Studies of geographic range shifts in response to climate warming that use data from Christmas Bird Counts or repeated state and provincial faunal atlases are better at detecting latitudinal shifts than altitudinal shifts because the coarse geographic scale of most citizen-science survey units masks the substantial elevational variation within their boundaries. To more directly measure altitudinal range shifts of forest-breeding bird species, we repeated an altitudinal transect survey conducted 40 years ago at Whiteface Mountain in the Adirondacks, New York, USA. We conducted roadside bird surveys at dawn and dusk at seven survey stations that ranged in altitude from 500 m to 1,425 m. We found considerable interspecific variation in the movement of altitudinal ranges, but document a preponderance of uphill shifts in both upper and lower boundaries of altitudinal breeding ranges. The shift of abundance-weighted mean altitudes for 42 species detected in both survey periods was\th82.8 m. These shifts are correlated with a regional trend toward warmer summers from Adirondack weather station data collected over this same time period. Upper range boundaries have shifted more than lower boundaries, resulting in novel bird communities at some elevations (e.g., we found 13 species at 1,425 m vs 7 species at this altitude in 1974), resulting in a flattening of the altitudinal gradient of species richness. At low elevations we encountered several species that were not recorded on the transect in 1974, a trend we attribute to climate warming and anthropogenic habitat change at low altitudes. Our resurvey shows that there have been substantial upward range shifts in most bird species on Whiteface Mountain in the last four decades, and provides a basis for reassessment of altitudinal range dynamics at the southern edge of the North American boreal forest in the coming decades.