Data sources used in the contemporary (C) and temporal (T) niche analyses, including years of coverage and raw sample sizes.

aAll sample sizes reflect the total number of georeferenced specimens or observations available at time of data acquisition, including multiple specimens or observations at a given site. Sample sizes are restricted to North America.bAll band and encounter records obtained through written permission from the USGS Bird Banding Lab (http://www.pwrc.usgs.gov/bbl/homepage/datarequest.cfm).cObtained from the USGS North American Breeding Bird Survey (http://www.pwrc.usgs.gov/bbs/RawData/Choose-Method.cfm).dObtained from the Christmas Bird Count database project (http://infohost.nmt.edu/~shipman/z/cbc/homepage.html).eAll observational data from the Cornell Lab of Ornithology were obtained from the Avian Knowledge Network (http://www.avianknowledge.net/content).