept watch on the nest boxes and sampled insect boluses – the ball of insects parents bring to feed their chicks – from each nest. But, in order to protect the chicks and keep their methods from affecting the health and survival of the nests, they did not sample more than three boluses from each nest in one day.

“We didn’t want to be the ones picking the insects; we wanted to look at the exact insects the parent brought to the chicks, and we did not want to have sacrifice the chicks to test their stomach contents,” Pelletier said. “This study is still an indirect proxy of what they have in their bloodstream, but this is a large-scale representation of the extent of pesticides exposure.”

The team found evidence of at least one of the 54 chemicals and pesticides they were tracking in 46% of boluses.

“We found pesticides everywhere, even in the less-pesticide-intensive farms,” Pelletier said.

Despite the omnipresence of pesticides, they found no direct effect of pesticides on nest survival, reproduction or nest success. However, Pelletier says, that is likely due to the fact that they could not directly quantify how much of each pesticide the chicks were ingesting; additionally, they did not account for other environmental and physiological factors that can affect a chick’s health and cause it to be more prone to pesticides’ effects.

“We sampled only a few boluses; that is very far from the total amount of pesticides to which they are exposed during nesting,” Pelletier said. “In order to really quantify the effects, you need to be able to measure how much pesticide they actually eat. This study was an enormous first step; you have to start somewhere.”

Journal article: Poisson M-C, Garrett DR, Sigouin A, et al. 2021. Assessing pesticides exposure effects on the reproductive performance of a declining aerial insectivore. Ecological Applications. doi.org/10.1002/eap.2415

Header image: Tree swallows, as insectivores that tend to favour farms as habitats, may be particularly susceptible to pesticides. Credit: Andréanne Lessard.