Intercolony and Intra‐Annual Variations in Isotopic Niche of Red‐Footed Boobies Sula sula in the Tropical Indian Ocean

ABSTRACT Foraging seabirds experience diverse environmental conditions, resulting in variation in their foraging ecology across different populations. Documenting this variability is important for understanding how seabirds adapt to environmental change. We examined the trophic ecology of red‐footed boobies ( Sula sula ) at three relatively remote colonies in the tropical Indian Ocean: Aldabra Atoll, Farquhar Atoll and Diego Garcia. We compared red‐footed booby diet and isotopic niches based on regurgitates and stable isotope compositions (δ 15 N and δ 13 C) of whole blood and body feathers, representing breeding and non‐breeding periods, respectively. At Diego Garcia, breeding red‐footed boobies consumed only flying fish, whereas at Farquhar and Aldabra they had a more diverse diet. Breeding red‐footed boobies at Diego Garcia had lower δ 15 N and δ 13 C values and a broader isotopic niche width along the δ 13 C axis than those at Aldabra and Farquhar. Considering the resident behavior of red‐footed boobies, we further explored intra‐annual variation in isotopic niches within each colony. All colonies showed minimal isotopic niche overlap between breeding and non‐breeding periods, which represent the northwest and southeast monsoon seasons, and had higher δ 15 N and δ 13 C values and larger isotopic niche widths during the southeast monsoon. Red‐footed booby isotopic niches were segregated from those of other seabirds, such as lesser frigatebirds ( Fregata ariel ) at Aldabra and brown noddies ( Anous stolidus ) at Farquhar. Our study shows trophic niche plasticity in red‐footed boobies in the tropical Indian Ocean. We provide ecological baselines to support monitoring of the impacts of environmental change on the diets and isotopic niches of red‐footed boobies across the rapidly changing tropical Indian Ocean.