White-crowned sparrows exposed to the insecticide imidacloprid eat less, lose weight and delay their springtime migrations— critical biological interruptions that could be contributing to the precipitous population declines observed among many North American migratory bird species, according to a new study.
The results published in the Sept. 13 issue of Science show that even in sublethal doses, consumption of the neurotoxic insecticide causes birds to linger significantly longer at stopover sites to forage and potentially recover from imidacloprid intoxication before moving on. Such behavior can seriously affect the birds' overall chance of survival.
The authors suggest that the findings are perhaps the first direct evidence of the grave impact of the agricultural use of neonicotinoid pesticides, including imidacloprid, on traveling songbirds that rely on agricultural habitats for their survival.
Neonicotinoids, also known as neonics, are a class of neurotoxic insecticides chemically similar to nicotine. Used globally since the mid-1990s, neonics have been linked to a number of adverse ecological effects, most notably colony collapse disorder in both wild bees and domestic honeybees.
However, because the insecticides were thought to pose a low risk to vertebrates and to be far less dangerous than other types of organophosphate and carbamate insecticides, neonicotinoids — specifically imidacloprid — have become the most widely used agricultural insecticide in the world.
Despite this, recent research has begun to provide evidence that suggests neonics may also have significant negative impacts on a number of vertebrate species as well, including birds.
"Birds have been overlooked in the regulatory reviews and re-evaluations for the safety of this chemical," said study co-author Christy Morrissey, an ecotoxicologist at the University of Saskatchewan.
Farmland bird species that use agricultural spaces as habitats or for migration stopovers are routinely exposed to agricultural pesticides, including various forms of neonics through sprays, treated soils, contaminated water or by eating imidacloprid-coated seeds. This is particularly true for North American migratory species whose spring migration often coincides with the sowing of neonic-treated agricultural crops across North America. Nearly three quarters of the more than 200 farmland-dependent bird species in North America — many of which are seed-eating songbirds — have experienced serious population declines over the past 50 years.
To learn more about the possible impact of neonics on these species, Margaret Eng, the University of Saskatchewan ecotoxicologist who led the study, and her research team tracked individual white-crowned sparrows — a common North American songbird — experimentally exposed to small, yet field-realistic doses of imidacloprid during spring migration across northern North America.
Previous research by Eng and her team evaluated the effects of sublethal imidacloprid exposure on the migratory behavior of captive white-crowned sparrows and discovered its role in suppressing feeding behavior and rapid weight loss, and observed that it caused temporary disorientation in the birds.
"Studying bird migration — particularly in small birds — is hard because most of the tracking technology uses larger tags that songbirds cannot carry. Birds also fly long distances and are hard to see as they migrate at night," said Morrissey. "So, it's really only in recent years that we have started to understand more about the migration patterns of songbirds using new technologies."
Technologies like the Motus Wildlife Tracking System, an automated radio telemetry network capable of tracking small, individual animals over thousands of kilometers, allowed Eng, Morrissey and others to expand upon the previous research to investigate the consequences of neonic exposure on small birds at larger, ecologically relevant scales.
Generally, when migrating birds visit stopover sites along their journey, they eat rapidly, refueling their energy stores for their next leg of sustained flight. They do not stay long, as dallying increases their chances of becoming prey or being caught in inclement weather. What's more, birds that arrive in their wintering areas late generally obtain poor nesting sites, breed later and produce fewer, less-healthy offspring.
The sparrows exposed to concentrations of imidacloprid no greater than what a bird would realistically consume if they ingested a few neonic-coated seeds at a stopover site greatly delayed their departure — in some cases by more than three days.
The chemical's anorexic effect and the associated rapid loss of body fat required the sparrows to forage longer to restore their greatly depleted fuel stores and to recover from the imidacloprid's neurotoxic effects before moving on.
"Our previous study was looking at migratory orientation in captive caged trials. We saw that birds became disoriented and expected that might happen in the wild too. But here, we assessed migratory navigation and did not see differences in navigation ability of free-living birds," said Morrissey.
"We expected that the birds might fly disoriented as seen in our previous captive trials," added Morrissey. "But in the real world, birds must make departure decisions to fly only when they have sufficient body fat to make the journey."
The heavy use of neonics — particularly as seed treatments — throughout migratory routes in the United States and southern Canada means that individual birds may suffer repeated exposure at successive stopover sites, thus amplifying migration delays and their population-level consequences, warn the researchers.
In addition to imidacloprid and other neonics, hundreds of other pest control products are on the market and are used widely in environments spanning the globe. Morrissey noted that most are not tested for their safety to songbirds.