High-throughput sequencing reveals prey diversity overlap between sympatric Sulids in the tropical Atlantic

Seabirds breeding at tropical latitudes suffer an increased pressure to forage efficiently, because oligotrophic waters have less abundant and more patchily distributed prey. In related species living in sympatry, trophic or spatial niche partitioning may emerge as a strategy to mitigate increased competition. However, studies using molecular methodologies such as high-throughput sequencing (HTS) to test for these strategies are still scarce. In this study we used HTS to assess prey diversity partitioning on two sulid species living sympatrically in the tropical Raso Islet (Cabo Verde). Brown boobies (Sula leucogaster) breed in Raso Islet throughout the year while Red-footed boobies (Sula sula) occur during molt, from June to October. We compared Brown boobies diet prey diversity between the two periods (non-coexistence vs. coexistence with Red-footed booby), as well as between both sulids while in coexistence. Overall, the diets of all groups were clearly dominated by a small number of Molecular Operational Taxonomic Units (MOTUs), with a predominance of flying fish. The diet composition of Brown boobies was significantly different between the two time periods, most likely due to temporal differences in prey availability or possible competition with other seabird species. Brown boobies exploited a wider taxonomic range of prey, while Red-footed boobies used a subset of Brown boobies' prey diversity. Furthermore, we did not find intersexual differences in the diet composition of both species. Overall, we did not find evidence that prey diversity partitioning occurred, probably due to the parallel occurrence of spatial niche partitioning.