A Comparison of the demography, body condition, and resource use of Emperor Geese (Anser Canagicus) at the geographic extremes of their wintering range

Abstract

Emperor geese (Anser canagicus) are a Bering Sea endemic species that have experienced long-term population declines and a recent harvest closure, prompting interest in the factors influencing their population dynamics. Understanding how environmental variation across the annual cycle affects demographic performance is therefore important for effective management. Emperor geese spend a large portion of their year on their winter grounds where they occupy geographically distinct regions that differ substantially in environmental conditions, yet the consequences of this spatial variation for survival, recruitment, and individual performance remain poorly understood. To address this, I evaluated variation in survival and productivity across two locations at the geographic extremes of the species’ winter range (Shemya Island and the Kodiak Archipelago) and examined differences in resource use and body condition among these same populations. From 2024 to 2026, I estimated age ratios as a proxy to productivity, and from 2020 to 2026, I used mark–resight data to estimate annual apparent survival. I also assessed the influence of regional climate conditions using the Aleutian Low–Beaufort Sea Anticyclone (ALBSA) index. I then used stable isotope analysis and morphometric data to quantify diet, isotopic niche structure, and body condition at each winter location during the winters of 2024 and 2025.Apparent survival and juvenile-to-adult ratios were consistently lower on Shemya than on Kodiak, with juveniles exhibiting reduced survival relative to adults, particularly on Shemya. Survival at both sites was related to seasonal climatic conditions, although the seasonal drivers differed between regions. Patterns of resource use and body condition also varied across wintering areas, with birds on Kodiak exhibiting a diet enriched in δ¹³C and δ¹⁵N signaling more marine based and protein rich resources and generally had higher body condition, while birds on Shemya appeared to rely on lower quality resources and exhibited lower body condition. Isotopic niche width differed between sites and years, with limited overlap between populations. These findings indicate that wintering location is an important source of variation in survival, recruitment, and individual performance in emperor geese. Spatial heterogeneity in winter environments may therefore influence population dynamics not only through movement and shifting distribution, but also through differences in demographic contribution among wintering regions. This work highlights the importance of winter ecology in shaping population processes and provides insight relevant to emperor geese management decisions.