The long-term monitoring of seabirds around proposed marine renewable energy (MRE) sites is vital to assess the large-scale and long-term environmental impacts of MRE installations. Marine radar could be a valuable tool to augment traditional seabird surveys but the problem of aspect dependency of the generic radar cross section (RCS) of live birds in flight must be understood before radar data is correctly interpreted. A marine radar multiple target tracking algorithm (‘GANNET’) was applied to data from an un-calibrated, horizontally polarised, 10kW X-band marine radar sited at the European Marine Energy Centre (EMEC) tidal renewable energy test site, Scotland U.K. From 24 days of data over 1.84 million target readings were recorded. For each target reading the radar aspect angle (bearing of radar beam incident on target), range and non-dimensional echo magnitude were derived allowing a view to be generated of the variation of echo magnitude with aspect angle for all tracked targets. The resulting polar diagram shows a significant change in echo magnitude with range between side-on and head/tail-on aspects indicating a large contribution of the RCS from the wings of birds in flight. The species-unspecific detectability of seabirds, especially at long range, is found to be strongly dependent on aspect angle. This has direct implications for the use of marine radar equipment for avian monitoring at proposed and active marine energy sites and must be taken into account if data from these radars are to be used to augment traditional bird abundance and area use surveys conducted by human observers.