Migrant blackbirds, Turdus merula, have higher plasma levels of polyunsaturated fatty acids compared to residents, but not enhanced fatty acid unsaturation index

Birds have been observed to have dietary preferences for unsaturated fatty
acids during migration. Polyunsaturated fatty acids (PUFAs) may increase
the exercise performance of migrant birds; however, PUFAs are also
peroxidation prone and might therefore incur increased costs in terms of
enhanced oxidative damage in migratory individuals. To shed light on this
potential constraint, we analysed plasma fatty acid (FA) composition and
estimated the unsaturation index as a proxy for susceptibility to lipid
peroxidation of migrants and residents of the partially migratory common
blackbird (Turdus merula) at a stop-over site during autumn migration. As
predicted, migrant birds had higher relative and absolute levels of PUFAs
compared to resident birds. This included the strictly dietary ω-3 PUFA
α-linolenic acid, suggesting a dietary and/or storage preference for these
FAs in migrants. Interestingly, the FA unsaturation index did not differ
between migrants and residents. These findings suggest a mechanism where
birds alter their levels of metabolic substrate without simultaneously
increasing the susceptibility of the substrate to lipid peroxidation. In
summary, our results are in line with the hypothesis that increased
exercise performance during migration might be constrained by oxidative
stress, which is manifested in changes in the composition of key FAs to
retain the unsaturation index constant despite the increased levels of
peroxidizable PUFAs.