摘要：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.La medición de la calidad de los sitios para las escalas de las aves durante su migración y el identificar los hábitats críticos para hacer dichas escalas son componentes importantes para conservar el ciclo de vida completo de las aves migratorias. Evaluamos la calidad del hábitat de las escalas ribereñas en los ríos San Joaquin y Mokelumne en el valle central de California, para las siguientes aves migratorias: los mosqueros sauceros (Empidonax traillii), las birijitas de coronilla anaranjada (Oreothlypis celata), las birijitas amarillas (Setophaga petechia) y las birijitas de Wilson (Cardellina pusilla). Para cada especie utilizamos tres enfoques diferentes para evaluar la calidad del hábitat: examinamos (1) los cambios en la masa corporal de los individuos que se recapturaron por lo menos una vez, (2) la condición corporal según la hora del día y a lo largo de la temporada migratoria, y (3) comparamos las tasas de cambio en la masa corporal por hora de las birijitas
