Age-structured model reveals prolonged immigration is key for colony establishment in Gentoo Penguins | Un modelo estructurado por edad revela que la inmigración prolongada es clave para el establecimiento de la colonia en Pygoscelis papua

Understanding the mechanisms of site colonization and range expansion is crucial to understanding population dynamics, particularly for colonial seabirds that may struggle to shift their breeding ranges under climate change. We provide an alternative and simple approach to estimating the number of migrating individuals among colonies when original mark–recapture datasets are not available for use in more integrated and established methods. Here we fit an age-structured population model with published vital rates and breeding success rates to simple point counts of abundance using rejection-based approximate Bayesian computation (ABC) to estimate the contribution of immigration to four recently colonized Gentoo Penguin (Pygoscelis papua) breeding sites on the Western Antarctic Peninsula. We found that sustained immigration over several years was required to generate the rapid population growth observed, with some sites even showing evidence of an accelerating immigration rate following initial colonization. We demonstrate that our method is capable of estimating the contribution of immigration to population growth in a species where mark–recapture datasets are unavailable. By leveraging census data that are relatively easy to obtain, our approach provides a new method for understanding how range expansions occur in species such as Antarctic penguins whose habitat is undergoing changing climate conditions. LAY SUMMARY Understanding how birds colonize new locations is important for learning how they may respond to climate change. While many methods exist to estimate the number of individuals that are moving among colonies, they cannot always be applied to certain species for which certain datasets are missing or unavailable. We overcame this issue by applying a common statistical approach to simulate growth of four new Gentoo Penguin colonies and compare these to the true growth patterns. Our model results showed that the rapid population growth we observed must have been supported by hundreds of immigrants coming from other colonies. These findings are interesting because these new colonies have led to a southward expansion of this species' typical breeding distribution and this recent range expansion may be a response to changing climate. We demonstrate that this method is effective at estimating movement among colonies when other similar methods cannot be applied due to lack of dataset availability and data collection limitations of the species of interest.