Arrival and departure dates

We review the current evidence that changes in arrival and departure dates of migratory birds have taken place, and the relationship of these changes to climate variability. There is little doubt that the timing of spring migration closely follows weather variations. This is more evident and/or stronger in short-distance migrants than in long-distance migrants, but the latter have also responded to climate change. Changes of spring arrival in birds depend on climate impacts at different latitudes along the route from wintering to breeding areas. Therefore, also migratory strategy, e.g., stopover tactics and migratory routes, may be under selective forces due to climate change. Changes in breeding environment depend on the climate there. The discrepancy between en route and breeding time impacts can induce poor fit between annual cycles of birds and their resources. Changes of timing of autumn migration are more variable and they are less well understood. Some species have advanced and others postponed their autumn migration. As long as seasonal variation of environmental constraints at species level remains elusive we cannot predict which species will delay autumn departure and which will advance it in synchrony with spring events. The net result of changes of phenology has often been the lengthening of the summer part of the annual cycle. Because the first analyses of changes of both migration periods and time span between them are based on bird station data, it is still too early to generalise this variation against ecological traits of species. Lengthening of time spent in the breeding area may relax some time constraints set by seasonality by allowing more time to breed and moult, but just the opposite is also possible depending on latitude and temperature regime at which changes are taking place.