Bio-logging has revealed much about high-latitude seabird migratory strategies, but migratory behaviour in tropical species may differ, with implications for understanding nutrient deposition. Here we use combined light-level and saltwater immersion loggers to study the year-round movement behaviour of adult red-footed boobies (Sula sula rubripes) from the Chagos Archipelago, tropical Indian Ocean to assess migratory movements and estimate nutrient deposition rates based on the number of days they spent ashore. Light levels suggest that red-footed boobies are resident in the Chagos Archipelago year-round, although there are large latitudinal errors this close to the equator. Immersion data also indicate residency with tracked birds returning to land every one or two days. Spending an average of 79.86 ± 2.80 days and 280.84 ± 2.64 nights per year on land allows us to estimate that the 21,670 pairs of red-footed boobies deposit 37.34 ± 0.56 tonnes year^-1 of guano-derived nitrogen throughout the archipelago. Our findings have implications for tropical seabird conservation and phylogenetics, as well as for assessing the impact of seabird nutrients on coral reef ecosystems.

RFBs were tracked from Barton Point, Diego Garcia, Chagos Archipelago between January 2018 and February 2020. Adults were fitted with combined geolocation and immersion loggers (Intigeo C330, Migrate technology, Cambridge, UK; 3.3g) attached to a multi-layered impact acrylic leg ring (Interrex-rings, Poland). Immersion data were processed using the ExMove toolkit (Langley et al. In press, Journal Animal Ecology) to create a standardised data frame.