Intensive agriculture and insect prey availability influence oxidative status and return rates of an aerial insectivore

Birds breeding in agricultural landscapes contend with potential reductions in prey availability that may alter behavior and incur physiological costs. Individuals may need to increase foraging intensity, producing elevated reactive oxygen species involved in oxidative damage. Food scarcity or low food quality may also reduce levels of diet-derived antioxidants. Here, we tested whether short-term changes in insect biomass on agricultural sites produce changes in the oxidative status of tree swallows (Tachycineta bicolor), with the potential to influence return rates. We sampled 368 adult and 374 nestling swallows at five agricultural cropland and grassland-dominated sites over three years from 2012 to 2014. Blood plasma was assessed for antioxidant capacity (OXY) and reactive oxygen metabolites (ROMs), a marker for oxidative damage. Overall, males appeared to have higher oxidative damage and therefore oxidative stress (calculated as the ratio between ROMs and OXY) than females. Consistent with our hypothesis, aerial insect biomass was a strong predictor of oxidative status in swallows. Adult plasma antioxidants were higher with greater insect biomass, while nestling plasma antioxidants, oxidative damage, and oxidative stress exhibited negative relationships with insect biomass. Annual return rates of adults were predicted by site type, age, fledging success, and oxidative status. In 2013, adult return rates were higher at grassland sites (41%) than at cropland sites (25%) and birds with lower plasma antioxidants the previous year were more likely to return. In 2014, adults were more likely to return if they had fledged more nestlings the previous season and if they had lower levels of oxidative damage the previous year. We conclude that even subtle changes in insect prey availability in agricultural landscapes caused by pesticides, intense cropping, and natural habitat loss can play a key role in swallow oxidative status, with subsequent effects on local return rates.