Biogeochemical markers in juvenile Leach's storm-petrels (Hydrobates leucorhous) stranded offshore are distinct from inshore stranded juveniles and nestlings.

Pelagic seabirds are one of the most threatened bird groups globally. Attraction to, and disorientation by, artificial light at night (ALAN) is a key factor reducing survival of nocturnal, burrow nesting seabird species. To investigate whether Leach's storm-petrel (Hydrobates leucorhous) nestlings and juveniles stranded due to ALAN could be differentiated using biogeochemical markers, we compared ten trace element concentrations (As, Cd, Co, Cu, Hg, Mn, Pb, Tl, V, Zn) and two stable isotope values (δ13C and δ15N) in juvenile feathers. We expected stranded juveniles to have different trace element concentrations from those sampled in the nest, and dietary indicators in the same feathers (δ13C and δ15N) to differ between groups. We found that offshore stranded juveniles had higher trace element concentrations than inshore stranded juveniles and nestlings while concentrations for nestlings and inshore stranded juveniles were more similar. Both stable isotope values were higher in nestlings than in stranded juveniles, with δ13C being similar between inshore and offshore stranded birds, while δ15N was lower in offshore compared to inshore stranded juveniles. Our study shows that inshore and offshore stranded juveniles have distinctly different biogeochemical markers, raising questions about differences in the mechanisms underlying the likelihood of stranding at inshore and offshore locations.