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.