I'm struggling to solve a NullReferenceException error involving the audioSource variable. Please help!

Watched this video and checked my code several times. Any idea as to what is going wrong?

Here's the exact code loaded as a script under the Bird Calls child of the Bird game object:

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

public class RandomSoundPlayer : MonoBehaviour
{
    private AudioSource audioSource;
    [SerializeField]
    private List<AudioClip> soundClips = new List<AudioClip>();
    [SerializeField]
    private float soundTimerDelay = 3f;
    private float soundTimer;

    // Use this for initialization
    void Start()
    {
        audioSource.GetComponent<AudioSource>();
    }

    // Update is called once per frame
    void Update()
    {
        // increment a timer to count up to restarting
        soundTimer = soundTimer + Time.deltaTime;

        // if the timer reaches the delay
        if (soundTimer >= soundTimerDelay)
        {

            // ... reset the timer...
            soundTimer = 0f;

            // ... choose a random sound... (soundClips.Count generates maximum for random selection range :)
            AudioClip randomSound = soundClips[Random.Range (0, soundClips.Count)];


            // ... and play the sound
            audioSource.PlayOneShot (randomSound);
        }

    }
}

And this is the error I receive every three seconds as the script attempts to play the random sound:

NullReferenceException: Object reference not set to an instance of an object RandomSoundPlayer.Update () (at Assets/Scripts/RandomSoundPlayer.cs:40)

the line it references is: audioSource.PlayOneShot (randomSound);

the tricky thing is at launch I get a Start error as well:

NullReferenceException: Object reference not set to an instance of an object RandomSoundPlayer.Start () (at Assets/Scripts/RandomSoundPlayer.cs:19)

referring to my GetComponent line: audioSource.GetComponent<AudioSource>();

I assume the problem has something to do with the AudioSource component having a blank AudioClip field but I can't seem to resolve the problem as placing anything in the field does not change the error (also nothing is placed in the field in the video walkthrough).

Any help would be greatly appreciated! Thank you!