In season 1 walkers can pick up rocks and break windows, climb fences and appear to look around. in later seasons they only walk and are easily distracted by noise. Why have their abilities changed?
The Walkers can grab and turn door handles also in Season 1. For example, the mother keeps returning to her own house and scratching at the door, also turning the door handle to try to get in.
The loss of this ability can maybe attributed to them not feeding on a regular basis or from getting "dumber" as time goes on. They just regress to the primal mode of moving to the sound of food.