Changes in climate that affect breeding phenology can have important ramifications for the population dynamics of migratory wildlife. In birds, predicted changes in global climate raise concerns over the ability of migratory species, especially long-distance migrants, to adapt to changes in spring weather conditions. At a riparian breeding site in southern Manitoba, we found that over 3 decades (1974 to 2003), migratory yellow warblers (Dendroica petechia) exhibited considerable plasticity in their timing of clutch initiation in response to mean May temperatures at the breeding site. Spring arrival dates estimated during the last 9 y of our study were also highly variable and correlated with mean May temperatures. By contrast, we found no support for an effect of the El Niño/Southern Oscillation, a climatic phenomenon known to affect weather conditions on wintering grounds of yellow warblers, on spring arrival dates or timing of breeding. This suggests that El Niño/ Southern Oscillation did not have carry-over effects on spring arrival dates or timing of clutch initiation. The results from our study are consistent with the timing of breeding in Neotropical migrant songbirds being flexible and closely coupled with spring temperatures (mean daily May temperatures) at breeding sites. These findings are important for predicting how sensitive long-distance migrant birds may be to changes in spring conditions caused by climatic warming.