Discussion
Our results demonstrate that within these northeastern forests, nest
survival of passerine birds is negatively influenced by abiotic factors
(i.e., intense rain bouts and minimum and average daily temperature) and
nests have a lower probability of surviving later in the breeding
season. These findings suggest important implications for montane
breeding birds as warmer temperatures and more frequent precipitation
events are likely to occur due to climate change (Murray et al., 2021;
Westra et al., 2014), both factors that have been documented to effect
species’ populations in this region (Duclos et al. 2019).
Montane birds face several tradeoffs when breeding along an elevation
gradient. For example, DeLuca (2013) documented that blackpoll warblers
are more likely to have a successful nest and overall higher annual
fecundity at high elevation breeding sites. However, high-elevation
breeding pairs of dark-eyed juncos (Junco hyemalis ) had overall
lower nest success, yet their offspring were in better condition than
their low elevation population (Bears et al. 2009), potentially because
of higher parental investment in these conditions (Badyaev & Ghalambor,
2001; Bears et al., 2009). We predicted that Swainson’s thrush nest
survival would have a higher probability of success at high elevations
due to the lower predator effects at high elevations (Boyle, 2008;
Camfield et al., 2010), and higher success from evidence DeLuca (2013)
found in blackpoll warblers, however, our model did not support this
prediction. Another purpose for examining this effect was because nest
depredation within the White Mountains has historically been affected by
red squirrel (Tamiasciurus hudsonicus ) densities on biennial cone
cycles (Townsend et al., 2015), creating challenges for nesting bird
species when these fir trees produce mass crops. Specifically, red
squirrel densities negatively affected nest survival of American
redstarts at a site approximately 64 kilometers west of our study site
(Hubbard Brook Experimental Forest, Sherry et al., 2015), raising the
possibility that high nest predation is possible during these
fluctuating seed-bearing trees (Bergeron et al., 2011; Holmes, 2011).
Furthermore, predation from birds may be more active in warmer
temperatures (Cox et al., 2013), potentially resulting in unusually
higher predation events in warmer, mass crop years. If montane species’
ranges continue to shift upslope (DeLuca & King, 2017; Van Tatenhove et
al., 2019), likely in response to climatic influences, they may avoid
the increased predation effects at lower elevations but consequently,
will be at risk of high elevation abiotic influences. Though we didn’t
monitor predator density due to equipment and time constraints, further
investigation of predator abundances and mass crops of fir trees along
the elevation gradient would help inform how influential predation could
be on nest survival of passerine species.
Extreme precipitation events have been shown to negatively influence
nest survival of several passerine species, including in great tits
(Parus major ; Bordjan & Tome, 2014; Schöll & Hille, 2020),
northern wheatears (Oenanthe oenanthe; Öberg et. al 2015), and in
mountain plovers (Charadrius montanus; Dinsmore et al., 2002;
Dreitz et al., 2012). Additionally, due to the orographic effect (i.e.,
colder high elevation air forcing clouds to release water), higher
montane elevations are likely to receive more rain, prompting us to
investigate the relationship between precipitation and nest survival. We
found a negative effect of light rain days (0.1-6.9mm daily cumulative,
Table 2) and rain intensity on DSR (Table 2, Figure 3) supporting
previously documented research and suggesting that rain bouts have
negative consequences on DSR of Swainson’s thrush. Also, we documented
large accumulations of precipitation between 30 June-4 July 2021 (Figure
2b) and had several nest failures throughout the elevation gradient (4
depredated, 4 abandoned) during this time. We examined an interaction
effect between seasonal time and daily cumulative precipitation to
evaluate if there were more nest failures after this rain event as well
as an interaction between nest age and rain intensity to determine if
the nest stage was affected by the amount of daily rain, but we did not
find support for these relationships. Nests that failed between 30
June-4 July 2021 had nestlings ~5+ days old, and perhaps
parents were unable to maintain their own energetic demands in addition
to caring for their nestlings (Martin, 1987; Martin, 1995) and chose to
renest after the inclement weather passed, though this is risky in that
fledglings born later in the season are less likely to survive (Shutler
et al., 2006). As the climate continues to warm, more intense
precipitation events will become more common (Trenberth, 2011; Westra et
al., 2014), putting these montane breeding birds at risk of these
effects.
Colder temperatures have been shown to negatively influence nest
survival of high elevation breeding bird species (Pierce et al. 2019)
and can delay spring insect emergence(Bears et al., 2009; Forrest &
Thomson, 2011), especially at higher elevation sites where there may be
persisting spring snow (Hahn et al., 2004). Additionally, warm
temperatures can promote arthropod abundance (Reneerkens et al., 2011;
Tulp & Schekkerman, 2008) as well as higher hatching success (Martin,
1987) and fledgling success (Reid et al., 2000). Despite our first
prediction, these studies raise the possibility that lower elevation
breeding populations may have an advantage over the colder, wetter high
elevation nesting pairs. We investigated several different temperature
metrics after predicting that minimum daily temperature would have a
negative effect on DSR and found strong support for this relationship,
in addition to a negative effect of average daily temperature on DSR,
but only in conjunction with elevation. This supports our prediction
that there are negative effects of temperature on DSR, and further
suggests a lower probability of Swainson’s thrush nest survival with
increasing elevation and colder temperatures. However, Dreitz et al.
(2012) reported higher nest success in mountain plovers in cooler
temperatures, and perhaps is preferable for high elevation ground
nesting breeding birds, since some predators may be more active when
temperatures are warmer (Cox et al., 2013).
Warming temperatures and inclement weather events are likely a product
of the changing climate (Hamburg et al., 2013; IPCC, 2014; Seidel et
al., 2009), resulting in negative direct and indirect effects on species
that breed in the White Mountains (Duclos et al., 2019) and causing both
upslope and downslope shifts in several montane bird species (DeLuca &
King 2017; Van Tatenhove et al., 2019). Although previous literature has
documented positive relationships with nest survival and warmer
temperatures (Hargrove & Rotenberry, 2011; Martin, 1987; Reid et al.,
2000), these climatic effects can trigger major phenological changes and
mismatches within bird populations (Leech & Crick, 2007). This could be
a disadvantage for single brood species, such as the Swainson’s thrush
(Mack & Yong, 2020) who rely on certain timing of events (i.e., insect
emergence, habitat availability, food abundance) during the breeding
season. It should be noted that this phenological phenomenon was not
documented at a nearby experimental forest within the White Mountains
(black-throated blue warblers, Setophaga caerulescens in Hubbard
Brook Experimental Forest; Lany et al., 2016). However, Swainson’s
thrush breed at higher elevations and occupy sensitive habitat, perhaps
putting them (and other high-elevation breeding birds) at greater risk
of climatic and phenological influences as a result of the warming
temperatures.
Evidence within the White Mountains overwhelmingly signals that the
climate is changing, leading to more frequent precipitation events in
the future (IPCC, 2014; Murray et al., 2021; Wason et al., 2017). Duclos
et al. (2019) found that Swainson’s thrush populations were directly
affected by precipitation, and indirectly influenced by changing
vegetation composition due to warming temperatures, though they were
uncertain what underlying mechanisms were driving these relationships.
Our results provide insight on this matter, indicating that
precipitation and minimum daily temperature negatively influenced nest
survival of Swainson’s thrush, thus, potentially affecting their
populations. Additionally, Swainson’s thrush rely on specific vegetation
composition that includes coniferous tree species such as red spruce and
balsam fir (Mack & Yong, 2020). Changes in vegetation composition have
occurred in this region (Foster & D’Amato, 2015), though red spruce may
be recovering, and thus expanding, due to shifts in land use within this
region (Battles et al., 2003). These climatic consequences to Swainson’s
thrush populations have both indirect and direct effects (Duclos et al.,
2019), and our results provide guidance on the potential underlying
mechanistic drivers these effects have.
This study provides evidence that Swainson’s thrush daily nest survival
is negatively affected by precipitation (light rain events and rain
intensity), as well as minimum and average daily temperature along an
elevational gradient. Several factors that were not measured in this
study could contribute to nest survival, including habitat structure,
predator abundance, and food availability. It is worth noting that other
studies have assessed the effects of temporal and abiotic variables on
nest survival and had considerably larger sample sizes than us (Dinsmore
et al., 2002; Dreitz et al., 2012; Johnson et al., 2018; Pierce et al.,
2019) and thus, could be one of the limitations this study faces. As the
climate continues to warm and cause more frequent rain bouts, as well as
shifts in plant and animal communities that may expose species to
factors they are not accustomed to, breeding birds will be at risk of
these shifts (Dunn & Winkler, 2010; Parmesan, 2007), especially for
species that occupy such restricted niches, like montane birds.
Specifically, one species of concern in northeastern forests is the
Bicknell’s thrush (Catharus bicknelli ), a rare and vulnerable
species that is facing population declines (Hill et al., 2019; Lambert
et al., 2008) and has already been negatively impacted due to climatic
effects (Lambert et al., 2008; Rodenhouse et al., 2008). (Freeman &
Montgomery, 2015) have documented dominant interspecific aggression from
Swainson’s towards Bicknell’s thrush, suggesting that Swainson’s thrush
may follow the “push” hypothesis and force Bicknell’s thrush out of
their climatic niche, a likely occurrence due to species range shifts
(DeLuca & King, 2017). Our inspiration for this study was considered
due to the vulnerability the Bicknell’s thrush faces, as well as the
finding that Duclos et al. (2019) reported. As we continue to better
understand the mechanistic drivers that influence these sensitive
montane species, this baseline knowledge will help inform land managers
about how passerine birds and their population sizes will cope with the
effects of climate change.