Tropicbirds are key pelagic seabirds in the Indian Ocean and breed on islands throughout the region. As top-marine predators, they are valuable indicators of ocean health, but their foraging ecology, and how they make use of ocean resources, remains poorly understood. This study aims to fill this gap in our knowledge of tropicbird biology by investigating the feeding ecology of the red-tailed and white-tailed tropicbirds Phaethon rubricauda and Ph. lepturus. More specifically, we determine the feeding distribution of birds using miniature GPS loggers, delve into their detailed at-sea behaviour by combining spatial data with depth and immersion loggers, and link these data with diet, using DNA metabarcoding techniques on faecal samples. To test whether feeding strategies vary between populations and marine environments, we compare two island populations of breeding white-tailed tropicbirds located in exploited and more pristine waters (Aride Island, inner Seychelles, and Aldabra Atoll, outer Seychelles, respectively). Finally, to evaluate how the two closely-related species avoid competition when breeding in sympatry, we compare the foraging strategies of red-tailed and white-tailed tropicbirds on Aldabra, one of the few places they coexist. We combine the latter with nest-based camera traps assessing potential differences in nest predation rates to identify the cause of recent red-tailed tropicbird declines and poor nesting success on Aldabra. Together, our findings provide the first complete picture of tropicbirds' foraging ecology during breeding, reveal differences between species and populations, address important ecological questions regarding competition avoidance between closely-related sympatric species, and provide important information for the conservation of tropicbirds and other seabirds breeding on Indian Ocean islands.