Diving ecology of a pelagic seabird, the Little Auk Alle alle, in a changing Arctic revealed by multiyear tracking and optical zooplankton data

Abstract In a rapidly warming Arctic, seabirds serve as sentinels of ecosystem change, reflecting shifts in oceanographic conditions and prey dynamics. The Little Auk Alle alle is a High-Arctic zooplanktivorous seabird that forages mainly on cold-water Calanus spp. during the breeding period. To assess its response to high interannual variability in environmental conditions and prey availability, we investigated the foraging ecology of chick-rearing adults breeding in one colony on Spitsbergen over a period of five hydrographically contrasting years. Based on previous studies we considered three potential foraging strategies adopted by Little Auks: optimal (optimal foraging conditions), effort (unfavorable conditions) and reactive (intermediate conditions). To investigate Little Auks foraging behavior we used Temperature-Depth Recorders (TDRs) combined with Global Positioning System (GPS) loggers. To assess prey availability, we used Laser Optical Plankton Counter (LOPC) coupled with a conductivity–temperature–depth (CTD) sensor to collect at-sea high resolution zooplankton distribution data across bird’s feeding grounds. Based on those data we modelled Calanus spp. abundance between and within years at different combinations of depth and temperature to show variability of foraging condition within the water column. We characterized diving patterns of Little Auks using a Self-Organizing Map, an artificial neural network, based on 10 TDR-based parameters of dives. We distinguished three diving modes differing in dive duration, depth and shape: short, shallow dives (SSM), intermediate dives (IIU), and deep long dives (DLV). Little Auks adopted two primary strategies: optimal-foraging, dominated by SSM targeting the preferred prey; and reactive-foraging, combining SSM and DLV and supplementing the preferred food with alternative prey. Birds switched between both foraging strategies across years depending on hydrographic conditions which influence the availability of preferred prey. Surprisingly, the optimal-foraging occurred in environmentally contrasted years: in the coldest (2021), the warmest (2023) and the mixed-regime (2024) year. The reactive-foraging prevailed in warm (2018) and mixed-regime (2022) years. In all years, the preferred prey Calanus glacialis was supplemented in various proportions by other prey items like Calanus finmarchicus , larvae of Euphausiacea or larvae of hermit crab Pagurus spp. Our study indicates foraging flexibility of Little Auks in a changing High Arctic environment.