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.