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