Contact information:
Sarah Deckel, sdeckel@umass.edu, 401-871-0973
Dr. William DeLuca, william.deluca@audubon.org
Dr. Alexander Gerson, argerson@umass.edu, 413-545-2782
Dr. David King, daveking@umass.edu, 413-545-6795
Abstract
Montane birds experience a range of challenges that may limit their breeding success, including nest predation and severe climactic conditions. The continuing effects of climate change are causing shifts in biotic and abiotic factors that may compound these threats to montane bird species. In northeastern montane forests, many bird species are shifting downslope, 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. Thus, montane birds might be faced with the opposing pressures of adverse climactic conditions at higher elevations and increased predation at lower elevations. We monitored nests of Swainson’s thrush (Catharus ustulatus ) along an elevation gradient in the White Mountain National Forest in New Hampshire in 2016, 2018, 2019 and 2021 to examine the effect of biotic and abiotic factors on nest survival. We found a significant negative effect of rain intensity (millimeters per hour per day) on daily nest survival, suggesting that heavier rain per hour decreases Swainson’s thrush daily daily nest survival. Moreover, we found a negative interaction effect of elevation in conjunction with minimum daily temperature and average daily temperature, suggesting that at higher elevations, temperature, specifically on colder days, decreases Swainson’s thrush nest survival. Our results provide evidence for a potential mechanism of how climate change will affect nesting survival of montane breeding birds 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.
Keywords: montane, nest survival, abiotic factors, White Mountains

Introduction

Montane bird species that breed within high elevation forests are particularly vulnerable to the effects of climate change due to the narrow elevational ranges they inhabit (Scridel et al., 2018), making them susceptible to extirpation (Sekercioglu et al., 2008) if their elevational ranges are compressed due to climate change and anthropogenic land uses (Rodenhouse et al., 2008). Research within northeastern forests of the United States confirms that the average temperature within this system has increased over the last several decades (Murray et al., 2021; Wason et al., 2017), which is likely causing recently documented downslope shifts in montane birds (DeLuca & King, 2017) and high-elevation vegetation (Foster & D’Amato, 2015; Wason et al., 2017).
Coincident with these changes, the elevational distribution of montane birds has also shifted downwards (DeLuca & King, 2017), although the motives for these shifts, and their consequences for montane breeding birds, is unclear. Within northeastern high elevation forests, populations of montane birds are declining (Ralston et al., 2015), potentially because elevational range shifts are exposing birds to unfamiliar resources and conditions along the elevation gradient that could affect their breeding success. For example, breeding success of blackpoll warblers (Setophaga striata ), is lower at low elevations, likely due to increased abundance of red-squirrels, a major nest predator in this system (DeLuca, 2013). Similarly, dark-eyed juncos (Junco hyemalis ) at high elevations produced nestlings in better condition and experienced higher survival rates than birds at lower elevations (Bears et al., 2009). These examples raise the possibility that lower elevations are less suitable for montane birds, and that the documented downward shifts in their elevational ranges may expose these species to elevated nest predation (Boyle 2008, Camfield et al., 2010; DeLuca, 2013).
Alternatively, abiotic factors, such as cold temperatures and increased precipitation, may affect reproductive success of birds nesting at high elevations (Pierce et al., 2019). Warmer temperatures promote larger egg size (Johnson 2000; Lessells et al., 2002), increased clutch size (Hargrove & Rotenberry, 2011; Rotenberry & Wiens, 1991), and improved hatching and fledgling success (Martin, 1987; Reid et al., 2000), and greater food availability (Bears et al., 2009; Svensson & Nilsson, 1995). Increased precipitation may also negatively influence nesting survival of breeding birds (Bordjan & Tome, 2014; Dinsmore et al., 2002; Dreitz et al., 2012; Fisher et al., 2015; McCain & Colwell, 2011; Öberg et al., 2015; Schöll & Hille, 2020), particularly during intense rain events (Öberg et al., 2015; Sexson & Farley, 2012). High elevation sites in the Northeast U.S. are experiencing more frequent rain events (Wright 2009; Karmalkar & Bradley, 2017). Precipitation is known to negatively affect avian nesting success and as precipitation intensities are increasing in frequency, this raises concern for montane birds nesting in this region.
Due to the reported vulnerability of montane birds and the threats that a changing climate imposes on high elevation species (Ralston & DeLuca, 2020; Ralston & Kirchman, 2013), we located and monitored nests of a representative montane bird species, the Swainson’s Thrush (Catharus ustulatus ) within the White Mountains of New Hampshire, USA. We considered three predictions: 1) nests at higher elevations would be more likely to be successful due to lower predation events (Boyle, 2008; Camfield et al., 2010), 2) average minimum daily temperature, minimum daily temperature, or higher temperature range (i.e., greater daily variability in temperature) would negatively affect daily survival rate (Pierce et al., 2019), and 3) increased daily cumulative precipitation, more intense rain bouts (i.e., more millimeters of rain per hour), or heavy precipitation events would negatively affect daily survival rate (Dinsmore et al., 2002; Dreitz et al., 2012; Pierce et al., 2019). Documenting the abiotic effects on reproductive success of birds along elevation gradients will contribute to a better understanding of future climatic responses by species (Martin, 2001; Tingley et al., 2012), something that is largely understudied within northeastern montane forests.

Materials and Methods