1. Sexual competition is increasingly recognized as an important selective pressure driving species distributions. However, few studies have investigated the relative importance of interpopulation versus intrapopulation competition in relation to habitat availability and selection.2. To explain spatial segregation between sexes that often occurs in non-territorial and central place foragers, such as seabirds, two hypotheses are commonly used.The 'competitive exclusion' hypothesis states that dominant individuals should exclude subordinate individuals through direct competition, whereas the 'niche divergence' hypothesis states that segregation occurs due to past competition and habitat specialization.3. We tested these hypotheses in two populations of an extreme wide-ranging and sexually dimorphic seabird, investigating the relative role of intrapopulation and interpopulation competition in influencing sex-specific distribution and habitat preferences.4. Using GPS loggers, we tracked 192 wandering albatrosses Diomedea exulans during four consecutive years (2016-2019), from two neighbouring populations in the Southern Ocean (Prince Edward and Crozet archipelagos). We simulated pseudotracks to create a null spatial distribution and used Kernel Density Estimates (KDE) and Resource Selection Functions (RSF) to distinguish the relative importance of within-versus between-population competition.5. Kernel Density Estimates showed that only intrapopulation sexual segregation was significant for each monitoring year, and that tracks between the two colonies resulted in greater overlap than expected from the null distribution, especially for the females. RSF confirmed these results and highlighted key at-sea foraging areas, even if the estimated of at-sea densities were extremely low. These differences in selected areas between sites and sexes were, however, associated with high interannual variability in habitat preferences, with no clear specific preferences per site and sex. 6. Our results suggest that even with low at-sea population densities, historic intrapopulation competition in wide-ranging seabirds may have led to sexual dimorphism | 2405

Seabirds are distributed widely over the world's oceans and have adopted a range of foraging tactics to secure food resources necessary for survival and reproduction. To better understand the foraging tactics and at-sea distribution of tropical seabirds, 38 Wedge-Tailed Shearwaters, Ardenna pacifica (WTS) from Réunion Island (21.375° S; 55.569° E) were tracked during 81 foraging trips using GPS loggers deployed over three breeding seasons (2016)(2017)(2018)(2019). Clustering algorithms, kernel density estimation and habitat models were applied to this tracking dataset. During incubation, WTS foraged in the open ocean towards the southeast of Madagascar. During chick rearing, however, WTS restricted their distribution and implemented a dual foraging tactic, where they executed several short trips near the colony before performing a single long trip (> 200 km) in a similar south-westerly direction observed for incubating birds. Birds did not seem to show a strong preference for specific environmental conditions or habitat features and arguably cue on marine predators, conspecifics, or fish-aggregating devices to find productive foraging grounds. This study confirmed that WTS foraged in areas that have previously been identified as 'hotspots' for other marine species which are threatened by anthropogenic pressures; further highlighting that these areas are important from a conservation perspective.