[SerializeField] not showing in Inspector for the Player and Bird game objects?

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PlayerMovement : MonoBehaviour {

    private Animator playerAnimator;
    private float moveHorizontal;
    private float moveVertical;
    private Vector3 movement;
    private float turningSpeed = 20f;
    private Rigidbody playerRigidbody;
    [SerializeField]
    private RandomSoundPlayer playerFootsteps;

    // Use this for initialization
    void Start () {

        // Gather components from the Player GameObject
        playerAnimator = GetComponent<Animator>();
        playerRigidbody = GetComponent<Rigidbody>();

    }

    // Update is called once per frame
    void Update () {

        // Gather input from the keyboard
        moveHorizontal = Input.GetAxisRaw("Horizontal");
        moveVertical = Input.GetAxisRaw("Vertical");

        movement = new Vector3(moveHorizontal, 0.0f, moveVertical);

    }

    void FixedUpdate () {

        // If the player's movement vector does not equal zero...
        if (movement != Vector3.zero) {

            // ...then create a target rotation based on the movement vector...
            Quaternion targetRotation = Quaternion.LookRotation(movement, Vector3.up);

            // ...and create another rotation that moves from the current rotation to the target rotation...
            Quaternion newRotation = Quaternion.Lerp(playerRigidbody.rotation, targetRotation, turningSpeed * Time.deltaTime);

            // ...and change the player's rotation to the new incremental rotation...
            playerRigidbody.MoveRotation(newRotation);

            // ...then play the jump animation...
            playerAnimator.SetFloat("Speed", 3f);

            // ...and play footstep sounds.
            playerFootsteps.enabled = true;

        } else {
            // Otherwise, don't play the jump animation.
            playerAnimator.SetFloat("Speed", 0f);

            // Don't play footstep sounds.
            playerFootsteps.enabled = false;
        }

    }

}

using System.Collections;
using System.Collections.Generic;
using UnityEngine;
using UnityEngine.AI;

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 () {
        // Set the bird's destination
        birdAgent.SetDestination(target.position);

        // Measure the magnitude of the NavMeshAgent's velocity
        float speed = birdAgent.velocity.magnitude;

        // Pass the velocity to the animator component
        birdAnimator.SetFloat("Speed", speed);

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

    }
}