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Most avian migrants alternate flight bouts, characterized by high metabolic rates, with stopovers, periods of fuel replenishment through hyperphagia. High-energy metabolism and excessive calorie intake shift the balance between damaging prooxidants and antioxidants toward the former. Hence, migration likely affects the oxidative balance of birds. Migratory flight indeed appears to cause oxidative damage; however, whether migration affects the oxidative state of birds at stopover is unclear. Therefore, we compared total nonenzymatic antioxidant capacity (AOX) and malondialdehyde concentration (MDA; a measure of lipid peroxidation) in the plasma of migrant and resident common blackbirds. We also determined plasmatic uric acid (UA) and fatty acid (FA) concentrations and calculated a FA peroxidation index. Birds were sampled during autumn migration at a stopover site that also supports a sedentary blackbird population. Migrants had higher AOX than residents, also after correcting for UA concentration. Migrants tended to have higher FA peroxidation indexes than residents, indicating that the energy source of migrants contains higher concentrations of peroxidizable FAs. However, the two groups did not differ in MDA concentration, also not after correcting for peroxidation index. Peroxidation-corrected MDA concentration was negatively correlated with UA-corrected AOX. In other words, individuals with low nonenzymatic AOX suffered more from lipid peroxidation than individuals with high nonenzymatic AOX. These results together indicate that migrant blackbirds invest in antioxidant defenses to reduce oxidative damage to lipids, likely representing an adaptation to diminish the physiological costs of migration.

The House Sparrow is a globally distributed species and is closely associated with anthropised environments. They are well-adapted to urban life; therefore the decline of their populations in Europe represents an unexpected event that demands an investigation into its causes. Causes that have promoted this decline are not well-known, but one of the highlighted hypotheses is an increase of oxidative stress linked to the toxicity of pollution in urban areas. From an ecophysiological perspective, oxidative damage, antioxidant defense, and oxidative balance are considered reliable indicators of environmental stressors such as pollutants. To carry out this study, blood samples were collected from House Sparrows in three different habitats that varied in terms of urbanization degree: urban, suburban, and rural; during the winter and breeding season. According to our results, urban sparrows showed higher levels of oxidative damage and higher activity of antioxidant enzymes, but lower antioxidant capacity in comparison with the rural birds; and these differences especially increase during the breeding season. The maintenance of oxidative balance increases in an urban environment in comparison to a rural one; we suggest that the high level of pollution and the poor quality diet linked to urban environments. The breeding season is expected to be particularly challenging for the oxidative balance of urban birds, when the reallocation of resources between self-maintenance and reproduction may be critical due to the scarcity of antioxidants found in urban areas. This study may contribute to determining the causes of the population decrease of House Sparrows in cities.