Bird Does not move at all.

private bool gameStarted = false;
[SerializeField]
private Text gameStateText;
[SerializeField]
private GameObject player;
[SerializeField]
private BirdMovement birdMovement;
[SerializeField]
private FollowCamera followCamera;
private float restartDelay = 3f;
private float restartTimer;
private PlayerMovement playerMovement;
private PlayerHealth playerHealth;


// Use this for initialization
void Start () {
    Cursor.visible = false;
    playerMovement = player.GetComponent<PlayerMovement> ();
    playerHealth = player.GetComponent<PlayerHealth> ();
    playerMovement.enabled = false;
    birdMovement.enabled = false;
    followCamera.enabled = false;
}

// Update is called once per frame
void Update () {
    if (gameStarted == false && Input.GetKeyUp (KeyCode.Space)){
        StartGame();
    }

    if (playerHealth.alive == false) {

        EndGame();
        restartTimer = restartTimer + Time.deltaTime;

        if(restartTimer >= restartDelay) {
            SceneManager.LoadScene (SceneManager.GetActiveScene ().buildIndex);
        }
    }
}
private void StartGame () {
    gameStarted = true;
    gameStateText.color = Color.clear;
    playerMovement.enabled = true;
    followCamera.enabled = true;
}

private void EndGame () {
    gameStarted = false;
    gameStateText.color = Color.white;
    gameStateText.text = "Game Over!";
    player.SetActive (false);

}

// Use this for initialization
void Start () {
    birdAgent = GetComponent<UnityEngine.AI.NavMeshAgent> ();
    birdAnimator = GetComponent <Animator>();
}

// Update is called once per frame
void Update () {
    birdAgent.SetDestination (target.position);
    float speed = birdAgent.velocity.magnitude;
    birdAnimator.SetFloat ("Speed", speed);

}