Extracting climate
We obtained monthly temperature and precipitation data from CHELSA version 2.1 (raster of data of ~1 km resolution; Kargeret al. 2021) between 2000–2018. Seasonal temperature and precipitation were obtained by averaging the climate data for each ~1km pixel over 3-month seasons – June–July for breeding and December–February for wintering – and over the 19 years of data. Seasonal temperature and precipitation were normalized using the z-score across the entire study region (i.e., Western Hemisphere).
Using a global dataset of terrestrial ecoregions (Olson et al.2001), which represent regions with distinct biotic characteristics, we assumed that ecoregions containing at least one sample of a given breeding population was occupied by this population. For ecoregions on the wintering ranges, individuals’ population assignment results were used to determine the association of ecoregions with breeding populations. We weighted the contribution of ecoregions to a given population \(j\) based on the relative abundance of the species and how much of this relative abundance belongs to this population, we assigned for each ecoregion \(k\) occupied by population \(j\) of a given species the following weight:
\begin{equation} W_{j,k}=A_{k}\frac{S_{j,k}}{\sum_{i=1}^{p}S_{i,k}}\nonumber \\ \end{equation}
Where \(A_{k}\) is the relative abundance of the species in ecoregion\(k\); \(S_{j,k}\) is the number of individual samples in ecoregion\(k\) that are assigned to population \(j\); and \(p\) is the total number of populations of the species.
Species’ relative abundance in ecoregions across the study region was estimated using predictions from spatiotemporal exploratory models (STEMs) based on observation data from eBird citizen-science program (Sullivan et al. 2014; Fink et al. 2020a). The survey completeness of eBird is particularly good in the broad region where the species used in the study are located (La Sorte & Somveille 2020). Estimates of relative abundance for the breeding and wintering seasons were obtained from eBird Status and Trends products (Fink et al.2020b), downloaded in raster format of 2.96 km resolution via the R package ebirdst . For each ecoregion, we extracted relative abundance values within the geographical boundaries of the ecoregion and then summed them to obtain ecoregion-level relative abundance estimates.