Arctic geese in newly colonised, colder breeding areas have higher spring body mass and breed earlier relative to the onset of spring

Global warming causes spring onset to advance, especially in the Arctic. Migratory animals may respond by advancing their phenology or colonising colder areas where spring starts later. The role of climate change in range expansion can be both driving (making traditional areas suboptimal) and facilitating (making new areas suitable). Recently, Pink-footed Geese (Anser brachyrhynchus) from Svalbard showed extreme range expansion by colonising the colder Novaya Zemlya as breeding ground, involving a new migration route. We examine potential costs and benefits associated with breeding in this new area. We use GPS-tracking, long-term population monitoring and remote sensing, to compare spring onset, migration timing and breeding performance between both flyways, and to evaluate how spring onset affects different reproductive stages in both breeding areas in these capital breeders. The traditional route showed challenges for migration timing, as spring had advanced in Svalbard (while arrival date had not kept up) but not on stopovers, and recently, early spring on Svalbard correlated with late spring on stopovers. On new stopovers, spring did advance. Spring was later in Novaya Zemlya than Svalbard, yet arrival dates were similar. In Novaya Zemlya, egg laying occurred later than in Svalbard, but still earlier relative to local spring onset (e.g. snowmelt and green-up), suggesting a smaller mismatch. The period between arrival and egg laying was longer in Novaya Zemlya than Svalbard, but breeding performance was similar. Finally, on the new route, geese were larger and (relatively) heavier than on the traditional route, thus possibly carrying larger capital body stores to cover harsher pre-laying periods. Our results suggest that colonising new breeding areas enables populations to regain phenological match with the environment, especially when advancement of migration timing was limited. Still, breeding in a colder area may require more parental investment, such as body stores. Thus, a benefit for offspring comes at a cost for parents. This mechanism can cause climate change to drive and facilitate colonisation especially in individuals capable of large investments. Consequently, variation in individual quality leads to heterogeneous effects of climate change within a population. These processes may play at the ‘cold’ edge of any range shift.