As global warming intensified toward the end of the 20th century, there was a northward shift in winter ranges of bird species in Cape Cod, Massachusetts, USA. These poleward shifts were correlated to local increases in minimum winter temperatures and global temperature anomalies. This evidence, plus other recent results, suggests that during the last two decades global warming has led to massive and widespread biogeographic shifts with potentially major ecological and human consequences. Local habitat changes associated with urban sprawl affected mainly forest birds with more northern winter distributions. In Cape Cod, the effects of warming on bird distributions are more substantial at the start of the 21st century, than those of habitat alteration, but as urban sprawl continues its importance may rival that of global warming.