Flexible foraging strategies of a tropical seabird in the western Atlantic Ocean

Abstract

Tropical oceans are typically oligotrophic but can feature productive environments, such as islands, which can promote high prey availability, influencing predators' foraging strategies in time and space. This study investigates interannual and sex-specific variations in the foraging ecology of the masked booby (Sula dactylatra) in the Fernando de Noronha Archipelago, western tropical Atlantic Ocean. During the 2017, 2018, 2019, and 2022 breeding seasons, blood from birds and muscle samples from regurgitated prey were analyzed for stable isotopes of carbon (δ¹³C) and nitrogen (δ¹⁵N). Simultaneously, birds were tracked with GPS devices and their dives were recorded by pressure sensors and later predicted by deep learning tools to estimate foraging areas. Tracking data revealed foraging areas eastward of the archipelago in all years, potentially driven by the Island Mass Effect. The δ¹³C and δ¹⁵N values varied significantly across the four years, but isotopic niches overlapped, except in 2022. Mixing models estimated the fourwing flying fish Hirundichthys affinis as an important prey source from 2017 to 2019, shifting to false herring Harengula clupeola in 2022. Simultaneously, foraging areas were closer to the archipelago in 2022, with shorter foraging trip duration and length. Stable isotope differences between sexes were occasional, with overlapping niches and foraging areas consistent across years. These findings showcase potential predictability in foraging areas, and also their interannual variability, suggesting shifts in prey availability and distribution around the archipelago. Our results highlight the ecological plasticity of masked boobies in tropical environments and their potential as monitors of fish community dynamics.