Distance from riparian edge reduces brood parasitism of southwestern willow flycatchers, whereas parasitism increases nest predation risk

Abstract

The southwestern willow flycatcher (Empidonax traillii extimus) is a federally endangered subspecies that breeds in increasingly fragmented and threatened habitat. We examined whether temporal and habitat characteristics were associated with risk of predation and probability of brood parasitism by brown-headed cowbirds (Molothrus ater) on flycatcher nests at 6 sites in southern Nevada and northwestern Arizona, USA. For nest predation, we found the most support for a model that included date and an interaction between parasitism status and nesting stage. Daily nest survival decreased from 0.87 (95% CI ¼ 0.81-0.93) to 0.78 (95% CI ¼ 0.72-0.84) through the season for parasitized nests but remained relatively constant for unparasitized nests (0.93, 95% CI ¼ 0.91-0.95 to 0.92, 95% CI ¼ 0.91-93). Parasitized nests had lower survival than non-parasitized nests during the incubation (0.85, 95% CI ¼ 0.84-0.86 vs. 0.92, CI ¼ 0.91-0.93) and nestling (0.79, 95% CI ¼ 0.77-0.81 vs. 0.91, 95% CI ¼ 0.90-0.92) stages. Of the variables included in our parasitism candidate models, model-averaged coefficients and odds ratios supported only distance to habitat edge; odds of parasitism decreased 1% for every 1 m from the habitat edge. Nests greater than 100 m from an edge were 50% less likely to be parasitized as those on an edge, however, only 52 of 233 nests (22%) were found at this distance. Where management and conservation goals include reducing nest losses due to parasitism, we recommend restoration of habitat patches that minimize edge and maximize breeding habitat further from edges. At sites where cowbirds have been documented as important nest predators, controlling cowbirds may be one option, but further study of the link between parasitism and nest predation and the identification of major nest predators at specific sites is warranted. ß 2011 The Wildlife Society.KEY WORDS brown-headed cowbird, Empidonax traillii extimus, habitat, lower Colorado River, Molothrus ater, nest predation, southwestern willow flycatcher.

The Influence of Meadow Moisture Levels on Activity of Small Mammal Nest Predators in the Sierra Nevada, California

“…Drawdown zone habitat might also function as an ecological trap because the modified habitat appears suitable but is less productive than habitat elsewhere. We did not measure the DSR of warbler and flycatcher nests outside the drawdown zone, but the DSR values that we recorded are similar to previously reported nest DSR for Yellow Warblers (0.94-0.99; Tewksbury et al 1998, Willson and Gende 2000, Galigan et al 2006, Richardson et al 2009, Latif et al 2012) and for Willow Flycatchers (0.78-0.98; Galigan et al 2006, Stumpf et al 2012). This suggests that drawdown zone habitat within the ALR does not Quinlan and Green (2012).…”

Poor reproductive success of Gray Vireos in a declining California population

“…Other factors such as nest predation could also be contributing to the decline, perhaps interactively with cowbirds, as reported for other species (e.g., Stumpf et al. ).…”

“…) or indirectly by attracting predators (e.g., Stumpf et al. ). Brown‐headed Cowbirds began spreading into California about 1910 (Willett , Laymon ), leading to parallel declines in other bird species susceptible to parasitism (Unitt ).…”

Declining Brown-Headed Cowbird (Molothrus ater) Populations Are Associated with Landscape-Specific Reductions in Brood Parasitism and Increases in Songbird Productivity

“…The presence of a cowbird nestling can result in zero host young fledging from on otherwise successful nest because of egg removal or destruction (e.g., [46]), host ejection [47], or host mortality due to competition [48]. In addition, louder and more frequent begging by cowbird young [9] and increased parental activity at nests with cowbird young [49] may result in higher predation risk [50] and contribute to lower rates of nest survival for parasitized nests as seen here and elsewhere [51], [52]. The negative effects of parasitism on fledging brood size and nest survival (and thus on our productivity measure) combined with the fact that cowbirds depredate nests more frequently in less forested landscapes [13] suggest that cowbirds may be a primary cause of the forest fragmentation effect on songbird productivity in the Midwest.…”