Songbirds exposed to imidacloprid, a widely used neonicotinoid insecticide, exhibit anorexic behavior, reduced body weight and delays in their migratory itinerary, according to a new study. This is perhaps the first direct evidence of a mechanistic link between the pesticide and declining migratory bird populations. The results suggest that, even in tiny sublethal doses, the presence of these neurotoxic compounds at critical stopover sites refueling birds visit on their cross-continental springtime journeys could be contributing to the overall population declines observed among many migratory species. Neonicotinoids are the most widely used class of agricultural pesticide. However, while the controversial neurotoxic insecticide is advertised to pose a low risk to vertebrates, a growing body of evidence has shown that neonicotinoids may have significant negative impacts on a number of species, including birds. This research has suggested that birds who use agricultural environments as habitats or for foraging stopovers during migration are routinely exposed to neonicotinoid pesticides. Many of these species are also undergoing precipitous population declines. Despite this association, the overall influence of neonicotinoids on wild migratory songbirds remains virtually unknown. Building on previous research, Margaret Eng and colleagues used automated telemetry to track individual white-crowned sparrows they experimentally exposed to sublethal yet field-realistic doses of imidacloprid during migration. The authors found that the pesticide acted as an anorexic agent within the birds, causing rapid losses in body weight and fat. This resulted in extended stays at stopover sites as the birds had to forage longer to restore their greatly depleted fuel stores and to recover from imidacloprid's neurotoxic effects before moving on. According to Eng et al., the use of neonicotinoids along migratory routes throughout North America means that birds may suffer repeated exposure at successive stopover sites, amplifying migration delays and their consequences.

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