Four millennia of long-term individual foraging site fidelity in a highly migratory marine predator

The study addresses how variation in foraging mobility and site fidelity influences conservation outcomes for recovering, highly migratory seabirds. Short-tailed albatross populations, once in the millions but reduced to near-extinction by feather hunting in the late 19th–early 20th centuries, are rebounding yet remain below 1% of historical levels. As populations grow, foraging behavior and spatial use may shift, altering overlap with threats such as fisheries by-catch. Individual foraging site fidelity (ISF)—consistent use of specific foraging areas by individuals over multiple years—can increase vulnerability to localized hazards. The prevalence and drivers of long-term ISF in short-tailed albatross are unknown due to limited multi-year tracking and low current population density. The authors test whether long-term ISF existed historically by using stable isotope analysis of archived tissues to infer long-term foraging geography over millennial timescales, focusing on birds exploited near Yuquot (west coast of Vancouver Island) at the eastern edge of the species’ range.

Prior work shows short-tailed albatross currently breed mainly on a few Western Pacific islands and forage broadly across the North Pacific, with adults focusing more near breeding areas and juveniles ranging more widely. Modern tracking and observations indicate generalist surface foraging and seasonal movements, and interactions with fisheries present significant risks that have motivated mitigation in known hotspots. Stable isotope ecology provides tools to infer trophic level (δ15N) and carbon source/foraging regions (δ13C), with bone collagen integrating diet over multiple years, enabling reconstruction of long-term behavior. Baseline δ13C and δ15N vary regionally due to oceanographic and climatic processes, making isotopes useful tracers of geographic foraging niches, though temporal baseline shifts are a known uncertainty. Previous isotopic studies of archaeological short-tailed albatross from California, Japan, Oregon, and Russia showed regional differences and suggested isotopic variation was driven more by foraging location than diet, with some amino-acid–specific isotope work supporting this interpretation.

- Study design: Analyze stable carbon (δ13C) and nitrogen (δ15N) isotope ratios in bone collagen from 95 archaeological short-tailed albatross specimens spanning multiple temporal zones over ~4,000 years, all recovered near the Yuquot site (west coast of Vancouver Island, BC, Canada). Include δ13C and δ15N from feathers of two 19th-century specimens (c. 1889 CE) from the nearby Strait of Juan de Fuca for qualitative comparison to late-period bone values. - Temporal grouping: Samples assigned to four zones: (1) Pre-2300 to 1000 BCE; (2) 1000 BCE to 800 CE; (3) 800 to 1789 CE; (4) 1789 to 1966 CE. - Comparative datasets: Compare Yuquot isotopic niche metrics to published archaeological short-tailed albatross datasets from California (Channel Islands), Japan (Hamanaka Islands/sites), Oregon, and Russia (Kuril Islands). - Collagen extraction and QC: Demineralize bone, remove contaminants (NaOH rinse), gelatinize and purify collagen following established protocols. Elemental concentrations and atomic C:N ratios screened; 98% (n=93) passed liberal QC criteria. Feathers cleaned with chloroform:methanol before isotope analysis. - Isotope measurement and calibration: δ13C and δ15N measured via elemental analyzer–IRMS. Calibrated to VPDB (carbon) and AIR (nitrogen) using USGS/UCSIA standards; accuracy/precision monitored with internal check standards and replicates. Reported systematic biases approximately ±0.09‰ (δ13C) and ±0.15‰ (δ15N); random errors ~±0.006‰ (δ13C) and ±0.13‰ (δ15N); standard uncertainty ~±0.11‰ for both ratios. - Statistics: Normality (Shapiro–Wilk), homogeneity of variances (Levene’s), and group comparisons (Welch’s t-tests; nonparametric tests as needed) across temporal zones. Isotopic niche metrics computed: total area (TA) and standard ellipse areas (SEA, SEA-B, SEA-C). Bootstrapping simulations used to assess the probability that observed narrow niches could arise by chance. Feathers’ δ13C adjusted to bone collagen equivalent (+1.66‰) for qualitative comparison (not pooled with bone due to tissue turnover differences).

- Quality control: 93 of 95 collagen samples (98%) passed QC. No δ13C–δ15N correlation (r = -0.051, p = 0.628) indicates low contamination. - Overall isotope means (Yuquot, n=93): δ13C = -14.6 ± 0.4‰; δ15N = +18.0 ± 0.7‰. - Temporal stability: Across four temporal zones, mean δ13C varied little and δ15N differences were small; no significant sequential period differences. A small δ15N difference between Zones 4 and 2 was detected (U = 0.613, p = 0.004) but magnitude was only 0.82‰. - Isotopic niche size: Yuquot birds exhibited a markedly smaller isotopic niche than archaeological samples from California, Japan, Oregon, and Russia. Reported niche metrics (means ± SD as given): - Yuquot (n=93): TA 4.237; SEA 6.398; SEA-B 1.791; SEA-C 1.829. - California (n=49): TA 6.398; SEA 1.791; SEA-B 1.763; SEA-C 1.829. - Japan (n=56): TA 5.515; SEA 1.396; SEA-B 1.365; SEA-C 1.422. - Oregon (n=17): TA 7.723; SEA 2.409; SEA-B 2.435; SEA-C 2.569. - Russia (n=35): TA 5.848; SEA 1.596; SEA-B 1.561; SEA-C 1.664. Relative to other regions, Yuquot birds’ niche breadth was roughly half or less, indicating a narrower and more geographically consistent foraging area. - Feather comparison: Two 1889 CE feathers (δ13C adjusted to bone equivalence) plotted near but slightly outside the Yuquot bone convex hull, increasing TA marginally (from 4.24 to 4.56). Similarity suggests continuity of foraging behavior into the late 19th century. - Interpretation: The small and temporally stable isotopic niche at Yuquot is inconsistent with selective harvesting alone and best explained by long-term individual foraging site fidelity (IFSF) focused on a limited set of regional hotspots near the west coast of Vancouver Island, persisting over millennia.

Findings show that short-tailed albatross harvested near Yuquot shared long-term, geographically consistent foraging habits, producing a small isotopic niche stable across four millennia. Selective human harvesting is unlikely to have produced such consistently narrow isotopic variation, especially given broader variation in nearby regions and over overlapping times. The parsimonious explanation is long-term IFSF, wherein individuals repeatedly exploited a limited number of hotspots with similar isotopic baselines. While the data cannot distinguish individual-level fidelity from colony-level spatial segregation, both imply geographic specialization with conservation relevance. Historically, the region likely supported foraging hotspots that are no longer focal today, aligning with archaeological dominance of short-tailed albatross remains and sparse historical accounts of abundance and year-round presence. The absence of strong long-term IFSF in modern observations may reflect currently low population density and reduced competition; theory and shorter-term studies in other seabirds suggest fidelity can be density-dependent. As populations recover, IFSF could re-emerge, increasing risk where hazards are localized (e.g., fisheries by-catch).

This study provides the first large-scale, deep-time evidence that short-tailed albatross exhibited long-term individual foraging site fidelity over four millennia, focusing on geographically limited hotspots near Vancouver Island. The narrow and temporally stable isotopic niche at Yuquot, relative to other regions, indicates persistent geographic specialization likely driven by density-dependent mechanisms. As the species continues to recover, similar fidelity could reappear among sub-populations, heightening vulnerability to localized threats. The authors recommend enhanced conservation monitoring of foraging behavior, expanded multi-year tracking from juvenile to adult stages, and integration with genetic analyses to evaluate heritability and colony-level effects. These insights also highlight the antiquity of sustainable Indigenous seabird harvesting and its compatibility with long-term seabird behavioral stability.

- Tissue-based inference: Bone collagen integrates diet over multiple years and cannot precisely locate foraging areas; feathers reflect shorter windows and were not pooled with bone. - Temporal baselines: Marine δ13C and δ15N baselines can change over time with climate and anthropogenic factors, introducing intertemporal uncertainty. - Life-stage unknown: Biological ages (juvenile vs. adult) could not be determined from skeletal elements, limiting life-stage-specific interpretations. - Spatial scope: Primary dataset comes from a single region (Yuquot); results may not generalize across the entire range without additional sites. - Behavioral resolution: Isotopes cannot distinguish individual-level site fidelity from colony-level spatial segregation. - Some reported statistical and niche-area values in the text are limited by archival dataset comparability and potential OCR/reporting inconsistencies; however, the main patterns (narrow niche, temporal stability) are robust.