Just as Mary's first real friend was the robin, Colin's first interaction with the outdoors comes in the form of these wild creatures. This suggests that animals like this are an easy entry into the outside world for someone like Mary or Colin who's never experienced either the joys of the natural world or a sense of camaraderie with another living being before. Specifically, when the lamb leads Colin to address Dickon, it shows that he's taking the same route as Mary to making friends with Dickon.