—We combined radiotelemetry, plasma metabolite analyses, and macro-invertebrate prey sampling to investigate variation in putative fatt ening rates (estimated as plasma triglyceride levels) at the fl yway scale in Western Sandpipers (Calidris mauri) migrating between Punta Banda, Mexico (31°N), and Hartney Bay, Alaska (60°N), a distance of 4,240 km. Birds were caught at a wintering site (San Francisco Bay) and eight stopover sites along this Pacifi c Flyway. Body mass was higher in females than in males at six sites, but variation was not correlated with latitude for either sex, and the relationship of change in mass by date within sites was uninformative with regard to possible latitudinal variation in fatt ening rates. At San Francisco Bay, triglyceride levels were higher in the spring than in the winter. Mean plasma triglyceride varied among stopover sites, and there was a signifi cant linear trend of increasing triglyceride levels with latitude as birds migrated north. At San Francisco Bay, length of stay was negatively related to triglyceride levels. However, plasma triglyceride levels at wintering or initial stopover sites (San Francisco and Punta Banda) did not predict individual variation in subsequent rates of travel during migration. We found no signifi cant relationship between triglyceride levels and prey biomass at diff erent stopover sites, which suggests that the latitudinal patt ern is not explained by latitudinal changes in food availability. Rather, we suggest that diff erences in physiology of migratory birds at southern versus northern stopover sites or behavioral diff erences may allow birds to sustain higher fatt ening rates closer to the breeding grounds. Received 24 February 2006, accepted 22 July 2006.