Study area and study species.
We researched nest survival of Swainson’s thrush along an elevational gradient in the White Mountains of New Hampshire, USA at Mt. Jefferson (approximately 500-1,250m; 44.3045° N, 71.3176° W) and Bartlett Experimental Forest (hereafter referred to as BEF; approximately 200-300m; 44.0556° N, 71.2973° W). The Swainson’s Thrush is a Nearctic Neotropical migratory songbird which is moderately common and breeds along a wide elevation gradient within this system (Figure 1, 200-1,250m; Mack & Yong, 2020). Though their populations were once documented to be declining (Holmes & Sherry, 2001; Robbins et al., 1989), recent research has shown their ranges to be increasing (Ralston et al. 2015) as well as shifting downslope (DeLuca and King, 2017), suggesting population increases. Nest survival data were collected from May through August 2016, 2018, 2019, and 2021 (field season canceled in 2020 due to COVID-19) in the White Mountains at Mt. Jefferson (all years) and BEF (2018, 2019, and 2021). The surrounding plant community on Mt. Jefferson below 700m in elevation consists of northern hardwood forest dominated by American beech (Fagus grandifolia ), paper birch (Betula papyrifera ), yellow birch (Betula alleghaniensis ), and sugar maple (Acer saccharum ). Between 700m and 1,300m is a mixed forest of deciduous and coniferous of birch trees and red spruce (Picea rubens ), balsam fir (Abies balsamea). As elevation increases above 1,250m, high alpine boreal tree species become more abundant before reaching treeline at ~1,350m. BEF ranges from 200-400m in elevation, though to capture low elevation breeding populations of Swainson’s thrush, nests were searched only between 200-300m. Tree species within BEF are primarily American beech, paper birch, red spruce and eastern hemlock (Tsuga canadensis ), though Swainson’s thrush typically build their nests in dense patches of small (<8 cm DBH) red spruce and eastern hemlock trees.
Swainson’s thrush typically arrive in New Hampshire between late May to early June and their nesting period is 28 days (+/- 2) with four stages: nest building (4 days), laying and incubation of the eggs (12 days), and provisioning and caring for nestlings before fledgling (12 days). Female Swainson’s thrush typically have one brood, though they can renest, and they build their nest in red spruce, balsam fir, and paper birch trees, ranging anywhere from 3-10m in height (Mack and Yong, 2020). Nests are typically built with bark from yellow birch or paper birch, as well as miscellaneous tree branches, twigs, and moss, and can occasionally have artificial materials in them (i.e., plastic bags, bottle caps).