Widespread decline of Yellow Warbler populations in California has led to increased interest in their conservation and management. However, because the species is now rare throughout much of its historic range in the state, there is relatively little demographic information about it. Predation and Brown-headed Cowbird parasitism are cited as causing declines, but their effects are poorly quantified. To address this information need, in 2008 we investigated the reproductive biology of the Yellow Warbler along the Pajaro River in Santa Cruz and Monterey counties, California, where the species is still relatively abundant. We examined predation and parasitism pressures by monitoring nests and recording reproductive success. In this heavily disturbed area, the Yellow Warbler’s nest success was very low (10%), revealing that one of the larger populations of this species remaining in the region may be threatened.