The Salas & Gomez and Nazca Ridges EBSA support a highly functional diversity of seabirds

Seabird Occurrence and Abundance Data Collection. Data on seabird occurrence and abundance at sea were obtained from the Seabirds at Sea Monitoring Program (SASMP; see Serratosa et al. (2020, https://doi.org/10.3389/fmars.2019.00838 for full methodology). Observations were conducted during 11 oceanographic surveys between 2014 and 2017, covering approximately 3,500 km of transects from the Chilean continental margin to Rapa Nui Island. Seabirds were recorded using standardized strip transect methods. An experienced observer, positioned on the vessel’s flying bridge, continuously recorded all seabirds within a 300-meter strip on one side of the ship (a 90° arc from bow to beam). Data were collected in continuous 10-minute intervals. For analysis, all 10-minute intervals recorded on the same calendar day were aggregated into a single daily transect, which served as the sampling unit. To account for variation in sampling effort across daily transects, seabird counts were standardized by the area examined during a day to obtain density estimates (individuals·km⁻2), which we refer to as abundance throughout this study. Functional Traits. We selected five standardized continuous morphological traits: beak length (mm), tarsus length (mm), wing length (mm), tail length (mm), and body mass (g); and then performed a Principal Component Analysis (PCA) to reduce redundancy among these correlated traits. The first two principal components explained 97% of the total variance and were retained as composite axes representing overall body size and shape variation. These axes (PC1_morph and PC2_morph) were used as continuous inputs in the functional trait space. In addition to continuous traits, we included ecological traits that were either categorical or fuzzy-coded. Categorical traits included primary lifestyle (terrestrial, aerial, aquatic, or generalist) coded as nominal as these categories reflect alternative ecological strategies without a functional hierarchy, and diet type (invertivore, omnivore, or piscivore/scavenger), coded as ordinal following a general trophic gradient: Invertebrate < Omnivore < Vertebrate/Fish Scavenger. Fuzzy-coded traits described the proportional association of each species with three foraging strata: below the surface, around the surface, and on the ground, summing to one per species. Trait values were extracted from Tobias et al. (2022, https://doi.org/10.1111/ele.13898) for morphology and lifestyle, and from Wilman et al.(2014, https://doi.org/10.1890/13-1917.1 for foraging strata and diet.