Why doesn't this display properly on iPad / iPhone?

Question

When I test this page on iPad / iPhone it only shows the first image and not the rest.

Is there something wrong with my HTML / CSS or am I going to have to start using media queries or something similar?

http://codepen.io/alsjohnstone/pen/oEKfc

Code snippet:

html,
body {
  height: 100%;
  width: 100%;
}

section {
  height: 1000px;
  position: relative;
  display: block;
}

.bg {
  background-position: center top;
  background-repeat: no-repeat;
  background-attachment: fixed;
  background size: cover;
  width: 100%;
  height: 100%;
  position: relative;
}


/* Main Navigation */

.navigation {
  position: fixed;
  top: 0;
  z-index: 99999;
  width: 100%;
  padding: 10px;
  background-color: #000;
}

.navigation ul {
  list-style-type: none;
  margin: 0;
  padding: 0;
}

.navigation li {
  display: inline;
  text-decoration: none;
}

.navigation a {
  display: inline-block;
  margin-left: 30px;
  vertical-align: middle;
  background-color: #000;
  text-decoration: none;
  color: #EFEFEF
}


/* section 1 */

#slide-1 .bg {
  background-image: url('http://i.imgur.com/lklWa7e.jpg');
}


/* section 2 */

#slide-2 .bg {
  background-image: url('http://i.imgur.com/fBzbkrr.jpg');
}


/* section 3 */

#slide-3 .bg {
  background-image: url('http://3.bp.blogspot.com/-IYmB20ch7EA/UhETyjIcndI/AAAAAAAADgU/hu8NEmCz_0c/s1600/1150736_10151773182603618_1631854368_o.jpg');
}


/* section 4 */

#slide-4 .bg {
  background-image: url('http://4.bp.blogspot.com/-mMxGA3yHWsY/UyW-z8W6pkI/AAAAAAAABDw/DFE7O1lLwEc/s1600/Koch.jpg');
}

<meta name="viewport" content="width=device-width" />

<!-- Navigation -->

<div class="navigation">
  <div class="inner">

    <ul>
      <li><a href="#slide-1">ONE</a></li>
      <li><a href="#slide-2">TWO</a></li>
      <li><a href="#slide-3">THREE</a></li>
      <li><a href="#slide-4">FOUR</a></li>
    </ul>
  </div>
</div>

<div id="skrollr-body">

  <!-- Section 1 -->

  <section id="slide-1">
    <div class="bg" data-top-top="background-position: 0% 0px;" data-bottom-bottom="background-position: 0% -100px;"></div>
  </section>

  <!-- Section 2 -->

  <section id="slide-2">
    <div class="bg"></div>
  </section>

  <!-- Section 3 -->

  <section id="slide-3">
    <div class="bg"></div>
  </section>

  <!-- Section 4 -->

  <section id="slide-4">
    <div class="bg"></div>
  </section>

</div>