Phenotypic changes in spring arrival: evolution, phenotypic plasticity, effects of weather and condition

Global climate change is having a major impact on the viability of species. Whether a particular population will survive or go extinct will depend on its potential to adapt to new environmental conditions. In migratory birds, the timing of spring migration is one of the major determinants of the timing of reproduction, and, as spring temperatures increase, we expect birds to return earlier to their breeding grounds. In accord with this expectation, the timing of spring arrival has advanced in many bird populations within the last 30 yr. Yet, despite the presence of genetic variation in the timing of migration and fitness advantages in birds arriving early, we still lack conclusive evidence for evolutionary change. One reason for this may be that weather conditions en route, particularly the direction and speed of winds, have an important influence on individual migration speed. Moreover, there is strong evidence that physical condition may modify the timing of departure from the wintering grounds, migration speed and ultimately the timing of arrival and breeding. We also expect nestling condition and maternal effects to influence spring migration timing. Long-term population studies monitoring individual arrival in combination with experimental approaches will be needed to tease apart genetic and different environmental effects on the traits determining migration timing. This knowledge will be crucial for predicting adaptive responses of the timing of spring arrival in the future.