Data from: Eggs brought in from afar: Svalbard-breeding pink-footed geese can fly their eggs across the Barents Sea

Many Arctic-breeding waterbirds are thought to bring nutrients for egg production from southern latitudes to allow early breeding. It has proved problematic to quantify the extent of such capital breeding and identify whether nutrients for egg production are brought in from nearby or from afar. Before reaching their breeding grounds on Svalbard, pink-footed geese Anser brachyrhynchus fly ∼ 1100 km across the Barents Sea from Norway. Using abdominal profile indexing (API) we scored body stores in individually marked geese just prior to migration from the northernmost staging area in Norway to Svalbard, followed by their breeding success on their non-breeding grounds in autumn. In productive breeding years leading to a high (> 13.8%) proportion of juveniles in the autumn population, there was a positive relationship between female API and number of young produced, suggesting that the geese are at least partial capital breeders. Moreover, focusing on the geographic origin of proteins used in egg synthesis and measuring nitrogen stable isotope ratios in pink-footed geese's eggs and food sources in Norway and Svalbard, we identified that capital breeding in this species is ∼ 50% on average but may potentially amount to as much as 100%, notably in females laying early. About 60% of this protein capital is carried in well-developed follicles across the Barents Sea, the remainder likely being stored in muscle tissues. Conditions on the wintering grounds and migratory stopover sites can have profound effects on an individual's fitness but the here presented link between the use of migratory stopover sites and breeding performance is particularly noteworthy. Apparently, some individuals accept the putative costs of carrying body stores over large distances to the breeding grounds. The data also highlights considerable variation in the reliance on capital for breeding, suggesting substantial individual scope to adjust breeding strategy to changing environmental conditions