Animals are breeding earlier as the climate warms, but some populations have kept pace with warming while others have fallen behind. Across 123 populations of tree swallows, we find relationships consistent with different climate change consequences despite a similar overall response to temperature. Northern populations have experienced greater recent warming and, as a result, face stronger time constraints on breeding. These populations have advanced breeding dates the most but still experience the strongest selection for earlier breeding and have declined in abundance. Our results suggest that vulnerability to climate change can arise even when populations have similar sensitivity to temperature due to differences in timing constraints and exposure.