Getting perfect recall with bird chasers (or squirrel chasers) is particularly hard. It is best not to have them off lead unless in a totally safe area and also once you know they will come back.
My two are 99% on recall -- but there is ALWAYS that 1%. I have learned that Leo in aprticular will bolt for someplace he knows well and wants to get to -- for example the dog pond in the Phoenix Park here -- and he has run hundreds of yards at high speed towards the pond, right across the access road (very litle traffic but still there are cars there). I now know never, ever to have him off lead within a few hundred yards of that pond; if he can smell it he will run. He once took off after the deer in the park too; something I thought he simply would never do, so I now leash him if we are getting close to the deer herds.
Most importantly, I practice, practice, practice recall with food rewards when we are on walks (still!). I have also taught Leo a hand signal for 'come' (which is basically waving my hands over my head) as he has trouble figuring out where a call is coming from and picking me out at a distance. On a typical walk I will call the dogs back many times and reward. Sometimes when they are a short distance, sometimes when they are a ways away. They need constant reinforcement of this crucial command. We are also working on 'close' (a loose form of heel). I also taught them the word 'treats' and strongly advocate having a word like this for when they get a treat. You would be amazed how many dogs won't come to 'come' but will come running for 'treats!' so this is just a second back-up way of getting them to come to me.
Both mine will stick close to me, never follow other dogs, generally pay attention and are very trustworthy off lead but ONLY in the right environment: so far from traffic that I know they cannot bolt towards it -- eg the beach, fenced areas, very large parks, the countryside. I would not trust Leo offlead near sheep now after his example with the deer (farmers can shoot a dog on sight for worrying sheep here). To get them to this point took lots of practice recallng when they are on a very long lead, like 50 feet of rope for example or one of those very long exercise leads.
PS Can you have a look at resizing your avatar just a bit? The max dimensions are here:
http://www.cavaliertalk.com/phpBB2/viewtopic.php?t=1706
As I can no longer run an auto-resizing program on this version of the board, I am trying to keep them below a max width as larger avatars stretch the whole thread out. It's pretty easy to do
My avatar is at the max width so you can gauge the size from that.