Stable isotopes reveal the common winter moult of central rectrices in a long-distance migrant songbird

By analysing the deuterium concentration in the scapulars and rectrices ( δ 2 H f ) of breeding and spring migrating Ortolan Buntings ( Emberiza hortulana ), we found a high correlation attesting that spring body and central rectrices have grown in similar isotopic environments. Furthermore, we failed to find a correlation between δ 2 H f of the rectrices and the amount-weighted growing season precipitation δ 2 H p of sites where we captured the birds. Winter-grown body coverts and rectrices displayed similar probabilistic assignments to origin. Further examination of 76 tails of breeding birds captured in Finland in May–June confirmed that breeding birds wear recently moulted central rectrices. The body coverts are known to moult during the winter partial moult in that species, but the rectrices have been reported to moult only once a year, during the complete post-breeding moult occurring on the breeding grounds in summer. Here, we reveal the common replacement of the central pair in winter, as well. The winter tail moult could occur beyond the central pair in some individuals, but this has still to be confirmed or refuted, by, e.g., further isotopic investigations.