Modified X-band marine radar was used to quantify flight heights, passage rates and flight behavior of Marbled Murrelets Brachyramphus marmoratus on the Olympic Peninsula, Washington, and to assess the collision risk associated with future coastal wind developments. Over three mornings, 287 height observations were collected for targets matching murrelet speed, flight height and density on radar. Mean height above ground level was 246 (SE 4.7) m. The lowest murrelet-type target detected was at 62 m, while the highest was recorded at 663 m. Fifty percent of murrelet-type targets were detected between 196 m and 286 m. A maximum-likelihood model of flight heights estimated that 4.6% of murrelets were flying at or below the average wind turbine rotor-swept height of 130.5 m as the birds transited to and from nest sites. From the same model, it was estimated that 0.5% and 0.01% of birds, respectively, were flying at or below the heights of typical communication towers and transmission lines. These data comprise three days of sampling at a single location, and thus may not be representative of height distributions found in other sites or regions. Flight heights likely vary with topography, distance from the ocean, weather, and other factors, and thus the proportion of birds flying below turbine height likely varies both spatially and temporally. Additional data on murrelet flight heights are now being collected to determine how the relative risk of collision changes with topography and weather conditions. This information should help predict the risk of collision with artificial structures, including wind energy developments that may pose a risk to murrelets.