Aim:To understand the population structure and its potential drivers at different spatial scales in a migratory bird, the black-fronted tern (Chlidonias albostriatus), a specialist of the spatially and temporally dynamic environments of braided rivers.Location: New Zealand. Methods:We used a three-pronged approach based on 17 microsatellites, two mitochondrial loci (cytochrome b/control region) and phenotypic data (head-bill length, bill depth, wing length, weight). We determined large-scale genetic structure throughout the whole breeding range (approx. 150,000 km 2 ), calculated genetic divergence of breeding colonies and tested for isolation-by-distance between colonies.We investigated the level of fine-scale genetic structure based on spatial autocorrelation analyses and assessed the presence of a body size cline based on phenotypic data. Lastly, we compared phenotypic divergence (P ST ) and the level of divergence by genetic drift (F ST ) among breeding colonies to test for underlying mechanisms of population differentiation.Results: Nuclear and mitochondrial DNA showed that across their range black-fronted terns were effectively panmictic, with low genetic divergence between breeding colonies overall and no isolation-by-distance. However, at fine geographical scales black-fronted terns accrued significant genetic structure for distances up to 75 km, primarily driven by males, indicating more frequent female dispersal. Furthermore, a phenotypic cline in accordance with Bergmann's rule was evident. P ST exceeded F ST in three traits, suggestive of local adaptation.Main conclusions: Significant fine-scale structure can be present in highly mobile, specialist species while not affecting spatial structures at larger scales. Hence, methodologies applied to both whole landscapes and local scales are important to appropriately estimate connectivity in dynamic metapopulations and investigate the processes behind connectivity. Conservation management will need to include protecting currently uninhabited patches to facilitate natural colonization of suitable | 17 SCHLESSELMANN Et AL.

Understanding how climatic and environmental changes, as well as human activities, induce changes in the distribution and population size of avian species refines our ability to predict future impacts on threatened species. Using multilocus genetic data, we show that the population of a threatened New Zealand endemic open‐habitat specialist, the Black‐fronted Tern Chlidonias albostriatus – in contrast to forest specialists – expanded during the last glacial period. The population has decreased subsequently despite the availability of extensive open habitat after human arrival to New Zealand. We conclude that population changes for open habitat specialists such as Black‐fronted Terns in pre‐human New Zealand were habitat‐dependent, similar to Northern Hemisphere cold‐adapted species, whereas post‐human settlement populations were constrained by predators independent of habitat availability, similar to other island endemic species.