We identified arthropods in fecal samples from 56 Southwestern Willow Flycatchers (Empidonax traillii extimus) at three localities in Nevada and Arizona with different plant communities during the 2004 breeding season. We concurrently collected arthropods in flight with Malaise traps and on different plant species by sweep net. These potential prey were identified to Order and counted. Fecal samples contained 57 taxa of spiders and insects including 32 families in 8 Orders. Flycatchers consumed similar diversities (numbers of taxa), but different taxonomic compositions (abundances in Orders) of arthropods among localities. Diets of E. t. extimus more closely resembled compositions of arthropods swept from plants than those trapped in flight with Malaise traps. Fecal samples at Upper Pahranagat Lake in southern Nevada contained arthropod compositions most related to those swept from Salix gooddingii. Fecal samples at the Virgin River near Mesquite in southern Nevada, where Salix exigua and naturalized Tamarix ramosissima grow, contained arthropod compositions most related to those swept from S. exigua. Fecal samples at Topock Marsh in western Arizona contained arthropod compositions most related to those swept from T. ramosissima, the dominant vegetation. The relation between flycatcher diet and arthropod composition on plants was least at Topock Marsh, suggesting prey from other communities are important in supplementing the fauna that develop on introduced Tamarix. The diverse diet of Southwestern Willow Flycatchers may take advantage of the increased nitrogen and sulfur contents of spiders and predaceous insects.