Genetic and morphological variation of the poorly known South American Tern Sterna hirundinacea

The South American Tern Sterna hirundinacea is a poorly known migratory seabird distributed along South American marine coasts, including the Falkland Islands (Malvinas). The species presents allochronic isolation (i.e., separation of populations by breeding time), with different populations occupying different environmental spaces during the reproductive periods. Additionally, southern populations appear to undertake long-distance directional migration, but there is limited knowledge of the migratory behaviour of northern populations. In spite of these temporal and spatial segregative behaviour, limited population structure has been inferred from genetic assessments, although only a few localities have been investigated. Morphological variation is also poorly known. Whether intrinsic, environmental, or anthropogenic factors are causing isolation and differentiation between populations is unknown. Here, we first characterize sexual morphological dimorphism in the South American Tern, and then evaluate morphological variation along the species range, using only measurements that did not respond to sexual dimorphism and with a representative number of specimens. We also evaluate genetic variation along the South American Tern distribution to test for population differentiation and estimate the demographic history and diversification time for the species. Overall, we found limited population structure in both morphological and genetic data, but with some differentiation in measurements assessed for specimens from the Southeastern Pacific. A trend for demographic stability after a population expansion was estimated for the species. The connectivity among species' colonies, followed by its likely non-philopatric behaviour, seems to maintain gene flow between South American Tern populations. However, genetic differentiation might still be undetected, given the estimated recent origin and demographic expansion for the species. Although the South American Tern is not considered globally threatened, it is regionally threatened, and conservation implications of our findings are considered.