MARBLED MURRELET GROUP SIZE AT SEA AS AN INDEX TO PRODUCTIVITY

Abstract 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 ...