Additional fieldwork details, GOF tests and E-SURGE implementation from High-Arctic family planning: earlier spring onset advances age at first reproduction in barnacle geese

Quantifying how key life-history traits respond to climatic change is fundamental in understanding and predicting long-term population prospects. Age at first reproduction (AFR), which affects fitness and population dynamics, may be influenced by environmental stochasticity but has rarely been directly linked to climate change. Here, we use a case study from the highly seasonal and stochastic environment in high-arctic Svalbard, with strong temporal trends in breeding conditions, to test whether rapid climate warming may induce changes in AFR in barnacle geese, Branta leucopsis. Using long-term mark-recapture and reproductive data (1991–2017), we developed a multi-event model to estimate individual age at first reproduction (AFR) (i.e. goslings produced). The annual probability of reproducing for the first time was negatively affected by population density but only for 2 year-olds, the earliest age of maturity. Furthermore, advanced spring onset (SO) positively influenced the probability of reproducing and even more strongly the probability of reproducing for the first time. Thus, because climate warming has advanced SO by two weeks, this likely led to an earlier AFR by more than doubling the probability of reproducing at 2 years-old. This may, in turn, impact important life-history trade-offs and long-term population trajectories.