Mercury Concentration and Distribution in Remiges, Rectrices, and Contour Feathers of the Barn Swallow Hirundo rustica

Feathers are commonly used to monitor trace elements in birds, including heavy metals. Typically, a single feather is analyzed to avoid harming living birds, assuming it reflects the organism’s overall contamination. To verify this assumption, we analyzed mercury concentrations in 12 flight and contour feathers from 25 barn swallows Hirundo rustica (16 adults and nine juveniles) that had died accidentally in a colony of the Po Plain (northern Italy). The median concentration in all feathers examined was 1.03 µg g−1 in adults (range 0.76 µg g−1–1.30 µg g−1) and 0.39 µg g−1 in juveniles (range 0.28 µg g−1–0.71 µg g−1), which is consistent with the results of similar research carried out on other world regions. No significant differences were observed between sexes, whereas marked differences were observed between adults and juveniles. In adults, mercury concentration was similar across remiges, rectrices, and contour feathers while in juveniles it was higher in contour feathers than in flight feathers. Mercury accumulation was highest in primary remiges and contour feathers, accounting for 67.6% of total mercury in adults and 77.5% in juveniles. However, primary remiges cannot be collected from live adults due to their importance in flight. In juveniles, contour feathers carry about 50% of total mercury, suggesting ventral and dorsal plumage may be useful for assessing mercury burden. Our findings are consistent with the hypothesis that mercury accumulation in feathers aids detoxification, with early-molted feathers (primary remiges and contour feathers) containing higher mercury levels than those replaced later (rectrices and secondary remiges).