Due to the fact you mention heather this suggests to me you are talking about moorland, grouse are good parents and would go back to chicks if they could not fly.(provided the spaniel did not damage the chicks)
Pheasants are poor parents and may not collect all the chicks
hope this helps
There is a strong possibility that eggs or chicks will be predated by carrion crows or buzzards in these circumstances. Dogs should be kept under close control ie. on a lead in the nesting season. Please read the Access Code!