ROS subscriber not using callback function

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 #include <boost/bind.hpp>
#include <gazebo/gazebo.hh>
#include <gazebo/physics/physics.hh>
#include <gazebo/common/common.hh>
#include <gazebo/common/Plugin.hh>
#include <gazebo/math/gzmath.hh>
#include <gazebo_msgs/ModelStates.h>
#include <stdio.h>
#include <ros/ros.h>
#include <std_msgs/String.h>
#include <string>

void model_states_callback(const gazebo_msgs::ModelStates::ConstPtr& msg)
{
    ROS_INFO("I heard the model state!");
}
namespace gazebo
{
  class kiwi_pos : public ModelPlugin
  {
    public: kiwi_pos() : ModelPlugin()
            {
            }
    public: void Load(physics::ModelPtr _parent, sdf::ElementPtr _sdf)
    {
            if(!ros::isInitialized())
            {
        ROS_FATAL_STREAM("A ROS node for Gazebo has not been initialized, unable to load plugin. "
        << "Load the Gazebo system plugin 'libgazebo_ros_api_plugin.so' in the gazebo_ros package)");
        return;
            }
            ROS_INFO("KIWI POSITION PLUGIN LOADED");
      // Store the pointer to the model
      this->model = _parent;
      // Listen to the update event. This event is broadcast every
      // simulation iteration.
      this->updateConnection = event::Events::ConnectWorldUpdateBegin(
          boost::bind(&kiwi_pos::OnUpdate, this, _1));
            ros::NodeHandle n;
            ros::Subscriber sub = n.subscribe("gazebo/model_states",1000,model_states_callback);
            //ros::spin();
    }
    // Called by the world update start event
    public: void OnUpdate(const common::UpdateInfo & /*_info*/)
    {
    }
    // Pointer to the model
    private: physics::ModelPtr model;
    // Pointer to the update event connection
    private: event::ConnectionPtr updateConnection;
  };
  // Register this plugin with the simulator
  GZ_REGISTER_MODEL_PLUGIN(kiwi_pos)
}

Here is my code, I am simply trying to listen in on the model states topic and publish a confirmation when I hear it. When I run the simulation I get the PLUGIN LOADED ROS_INFO but nothing else is outputted.