Assessment of genetic diversity in a locally endangered tern species suggests population connection instead of isolation

Many bird populations are declining globally, with migratory sea- and waterbirds among the most threatened groups. Increased isolation between populations, due to habitat loss leading to limited connectivity and reduced gene flow, may accelerate population declines or even cause local extinctions. To address this threat and inform conservation efforts, assessment of the genetic diversity of endangered populations is required. In Central Europe, gull-billed terns ( Gelochelidon nilotica ) have experienced a severe population decline in the past century, with only one last population of c. 40 breeding pairs remaining. Here, we aimed to characterise the genetic diversity of this population, and to compare it to that of two separate Mediterranean colonies to assess potential population connectivity. To achieve this, we developed a PCR protocol to amplify and sequence the complete mitochondrial genome (mtDNA). We found high genetic diversity in all three populations, and no population structure based on breeding location, as well as moderate population differentiation between Germany and Italy, and low differentiation between Germany and Spain, and between Italy and Spain. The German population, however, showed signs of population restriction, while both Mediterranean colonies showed evidence for population expansion. These results indicate that the last population of gull-billed terns in Central Europe is not acutely threatened by loss of genetic diversity, but instead shows signs of connectivity with other European colonies, which in turn suggests that conservation efforts should best focus on habitat and predation management.