Summary: | Combined effects of multiple, climate change-associated stressors are of mounting concern, especially in Arctic ecosystems. Elevated mercury (Hg) exposure in Arctic animals could affect behavioural responses to changes in foraging landscapes linked to climate change, generating interactive effects on behaviour and population resilience. W e investigated this hypothesis in the little auk ( Alle alle ), a keystone Arctic seabird. We compiled behavioural data using accelerometers, and quantified blood mercury and environmental conditions (sea surface temperature (SST), sea ice coverage (SIC)) across multiple years. Warm SST and low SIC reshaped time activity budgets (TABs) and diving patterns, causing decreased resting, increased flight, and longer dives. Mercury contamination was not associated with TABs. However, highly contaminated birds lengthened inter-dive breaks when making long dives, suggesting mercury-induced physiological limitations. A s dive durations increased with warm SST , subtle toxicological effects threaten to increasingly constrain diving and foraging efficiency as climate change progresses, with ecosystem-wide repercussions. Datasets can be opened in Excel and R. Funding provided by: H2020 Marie Skłodowska-Curie Actions Crossref Funder Registry ID: http://dx.doi.org/10.13039/100010665 Award Number: 101025549, 896866, 101024166 Funding provided by: Institut Polaire Français Paul Emile Victor Crossref Funder Registry ID: http://dx.doi.org/10.13039/501100004796 Award Number: ADACLIM program (388) Funding provided by: Association Nationale de la Recherche et de la Technologie Crossref Funder Registry ID: http://dx.doi.org/10.13039/501100003032 Award Number: ANR‐20‐CE34‐0006, ANR‐16‐CE34‐0005
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