Body mass variation in Marbled Murrelets in British Columbia, Canada

Body mass was measured in Marbled Murrelets Brachyramphus marmoratus from May to August, 1994–1998 at Desolation Sound, and during June and July 1997 at Mussel Inlet, British Columbia, Canada, to assess seasonal, sexual, site and intra-annual variation. Birds were captured by mist-net, and by night-lighting, and were sexed using molecular techniques. The breeding status of females (gravid or not) in 1997 was determined from plasma levels of egg-yolk precursors. Adult males weighed 203.7 ± 14.8 g, n  = 495 (juveniles 166.6 ± 28.8 g, n  = 31) and females weighed 201.2 ± 20.5 g, n  = 344 (juveniles 148.3 ± 23.5 g, n  = 20). Murrelets caught in mist-nets were significantly lighter than those caught by night-lighting. Female mass was constant across years of the study, but males caught by night-lighting were heavier in 1998 than in 1997. Females declined in mass during the egg-laying period, but when gravid females were removed from the analysis, or when the post-laying data were analysed, no declines were found. Marbled Murrelets appear to maintain mass at a constant level all season, and are probably more income than capital breeders. Variation in body mass in this species may be constrained by the need to fly with speed and agility to avoid aerial predators.