Head up to the Jews Gate bird observatory which is just inside the upper rock national park.
If you want to avoid the entry fee get there before 9am.
I found it best to email the Gibraltar bird society first to ask if it was ok to observe migration with them as they also ring birds there so thought it best to ask first.
Failing this anywhere on the upper rock would be great but a decent Westerly wind will help big time.
A lot of easterly winds in the area at the moment so Tarifa may be better but found in the last week that a lot of birds preferred west winds for crossing.
Hope that helps.