Non-Invasive Sampling Reveals Colony-Scale Spatial Heterogeneity in Cadmium Concentrations in Atlantic Puffin Fratercula arctica Droppings in Iceland

Cadmium (Cd) concentrations in Atlantic puffin Fratercula arctica droppings were assessed during chick rearing using non-invasive sampling at nine Icelandic colonies. One fresh dropping was collected immediately after directly observed defecation from each of 159 different individuals in June–July 2018, and Cd was quantified by graphite furnace atomic absorption spectrometry on a dry-weight basis. Reported Cd concentrations were strongly right-skewed (median, 155.2 µg kg−1 dry weight; range, 1.1–3539.2 µg kg−1) and differed among colonies in the primary unadjusted comparison (Kruskal–Wallis, p < 0.001), with Holm-adjusted Dunn tests identifying 9 of 36 significant colony pairs. In a secondary model including dry sample mass, the colony effect remained strong (Wald χ2(8) = 108.75, p < 0.001), and adjusted geometric means spanned 24.2–506.9 µg kg−1 dry weight (20.9-fold). Variance partitioning yielded an ICC of 0.261 (95% confidence interval, 0.104–0.519), which is interpreted as a model-dependent estimate given the nine colonies. No association with chick-provisioning prey composition or breeding performance remained detectable after false-discovery-rate correction. The 2018 dataset therefore shows pronounced colony-scale contrasts in reported Cd concentrations in droppings. Mechanistic attribution requires direct prey measurements and multi-year replication.