2009, COWRIE, A Review of Assessment Methodologies for Offshore Windfarms (Birds) series https://portal.medin.org.uk/portal/start.php?tpc=015_8e4e0b4a23c65a3aab78c24402576023 COWRIE previously published guidelines for ship and aerial survey of marine birds for offshore windfarm assessments (Camphuysen et al. 2004). The purposes of the current review are fourfold: a. to assess the extent to which these recommendations have been followed; b. to identify any differences in interpretation of the guidance between sites; c. to test the practicalities of using high definition cameras during aerial surveys; d. to provide more rigorous guidelines for the type of analytical techniques that should be used. The BTO and COWRIE undrtook a desk based study using datasets obtained from previous COWRIE surveys. 20090101 20090101