Reconstructing and forecasting the dynamics of an internationally important population of Manx shearwaters

The Isle of Rum (Scotland) holds ~30% of the world’s Manx shearwater population. The status and vulnerability of this internationally important and remote population are currently in question, mainly because of variations in field survey methodologies used on Rum across the years. This is particularly urgent in light of proposed offshore wind farm developments in the area. Here, we aimed to reconstruct population and demographic trends and quantify future sensitivities using a Bayesian state-space model. Fitting the model to simulated data of a similar nature and extent as the real data confirmed our ability to retrieve hidden parameters and reconstruct latent population trends in partially observed or wholly unobserved demographic time series. Applying these methods to the real data revealed that the population has been increasing since the 1980s but may now be starting to plateau. We extended the temporal horizon to a 100 yr forecast and ran several counterfactual scenarios relating to anthropogenic impacts on adult mortality and fecundity. These experiments indicated that the population is robust to strong pulse perturbations (e.g. wind farm construction or epidemic outbreaks), but vulnerable to small, sustained perturbations in adult survival (e.g. low-level mortality due to nearby wind farm operation). By integrating different data types collected by varying field methods over mismatched time windows, we have gained valuable insights into the status of this difficult-to-monitor species. Impact assessments for planned offshore wind developments around Rum should focus on collision and displacement costs to provisioning adults borne by wind farm operation (rather than construction).