White-crowned Sparrow. Photograph by Bettina Arrigoni (CC BY 2.0)
It is a tough world out there if you are a migratory bird. Loss of habitat for breeding, migration, and wintering can force compromises in vital life events. So can reduced food supplies due to invasive plants, agricultural practices, urbanization, climate change, and other blights. The likelihood of death by feral cats, other human-increased mammals, glass windows, automobiles, and adverse weather, not to mention disease and disability, increases with longer stopover time as well. If you noticed agricultural practices on the above list, let me tell you about an interesting paper I read recently from a research group I have written about before (Larson 2018).
Eng et al., 2019, investigated the effects of neonicotinoid insecticides on spring migrating White-crowned Sparrows (Zonotrichia leucophrys). Neonicotinoids (neonics) are a class of neurotoxic insecticides that are widely used in modern agriculture for reducing pest loads on crops. They bind to a type of neurotransmitter receptor, thereby overstimulating the nervous system. This can lead to death for insects—oh wait, that means less food for birds. Neonics are more effective on receptors in insects than those in vertebrates, but they are controversial because they kill beneficial insects as well as pests and they are acutely toxic to birds. Birds can be exposed by sprayed neonics, contaminated soil and water, and by consumption of treated seeds. The actual mechanisms of toxic effects of neonics on birds and the extent of long-term damage are poorly known. In order to elucidate some of the effects, Eng et al., combined controlled dosing of wild-caught birds with telemetry, using nanotags and the Motus Wildlife Tracking System, to follow migration movements in the wild. Sparrows who consumed even very small doses of the neonic imidacloprid showed negative effects on fueling and migration. A single dose of imidacloprid—3-10% of predicted median lethal dose—caused a weight loss within 6 hours; 3% in the lowest dose and 6 % in a higher dose. The loss of weight was due to loss of fat, the critical fuel for migration. Loss of weight or fat was due to appetite suppression, possibly due to acute toxicity, with high-dose sparrows eating 70% less food than controls. Exacerbating changes in nutrient uptake or general toxicity are possible. Once birds had recovered, they resumed a normal migration with no apparent change in directionality or speed of movement.
Imidacloprid exposure significantly increased stopover duration. It seems likely that the birds had to recover from the toxicity of the neonic, and then resume migratory hyperphagia to rebuild fat stores for the next leg of their migratory journey. In previous studies, this research group has shown that sparrows recover body mass and orientation ability within two weeks of a high dose of imidacloprid. Delaying migration, even by a short period, has clear deleterious effects at the individual and population level. Extending stopovers increases the chance of predation, disease, and encountering adverse weather. Delays run counter to selection pressures to minimize migration time in the spring and have been associated with decreased number of offspring in late breeding in poor territories.
Neonics are the most extensively used insecticides in the world, and their abundant availability along bird migration routes presents a worrisome possible barrier to normal migration in this world of too many obstacles to the survival of birds.
- Eng, M.L., B.J.M. Skutchbury, and D.A. Morrissey. 2019. A neonicotinoid insecticide reduces fueling and delays migration in songbirds. Science 365:1177–80.
- Larson, D.M. 2018. Gleanings: Getting Lost. Bird Observer 46 (1):36–8.