Size variation in mid-Holocene North Atlantic Puffins indicates a dynamic response to climate change.

Seabirds are one of the most at-risk groups, with many species in decline. In Scandinavia, seabirds are at a heightened risk of extinction due to accelerated global warming. Norway is home to significant portion of the European Atlantic Puffin (Fratercula arctica) populations, but Norwegian populations have declined significantly during the last decades. In this paper we use biometric data from modern and archaeological F. arctica specimens to investigate patterns in body size variation over time of this iconic species. We aimed to set out a baseline for our archaeological comparison by firstly investigating whether modern subspecies of F. arctica are reflected in the osteological characters and are enough to distinguish subspecies from the bones alone. We then investigated if archaeological remains of F. arctica differ in size from the modern subspecies. Our results show that the subspecies Fratercula arctica naumanni was distinctly larger than the other subspecies. However, Fratercula arctica arctica and Fratercula arctica grabae were difficult to separate based on size. This generally supports ornithological observations. Post-Medieval F. arctica bones from Måsøy were similar to modern F. a. arctica populations. The mid-Holocene remains from Dollsteinhola overlaps with the modern size ranges of F. a. arctica and F. a. grabae but are generally shorter and more robust. Dollsteinhola is located close to the borders of the modern breeding ranges of both F. a. arctica and F. a. grabae. We consider it therefore likely that given the mid-Holocene climatic oscillations, breeding ranges of the two subspecies shifted north or south accordingly. However, this does not explain the different proportions of the Dollsteinhola specimens. Our data provide the first evidence for shifting distributions in ancient Atlantic Puffins and represent the first osteological analysis of Fratercula arctica subspecies.