Bizarre pack behavior

Question
I have three shelties.  All of them behave perfectly when taken out by themselves.  However, if I take them out together, they turn into a barking pack of hystercal demons throwing themselves on the ends of their leads at every cat or other dog we pass, not to mention ducks and swans.

However, it's even more bizarre because off the lead they don't do it.

If one of the three (any one)is off the lead, none of them do it.

If one wears a anti-bark spray collar, none of the do it.  It isn't that it starts and when one doesn't join in, they stop.  None of them start at all.  They will walk past 1 ft from a cat perfectly to heel on loose leashes and won't even acknowledge it is there.

Do you know how to stop this pack "up for a chase" behavior?  Have you any idea why it takes three cooperating dogs to make a chase pack, or how the other two know if one of them is wearing a spray collar so can't participate?

I have watched them very carefully and noticed that if one has the spray collar it is on best behaviour and is heeling just slightly better than the other two - more attentive to my changes in speed etc.  Can it be as simple as that the other two think that if the third is behaving that well he/she wont join in so it's not worth starting? Could it be that they are slightly hyping each other up all the time normally and it takes 3 of them competing for who can walk quarter of an inch further in front than the others etc, to get them hyped up enough to fancy a cat chase?

Any ideas?  

Answer
Hi Vicky!

My wife and I are laughing and re-reading your question over and over again!  This question is what makes it fun!

Ok, let's analyize this:
When all are on leads they act like heathans.
When not on leads they don't
When alone they don't
When 2 on leads, one off, they don't
When one has bark collar, they don't

(sorry, laughing again)

If your Shelties had oppsable thumbs I would be terrified right now!

My (our) best guess at this point is you have exceptionally intelligent Shelties.  Sure, there is some weir---no, more correctly: "advanced" pack dynamic going on here.  I would speculate that this behavior built from a core starting point a very long time ago.  The clue in my mind is the bark collar behavior.  

Here is my thinking: they all know what that collar is.  When they bark, it beeps, and it is unpleasant.  Their conditioning has evolved from unpleasant association to total obedience when the collar is out.  Because if they hear the beep in close proximity they associate it with the negative reinforcement against barking.  Now that collar brings them under control just by being on-or seen?

Could they know that it is bad to encourage the one with the collar to bark and bring on the punishment?  That's pretty advanced thinking for dogs.  However, Shelties are noted for their abilities to perform in ways that we associate with multi-level thinking.

Again, speculation says there were stepped events leading up to this activity.  Surely it did not spontaneously happen one day without some kind of preceeding behavior or events?  Which would throw out everything I just typed.

My closing thought-and the simple answer is usually the best one: is that when they are on leash they think they are not responsible.  When off leash they are "working" and go to their "please Vicki by being what she taught us to be" mode.  On leash is free time.  Off leash is work.  If one is working, they all work.  

The only true answer would have to be a combination of both speculations when tested against the leads with spray collar.  Or I could be completely wrong and they are totally aware of how confusing this is to you and are perpetuating it as some kind of Sheltie joke.

We will send it out to some friends to see if there has been any similar activity noted.  Thank you again for that fantastic question!  I look forward to hearing from you again!

'sneezes
Dave