The Scottish Government has set a target of 100% of Scottish demand for electricity to be met by renewable sources by 2020. Offshore renewables have the potential to make a significant contribution to achieving this target. However, the Scottish Government has a duty to ensure that offshore renewable developments (ORDs) are achieved in a sustainable manner, by protecting habitats and species from adverse impacts. ORDs may negatively affect seabirds, in particular due to collisions with turbine blades, displacement to less favourable habitats and barrier effects to movement. In assessing the potential effects of ORDs on the environment the potential for impacts on protected populations for seabirds is assessed. To inform environmental assessments ORD developers conduct at sea surveys of seabirds. Most seabirds at sea data are collected during surveys from ships or planes, but the breeding colony a seabird has come from cannot be identified from these data. It is important to understand which breeding colonies birds at sea come from, to ensure that the impacts upon the appropriate colony can be assessed as part of the licensing process. The process by which the origin colonies of seabirds at sea is identified is called apportioning. In a project led by the UK Centre for Ecology & Hydrology a new tool was developed that built upon an existing tool that attributes seabirds at sea to specific breeding colonies. By comparing several new approaches to analysing seabirds at sea data new improved methods were developed. The resulting apportioning tool allows users to produce a more accurate estimate of the relative proportions of seabirds present in a specific location that can be attributed to different breeding colonies. This research is part of the Scottish Marine Energy Research Programme (ScotMER).