With increasingly warming climate, many bird species have been forced to respond to environmental changes. It has been suggested that increased temperature results in decreasing population sizes in a number of avian species (Miller-Rushing et al. 2008) and seabird species in particular have had a difficult time (Dias et al. 2019). Sea birds have been considered important indicators of the health of marine ecosystems (Parsons et al. 2008) and a recent assessment by Dias et al. (2019) indicates that major threats to sea birds include invasive alien species, bycatch in fisheries and climate change, and these threats have had an increased effect for the past decade. With a changing climate and more severe weather events, the marine ecosystems are at risk, and therefore the food availability for seabird species (Dias et al. 2019, Skov et al. 2011). It might also affect the migratory routes and migration timing of waterbird populations (Skov et al. 2011). The populations of the long-tailed duck (Clangula hyemalis) have been in decline the past few decades (Wilson et al. 2016, Skov et al. 2011, Bellebaum et al. 2014) and the species is classified vulnerable (VU) globally and near threatened (NT) in both Iceland and Sweden (BirdLife International 2018, SLU Artdatabanken 2020, IINH 2018). The long-tailed duck is a circumpolar, migratory sea duck and with many speculations as to why the global population is decreasing, it is important not only look at the ecology of the species, but also its genetic diversity, evolutionary history and population differentiation in regard to conservation.