When Land Changes, Wetlands Provide for Insectivores

Debbie Blicher is senior producer of Ray Brown's Talkin' Birds. 

As human population increases, we need more food. To increase food production, we often claim wild grassland and make it into farmland, which means treating it with pesticides, fertilizers, or both. When we treat the land, insect populations diminish and become less diverse, which affects all animals that feed on those insects—including birds. In fact, aerial insectivores—birds that hunt for insects on the wing—are declining across North America. A forthcoming study in The Condor: Ornithological Applications takes a closer look at precisely how Tree Swallows are affected by agricultural practices. 

In 2012 and 2013, Chantel Michelson, Robert Clark, and Christy Morrissey—all researchers at the University of Saskatchewan—monitored Tree Swallow nest boxes at agricultural and grassland sites. To find out what the birds were eating, they collected blood samples and measured isotope ratios. (Measuring certain chemical elements in tissues is an established way to glean diet data.) Tree Swallows usually eat aquatic insects. Since water receives pesticide runoff, the researchers expected that the swallows living near cropland would be forced to eat more land-based insects than the swallows living near wild grassland.

What the researchers found surprised them. The Tree Swallows at both locations ate more aquatic insects than expected. In fact, in 2012 the birds at agricultural sites ate more aquatic insects than the birds at the wild sites. In other words, even though the agricultural sites had been treated for farming, the swallows still preferred the insects there to the insects living by the water. The researchers think this might mean that wetlands provide some cushion for birds against changes brought about by farming.

One other surprising finding: The swallows living at agricultural sites weighed less than the swallows living in wild grassland. They were healthy, just smaller, which might mean they had a harder time finding food. But in both locations, adult swallows seemed to rely on aquatic insects for themselves and their young, illustrating the importance of wetlands for bird survival.