Combination of At-Sea Activity, Geolocation and Feather Stable Isotopes Documents Where and When Seabirds Molt

Key facets of the foraging ecology of seabirds during the inter-breeding period still remain poorly understood because of the difficulty of studying them at sea, including during the energy-demanding molting stage. Here, the extent to which three sympatric petrels (Antarctic and thin-billed prions, and blue petrel) from the subantarctic Kerguelen Islands modify their foraging ecology during molt was investigated using a combination of complementary tools, namely miniaturized saltwater immersion geolocators (GLS) and the isotopic method. Firstly, molting behavior was first characterized in the blue petrel, a reference species that is known to renew its plumage in autumn. GLS and feather stable isotopes ( 13 δ C as a proxy of the birds’ foraging habitat) indicated that the post-breeding molt of blue petrel occurred in Antarctic waters. Importantly, activity recorders showed that molt was marked by a strong peak in time spent daily sitting on water, which thereafter declined to lower values during the remaining winter months. Secondly, the peak in time spent sitting on water was used as a proxy to characterize the contrasted molt strategies of the two prion species. As blue petrels demonstrated, thin-billed prions molted during the post-breeding period in cold Antarctic waters where they fed primarily on low trophic level prey, most likely Antarctic krill ( 15 δ N as a proxy of the birds’ diet). By contrast, Antarctic prions presented an unexpected pre-breeding molt of longer duration that took place further north, in warm subtropical waters. Interestingly, the two Antarctic molting species, the blue petrel and thin-billed prion, renewed their plumage at the same time and within the same oceanic zone that is likely to be a previously undescribed hot spot of seabird diversity during the Austral autumn. The study contributes to a growing body of evidence that closely-related species exhibit various foraging strategies allowing ecological segregation and sheds new light on the poorly known critical molting stage of seabirds.