Figure 40.1: Migratory Birds Have Returned Earlier to the Netherlands in the Spring as Temperatures Have Risen Pied flycatchers (Ficedula hypoleuca) overwinter in Africa and migrate back to the Netherlands in the spring. Over a 34-year period, air temperature in the Netherlands has increased because of climate change, and males have arrived earlier. There has been a statistically significant correlation between average arrival date and average air temperature, as this graph shows. Each symbol corresponds to one year. Air temperatures were measured each year between March 16 and April 15, and all the measurements for each year were averaged to get the temperature plotted for the year.