Predicting demographic impacts from sublethal cumulative effects of offshore renewable developments on breeding seabirds

1. Offshore renewable developments (ORDs) are often located in habitat used by protected seabird species and may cause sublethal effects by altering movement patterns and displacing individuals from key resources. Predicting how these effects translate into population-level impacts is challenging for long-lived species because demographic consequences emerge from complex, state-dependent behavioural and energetic processes. Individual-based models (IBM) provide a mechanistic framework to link behavioural responses to fitness and demography. 2. We present an IBM, ‘SeabORD’, to estimate the demographic consequences of sublethal displacement and barrier effects from ORDs. The model simulates time-energy budgets of individual seabirds during the chick-rearing period under scenarios with and without wind farms. From these simulations, we predict breeding success, adult mass change and year-round survival, allowing assessment of cumulative effects from multiple developments. We apply the model to black-legged kittiwakes Rissa tridactyla and common guillemots Uria aalge breeding at a North Sea colony, examining how impacts scale with increasing number of hypothetical ORDs. We also conduct a sensitivity analysis focussing on parameters with limited empirical support. 3. Model predictions indicate that demographic impacts do not increase linearly with exposure to ORDs. The strength and form of cumulative effects emerging from interaction between behavioural decisions and energetic constraints differed between species, highlighting the benefit of adopting a mechanistic approach in this context. The results of the sensitivity analysis indicated high sensitivity to some parameters, such as an adult body condition threshold below which adults abandon their breeding attempt. However, the emergent properties of model outputs were biologically plausible, and serve to highlight areas for future empirical work. 4. Synthesis and applications. SeabORD provides a transparent, mechanistic approach for predicting population-level consequences of sublethal interactions with ORDs, with direct relevance for environmental impact assessment and marine spatial planning. Our results demonstrate that cumulative effects may be non-additive and species-specific, highlighting limitations of current simplistic assessment approaches. Sublethal effects are the result of complex, interacting state-related behavioural decisions, and IBMs provide a platform for estimating stressor impacts and facilitating exploration into underpinning processes and key areas for future research.