Winter weather and land cover
From relocations and original trap locations, we constructed 75% minimum convex polygons (MCPs) using the R package ‘adehabitatHR’ to represent individuals’ core area of use throughout the winter for birds with greater than or equal to 10 locations (Fig. 2, n = 64, R Core Team 2021, Calenge 2006). For individuals with less than 10 locations, we created buffered centroids equal to the mean MCP area (12.02 ha, n = 30). We explored 50%, 75%, and 95% MCP and kernel density estimates for calculating winter ranges. We chose to use MCPs at the 75% level to represent the core area of an individual’s winter use areas as peripheral areas in an animal’s home range are difficult to identify with telemetry data and may have reduced biological significance (Powell 2000, Whitaker et al. 2007). Additionally, we found no evidence for a linear relationship between the estimated 75% MCP area and either number of radiolocations or duration of monitoring (F 1,69 = 3.30, p = 0.08).
To characterize environmental conditions encountered by individuals throughout winter, we extracted daily snow data and minimum daily temperature data at 1-km resolution from Snow Data Assimilation System (SNODAS, National Operational Hydrologic Remote Sensing Center 2004) and Daymet, accessed through the ‘daymetr’ package in R (Hufkens et al. 2018, Thornton et al 2020, 2021). We used snow depth from SNODAS to determine snow presence or absence. Values were spatially averaged across individual winter use areas using the ‘exactextractr’ package (Baston 2022).
To capture important landscape metrics within an individual winter use area, we calculated class-level landscape metrics on reclassified rasterized data from the Wisconsin Forest Inventory Map (WisFIRS; WDNR). The reclassified data layers were adapted from methods reported in Wilson et al. 2019 and contained 5 cover types: “Dense Cover” (< 20 yr aspen or alder stands), “Mature Forest” (> 20 yr stands), “Open” (emergent vegetation, lowland brush/willow, and lowland grass), “Clear Cut” (< 4 yr since harvest), and “Other” (all other cover classes; mostly open water and marsh). We classified areas as Clear Cut for three years following harvest as recruitment of aspen suckers into the sapling stage begins roughly four years after harvest (Bose et al. 2014). We classified small areas not covered by WisFIRS (e.g., private lands) using leaf-on and leaf-off NAIP imagery and field data (OCM Partners 2022). We rasterized the land cover data and using a land cover raster for each winter field season, we calculated class-level landscape metrics for each individual’s winter use area with the ‘landscapemetrics’ package in R, an extension of FRAGSTATS (McGarigal 1995, Hesselbarth et al. 2019).