The recent discussion under "Robins and Sparrows" got me
thinking about just when migratory birds leave and arrive.  While predicting
a hard winter based on early migration seems fairly silly, just how much
effect does current weather/climate have on departure dates?  Do birds leave
earlier during cold and wet autumns?  I always thought that migratory
movement was basically driven by sun angle;  how much is caused by local
weather conditions?