Population demographics of marbled murrelets (Brachyramphus marmoratus) are difficult to measure. Population size can be estimated from at-sea densities, and productivity indices are sometimes derived from ratios of hatch-year to after-hatch-year birds observed during systematic surveys on the ocean. However, one cannot determine from marine surveys alone what proportion of a marbled murrelet population sampled at sea is nesting in any given year, which would allow a more meaningful interpretation of productivity ratios. We hypothesized that group size (the number of murrelets occurring together on the water) could provide such an index if it could be demonstrated that single murrelets detected on the ocean during the incubation phase of the breeding season represent breeding birds. We monitored radio-tagged murrelets in Desolation Sound, British Columbia, from 29 May through 19 June 2001, using an incubating pair's distinct 24-h on-off occurrence on the water to determine nesting status. Of 160 murrelet groups comprised of at least 1 individual of known nesting status, there was a significantly greater percentage (37%) of single birds among incubating murrelets than among non-incubating birds (20%). Annual variation in the proportion of single murrelets recorded on marine surveys in Puget Sound during peak incubation corresponded with the annual productivity index in 4 of 5 y from 1997 to 2001. Our results suggest that group size, especially the proportion of single-bird groups, may help assess the proportion of murrelets that are nesting. Multiple-year comparisons of group size with nesting rates are needed to validate and interpret these results, and we need to continue to explore new methods to measure murrelet productivity.