Ornithological offshore wind farm impact assessments are complicated, with many steps and associated uncertainty. One of these steps involves the apportioning of predicted impacts on species to relevant Special Protection Areas (SPAs) where they are designated features for Habitats Regulations Assessments. 

In the non-breeding season, Biologically Defined Minimum Population Scale (BDMPS) data provide information on the composition of populations in specific sea areas. This information can be used to inform the apportioning of impacts to SPAs. However, the information underpinning this approach requires periodic updates to incorporate new evidence. To address this, Natural England, on behalf of the other Statutory Nature Conservation Bodies (SNCBs), and with funding from the Department for Environment, Food and Rural Affairs (DEFRA), commissioned BTO to carry out an independent review of the BDMPS approach. 

The aim of this review is to inform methodological refinements and identify contemporary sources of data for use in an update. This review has taken into consideration the evidence requirements of the SNCBs and has engaged with a representative cross-section of industry stakeholders. This evidence will be used to inform a subsequent update of the original BDMPS work for specified seabird species that will contribute to new guidance and best practice for future offshore wind farm impact assessments.

This review assesses the BDMPS approach originally developed by Furness (2015) and its application in impact assessments of seabirds for offshore developments. The original methodology relied on expert opinion in the definition of seabird population regions, based on their seasonal distributions and connectivity to UK waters. With the greater availability of tracking data and updated census data, the information applied to the original framework can be updated. 

The prevalence of updated data supports a shift in methods to a more quantitative and repeatable approach. The findings of this review should inform planned future work. It is anticipated that future updates will further refine the methodology of calculating population estimates, with consideration of an accessible interface for conducting apportionment.

This Excel spreadsheet forms Appendix C of the report.

1. UK Statutory Nature Conservation Bodies (SNCBs) require estimates of seabird populations during the non-breeding season to apportion impacts of offshore wind farm developments to Special Protected Areas (SPAs). Furness (2015) formed the Biologically Defined Minimum Population Scales (BDMPS) method, the primary tool in apportioning impacts of offshore wind farms to SPAs for Habitats Regulation Assessments (HRAs) during the non-breeding season. The outputs of the report have also been used to inform reference population values for Environmental Impact Assessments (EIA) and seasonal definitions for assessments.
2. This review (Phase 1) aims to evaluate the methods used within the original Furness (2015) report. We highlight potential refinements to the current BDMPS approach, including methods, definitions, and data resources, to improve future ease of use and accuracy of estimates. The review included workshops to consult SNCBs and industry stakeholders to discuss refinements to populations, seasons, and regional designations within the BDMPS framework.
3. Where feasible, we propose the use of a quantitative and hierarchical approach (based on resolution of available data) for defining regions and seasons. Designation of regions should be informed by tracking data (geolocators, GPS) and updated ring recovery data from repositories such as SEATRACK (https://seatrack.net) and The Eurasian African Bird Migration Atlas (https://migrationatlas.org). Seasonal definitions may incorporate monthly data to allow greater flexibility in assessments.
4. We suggest that the assessment of uncertainty in population estimates uses a quantitative approach, moving from expert judgement to a data-driven assessment of confidence based on the availability and quality data.
5. Case studies updating population estimates of Guillemots and Great Black-backed Gulls display the application of updated demographic data and highlight the importance of updating proportions of overseas populations in future, for example, using updated population models and tracking data.
6. Discussions within the workshops highlighted the challenge in balancing high-resolution data, biologically meaningful population estimates, and the need to make apportioning based on BDMPS methods practical, ensuring that assessments remain ecologically meaningful and compliant with regulations.
7. Specific discussions focused on potentially adopting a gridded approach to defining BDMPS regions, allowing for finer-scale apportioning of impacts. Additionally, there was discussion on how different SNCBs apply BDMPS data, with some using bespoke seasonal and regional definitions, which potentially require standardisation. Consideration was also given to the different seasonal definitions being used by SNCBs. The need to address cross-border impacts was discussed, particularly in relation to the future inclusion of the Republic of Ireland given the increasing offshore wind development within the Irish Sea.
8. Future work (Phase 2 and beyond) should implement the suggested updates to the BDMPS methodology, address knowledge gaps, update data sources, and provide guidelines on how stakeholders should interpret and use updated BDMPS data to maintain consistency in impact assessments.
9. Ongoing collaboration between conservation bodies, researchers, and industry stakeholders is needed to facilitate updates to the BDMPS methodology and application in future impact assessment.