Factors affecting the nesting success of Swainson's thrush (Catharus ustulatus) along an elevational gradient

Montane birds face several ecological challenges during the breeding season, including nest predators, competition for territory, and food availability. The continuing effects of climate change are causing shifts in these plant and animal communities, as well as changes in abiotic factors such as increased temperature and precipitation effects, adding additional stressors that could affect nest survival, putting their populations at risk. The Swainson’s thrush (Catharus ustulatus) is migratory songbird which is moderately common and breeds along a wide elevation gradient within this system (200-1,250m). Populations of Swainson’s thrush were once declining, though recent research shows their ranges to be shifting downwards, potentially as the result of increased precipitation and temperature at higher elevations. Although lower elevations might be more favorable in terms of climactic conditions, nest predation is higher at lower elevations in other systems. Thus, this species might be faced with the opposing pressures of adverse climactic conditions at higher elevations and increased predation at lower elevations. Nest survival is a fundamental process to population size, and therefore, is important in evaluating how montane breeding birds are adjusting to the changing climate. We monitored nests of Swainson’s thrush along an elevation gradient in the White Mountain National Forest in New Hampshire in 2016, 2018, 2019 and 2021 at two sites: Mt. Jefferson (500-1,250m) and Bartlett Experimental Forest (200-300m). We found a significant negative effect of rain intensity (millimeters per hour per day) as well as a negative interaction effect of elevation with minimum daily temperature and average daily temperature, on the daily survival rate of Swainson’s thrush. Our results provide evidence that nesting survival of montane breeding birds could be at risk as heavier precipitation events become more frequent and intense, a likely outcome due to the changing climate within the White Mountains and other montane ecosystems, putting other passerine species at risk in this system.