A Ruddy Oxyura jamaicensis × White-headed Duck O. leucocephala hybrid diagnosed in Denmark

The endangered White-headed Duck Oxyura leucocephalus is threatened by the prospect of hybridisation and genetic introgression from the invasive Ruddy Duck O. jamaicensis, which became established in Europe from a small captive population imported from North America to England. Previous studies have demonstrated that these species readily hybridise in the wild if they come into contact, but the prospect of hybrids occurring far outside the range of leucocephala has not previously been described. Here we use whole-genome-sequencing to confirm the identity of a female F1 jamaicensis × male leucocephala hybrid culled in Denmark in 2022. O. leucocephala is a rare vagrant in Denmark, with the species' typical range limited to Central Asia, southern Europe and North Africa. The presence of a hybrid so far outside this core range highlights the importance of international co-operation to extirpate invasive species that threaten native wildlife, as well as the importance of carefully identifying female Oxyura when assessing management choices and quantifying the frequency of hybrids in Europe.