Null Reference Exception - Bird Movement.Update()

using UnityEngine;
using System.Collections;

public class birdMovement : MonoBehaviour {

[SerializeField]
private Transform target;
private NavMeshAgent birdAgent;
private Animator birdAnimator;
[SerializeField]
private RandomSoundPlayer birdFootsteps;
// Use this for initialization
void Start () {
    birdAgent = GetComponent<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);

    if (speed > 0f) {
        birdFootsteps.enabled = true;
    }
    else
    {
        birdFootsteps.enabled = false;
    }
}