Figure 2. Observed delay in onset of spring migration.

The departure dates from the wintering grounds in the Netherlands, shown as the relative cumulative percentage of departure as a function of days since the median departure date in the 1970's. Data points represent per day the mean relative cumulative passage count at Ottenby bird observatory over a certain period (circles: 1970–1979, triangles: 1980–1989, open squares: 1990–1999, solid squares: 2000–2004). The median departure date in the 1970's was April 12.