The relative contributions of past and present evolutionary processes in shaping population genetic differentiation can be difficult to ascertain, especially in highly mobile animals. The Northern Gannet (Morus bassanus; hereafter, gannet) is a migratory colonial seabird that is widely distributed across the North Atlantic Ocean. Despite strong dispersal abilities, decades of banding and tracking studies indicate that the North Atlantic is a barrier for seasonal migration of gannets: gannets breeding in North America winter along the southeastern coast of USA and Mexico, while European breeders winter along the western European and African coasts. However, telemetry recently revealed that some gannets migrate across the ocean, suggesting that trans-Atlantic gene flow is possible. We investigated patterns of genomic variation among gannets from 12 colonies across the species’ range using double digest restriction-site associated sequencing (ddRADseq). Indices of genetic differentiation, principal component analysis, a Bayesian clustering method, and discriminant analysis of principal components all indicated that gannets breeding in North America versus Europe differ genetically, in accordance with segregation at both breeding and non-breeding areas. However, Bayesian assignment methods indicated that low, unidirectional introgression occurs from Europe into North America, suggesting that the North Atlantic is a semi-permeable barrier to gene flow in gannets. Evolutionary modeling suggested that the two genetic populations originated in separate refugia during the Pleistocene and underwent secondary contact during the Holocene. These results are consistent with results of previous studies and provide direct evidence that seasonal migratory behaviour can influence population genetic structure in a highly mobile organism.

Conservation of breeding seabirds typically requires detailed data on where they feed at sea. Ecological niche models (ENMs) can fill data gaps, but rarely perform well when transferred to new regions. Alternatively, the foraging radius approach simply encircles the sea surrounding a breeding seabird colony (a foraging circle), but overestimates foraging habitat. Here, we investigate whether ENMs can transfer (predict) foraging niches of breeding tropical seabirds between global colonies, and whether ENMs can refine foraging circles. We collate a large global dataset of tropical seabird tracks (12000 trips, 16 species, 60 colonies) to build a comprehensive summary of tropical seabird foraging ranges and to train ENMs. We interrogate ENM transferability and assess the confidence with which unsuitable habitat predicted by ENMs can be excluded from within foraging circles. We apply this refinement framework to the Great Barrier Reef (GBR), Australia to identify a network of candidate marine protected areas (MPAs) for seabirds. We found little ability to generalise and transfer breeding tropical seabird foraging niches across all colonies for any species (mean AUC: 0.56, range 0.4-0.82). Low global transferability was partially explained by colony clusters that predicted well internally but other colony clusters poorly. After refinement with ENMs, foraging circles still contained 89% of known foraging areas from tracking data, providing confidence that important foraging habitat was not erroneously excluded by greater refinement from high transferability ENMs nor minor refinement from low transferability ENMs. Foraging radii estimated the total foraging area of the GBR breeding seabird community as 2,941,000 km2, which was refined by excluding between 197,000 km2 and 1,826,000 km2 of unsuitable foraging habitat. ENMs trained on local GBR tracking achieved superior refinement over globally trained models, demonstrating the value of local tracking. Our framework demonstrates an effective method to delineate candidate MPAs for breeding seabirds in data-poor regions.

Bio-logging has revealed much about high-latitude seabird migratory strategies, but tropical species are comparatively understudied. Here we use geolocators to study the year-round movement behaviour of adult red-footed boobies (Sula sula rubripes) from the Chagos Archipelago, tropical Indian Ocean. Light levels suggest that red-footed boobies are resident in the archipelago year-round, although there are large latitudinal errors this close to the equator. However, immersion data revealed tracked birds returned to land year-round, with no extended at-sea periods, further indicating this population is non-migratory. Our findings have important implications for seabird conservation and phylogenetics, as well as for assessing the impact of seabird nutrients on coral reef ecosystems.