A system

Lately I'd noticed a collection of feathers build up on a gate post near the Relief Channel. Each day there seemed to be another 1 or 2 added to the group. 
A puzzle, but I have the answer although I still don't fully understand the logic. 
I often stop and chat to a guy who takes his mother's cockapoo for a walk there. He is a font of local knowledge so I thought I should ask him if he new the background to this particular mystery. It turns out that he not only knew but was actually one of the instigators of it. 
He often shares his walk with a guy who use to walk his own dog along here but unfortunately lost his dog to illness some time ago. Sometimes they share the walk but sometimes they take their walks at different times of the day. If this happens then one or the other, or possibly both stick a feather in the gate post to alert the other that they have already walked along there that day. 
I'm not convinced that this system is foolproof but it adds a bit of colour to things and no doubt provides food for thought for others who walk along here.