We present datasets from long-term studies of brood parasitism of Southwestern Willow Flycatcher (Empidonax traillii extimus) populations at the South Fork Kern River (SFKR), California, the Grand Canyon, Arizona, and from other intensive flycatcher studies in Arizona. In the two main study areas, we recorded high parasitism rates for the flycatcher. We found that 75 % of Willow Flycatcher nests failed completely when parasitized and that an extremely low percentage of Willow Flycatcher eggs survived to fledging in parasitized nests (11% vs. 47% in unparasitized nests). Our data show that cowbird parasitism also delayed the fledging of young flycatchers. However, contrary to our expectations, we did not find a significant difference between the return rates of “early” versus “late” fledged birds. To evaluate how important cowbird parasitism is to the population decline of the endangered Southwestern Willow Flycatcher, we reviewed the current level of parasitism on this species throughout its range in six states using a large number of datasets from different sites. We also reviewed the historic pattern of increase in Brown-headed Cowbird (Molothrus ater) populations in the southwest between 1872-1997 using both nest record and egg collections and documentary evidence. Given the level of impacts to flycatcher productivity inflicted by cowbird parasitism that we observed at SFKR and Grand Canyon, it is likely that cowbirds played a role historically in reducing many local Southwestern Willow Flycatcher populations. Also, cowbirds continue to play a role in slowing or preventing the recovery of this subspecies.