This is definately a tricky one because he obviously gets a lot of natural reinforcement for chasing the birds and you are right, it is good exercise.  First of all, when it is appropriate to chase them, come up with a specific command, like "birds" and let him have at it.  At the very least, it's good that he thinks it's your idea and he' s not getting away with anything.

The remote citronella would be a good option here.  Use a command that he is most familiar with at first.  My guess would be the "here" (or come) command.  When he comes, make a big deal about it and if possible, let him go chase the birds again, on your command.  You want to be careful that he doesn't see the "here" as an end to all his fun, all the time.  It's ok if it's an end some of the time, but not all the time! 

Some dogs are really good at a down at a distance.  That would be another option down the road if you wanted to practice it.  Many owners have lots of success with an "emergency drop" instead of a recall.  The "wait" command would be the closest thing to a "stop" command and if he knows wait, then use that instead of trying to introduce a new command that essentially means the same thing. 

Good Luck....