Known for their metallic blue, white and brown plumage, tree swallows are in steep decline in eastern Canada, particularly in the agricultural regions of southern Québec. One possible cause is the intensification of farming practices, which is believed to have a detrimental effect on reproduction. But in what way?
To learn more, a team of scientists from the Université de Sherbrooke comprising Marc Marc Bélisle, Fanie Pelletier and Dany Garant set out in 2004 to track swallows nesting on some forty farms in Montérégie and the Eastern Townships.
Over 16 years later, the data show that these birds have a less abundant food supply in agro-intensive areas. In addition, their primary diet – flying insects such as flies, mosquitoes and horseflies – is more contaminated in areas dominated by corn or soybean monocultures, which employ a range of chemical inputs. With less prey to feed on, swallows lay fewer eggs, and their chicks have lower growth and survival rates.
Comparing different crops in Sherbrooke and Montréal, it became clear that swallows are indirectly exposed to a large number of pesticides: around half of the “insect balls” the parents fed to their young were found to have traces of at least one of the 47 chemical compounds being tested for.
The pesticides identified included “bee-killing” neonicotinoids used to treat corn and soybean seeds. These have recently been replaced by other similar compounds, diamides. Could these insecticides have an impact on avian reproduction? For the moment, the scientists do not know if the quantities found in the insects are sufficient to directly affect the swallows’ reproductive performance.
The research team also found that only 30% of adult swallows return to lay eggs in the same location the following year. Moreover, less than 2% of their chicks return to breed in the habitat where they were born. It remains to be seen whether the birds are dying or fleeing an area with a depleted food supply in search of a better breeding site.
Sources
Garrett, D.R. et al. (2022). Combined influence of food availability and agricultural intensification on a declining aerial insectivore. Ecological Monographs, 92(3). https://esajournals.onlinelibrary.wiley.com/doi/abs/10.1002/ecm.1518
Garrett, D.R. et al. (2022). Interacting effects of cold snaps, rain, and agriculture on the fledging success of a declining aerial insectivore, Ecological Society of America Journals, 32(7). https://esajournals.onlinelibrary.wiley.com/doi/10.1002/eap.2645