Figure 1.

Potential effects of relative climatic and phenological changes on ecological mismatch of migratory birds. Curves represent the progress of spring (degree-days) in 2 years. The curve for the recent year (red line) lies above that for the past (blue line) because of winter and spring warming. Migratory birds show no change (red dot), advancement or delay (red arrows) in arrival date. Species that now arrive at the same or later dates face higher degree-days and are thus ‘thermally delayed’ relative to spring phenophases. Even species that have advanced their arrival may experience a thermal delay, if advancement does not fully compensate for increasing temperatures. Only large advancement in arrival can fully compensate for climate change.