Movement and diet of brown boobies in a remote protected area suggest temporal variations in foraging strategies

Abstract

Understanding tropical seabird foraging strategies is essential for evaluating ecosystem functioning within Very Large Marine Protected Areas (VLMPAs). We investigated spatial and trophic behavior of brown booby ( Sula leucogaster ) at the Saint Peter and Saint Paul Archipelago (SPSP), a remote breeding site within a VLMPA in the equatorial Atlantic Ocean. Using GPS tracking from 207 individuals (706 trips, 2014–2015 and 2023–2024), stable isotopes ( δ 13 C and δ 15 N) from 58 blood samples, and 49 regurgitated samples, we assessed variation in foraging effort and diet across sex, breeding stage, and temporal scales. Despite pronounced reversed sexual size dimorphism, no intersexual differences were detected in movement parameters. However, females exhibited significantly higher δ 15 N values, indicating subtle trophic segregation. Breeding stage showed minimal influence, suggesting that extreme nest density and continuous territorial defense override intrinsic reproductive demands. Pronounced interannual variation in foraging effort was detected, with 2023 showing nearly double the ranges compared to 2015. Seasonally, trip metrics did not differ, but time allocation shifted between austral winter and summer: traveling decreased (48.1% to 29.7%) while searching increased (40.3% to 58.4%), coinciding with dietary shifts from flying fish to margined flying fish ( Cheilopogon cyanopterus ) spawning. Daily foraging directions were highly variable, reflecting opportunistic responses to ephemeral prey distributions. The interannual increase in foraging effort coincided with recovery of large pelagic predators, though oceanographic mechanisms cannot be excluded. These findings demonstrate that colonial constraints override sexual dimorphism while positioning brown boobies as ecological indicator for adaptive management of remote VLMPAs.

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