Measuring site quality for birds at migration stopover grounds and identifying critical stopover habitats are both important components of gauging the full life cycle conservation of migratory birds. We evaluated riparian stopover habitat quality on the San Joaquin and Mokelumne rivers in California's Central Valley for migrant Willow Flycatchers (Empidonax traillii), Orange-crowned Warblers (Oreothlypis celata), Yellow Warblers (Setophaga petechia), and Wilson's Warblers (Cardellina pusilla). For each species, we used 3 approaches to assess habitat quality: (1) we examined change in mass of individuals that were recaptured at least once; (2) we tracked body condition over the time of day and over the course of the migratory season; and (3) we compared the rates of hourly mass change for Wilson's Warbler to similar studies in Canada and the United States. On average, individuals of all species recaptured at stopover sites increased mass from initial capture to final capture (0.49 g to 0.88 g). Over the course of the day, the average condition of individuals of all study species either showed increases or remained stable, although most increases were driven by after-hatch-year birds in the population. Only Yellow and Wilson's warblers at the San Joaquin sites showed a positive relationship between capture date and condition. For Wilson's Warblers, rate of mass change was higher at California riparian sites than at other stopover sites in Canada and the United States. Our results demonstrate the importance of riparian floodplain forests in California as a stopover for migrating landbirds.