Decking and sealing / cleaning problem

Fence off all the decking to prevent access by the chickens, except one very short route across it?

You could use another surface for that short route, something that was separated from the surface of the other decking, for example an non-slip epoxy coated marine plywood with a low kerb each side. You could then hose this down to clean it without affecting the rest of the decking.

The pheasant droppings I get on my deck clean off very easily.

Do they leave a stain, if left to dry on.

Then how easy to chip them off the decking grooves, or would a water hose be sufficient..

No. The deck is hardwood and is regularly treated with teak oil.

Generally, a hose is sufficient. Occasionally I have to use a stiff broom on them. You never have to "chip" them off. Once a year I pressure wash the whole thing, but that's to get rid of the algae, rather than the pheasant droppings.