Riparian habitat restoration increases the availability and occupancy of Yellow-breasted Chat territories but brood parasitism is the primary influence on reproductive performance

Implementation and evaluation of conservation efforts requires an understanding of the habitat selection and reproductive success of endangered populations. As populations recover, established territory holders may force new arrivals into lower-quality habitat, which can reduce reproductive success, especially in disturbed landscapes where suitable habitat is scarce. The endangered Western Yellow-breasted Chat (Icteria virens auricollis) population in the fragmented riparian zone of the Okanagan Valley, British Columbia, Canada, has rapidly increased in response to habitat restoration. During this population increase from 2002–2014, we monitored 485 chat nests in 157 breeding territories to evaluate the influences of habitat selection, habitat restoration, and brood parasitism by Brown-headed Cowbirds (Molothus ater) on reproductive performance. We found that, in protected reference sites, breeding territories that were occupied in the early years of the study had higher percent shrub cover than territories that were first occupied in the later years of the study, indicating that chats preferred territories with high shrub cover. Conversely, in restoration sites, later-occupied territories had similarly high shrub cover as earlier-occupied territories, suggesting that restoration activities enabled chats to continually settle in territories with high shrub cover. Yet, we did not find strong evidence that nest site vegetation characteristics or habitat restoration influenced reproductive performance. Instead, the high rate of brood parasitism (49%), which reduced nest success and productivity, was the dominant influence on reproductive performance. However, this recovering population still had high daily nest survival (0.974) and productivity (2.72 fledglings per successful nest) compared to other riparian songbirds and the high parasitism rate did not prevent the population from increasing. Thus, conservation efforts for Yellow-breasted Chats should focus on restoring riparian shrubs, even within heavily developed landscapes, to increase the number of potential breeding territories, while also quantifying how brood parasitism influences reproductive performance.

Methods
Methods and analyses are described in the manuscript.