Comparison of Aerial and Boat-based Survey Methods for Marbled Murrelets Brachyramphus Marmoratus and Other Marine Birds

We compared density estimates for marine birds off central California from simultaneous aerial and boat-based surveys, with a special emphasis on Marbled Murrelets Brachyramphus marmoratus. We surveyed 44 eight-kilometer transects in nearshore waters of Monterey Bay, California, from December 2005 through March 2006. We found that density estimates of all birds combined and of Western Aechmophorus occidentalis and Clark’s A. clarkii Grebes were significantly greater from the air than from the boat, but no significant differences in density were noted for four other taxa. Density estimates for Marbled Murrelets were nearly identical from the two platforms, with a high degree of correspondence correlation (0.82). Mean species richness was significantly greater from the boat than from the air. For taxa that occurred in large flocks (grebes and gulls), density estimates from the air were substantially greater when abundance was greater. Our results indicate that aerial surveys may provide more accurate density estimates than do boat-based surveys for some taxa, under certain conditions. However, density estimates were similar for most taxa, and density estimates from the two platforms based on certain survey methods may be similar enough to be considered directly comparable.