METHODS
We compiled data on social pairs from studies published by five of the authors. In addition, we included data from Ficken and Ficken (1968), and from Will (1986) with supplemental data from Will (personal communication ). This provided a total of nine, chronologically distinct studies in eight study areas. For each study area and for pooled data, we compiled the frequency of social pairing for Golden-winged and Blue-winged warblers and hybrid phenotypes. Not all studies could be used for all calculations because of limitations in the recorded data.