jQuery - Sticky header that shrinks when scrolling down

I wonder how to make a sticky header shrink(with animation) when you scroll down the page and goes back to normal state when the page is scrolled up to the top. Here are two examples to clearify:

http://themenectar.com/demo/salient/

http://www.kriesi.at/themes/enfold/

I get the part to make it fixed, but how should I do to shrink my header when the user scrolls down?

Thanks a ton

This should be what you are looking for using jQuery.

$(function(){
  $('#header_nav').data('size','big');
});

$(window).scroll(function(){
  if($(document).scrollTop() > 0)
{
    if($('#header_nav').data('size') == 'big')
    {
        $('#header_nav').data('size','small');
        $('#header_nav').stop().animate({
            height:'40px'
        },600);
    }
}
else
  {
    if($('#header_nav').data('size') == 'small')
      {
        $('#header_nav').data('size','big');
        $('#header_nav').stop().animate({
            height:'100px'
        },600);
      }  
  }
});

Demonstration: http://jsfiddle.net/jezzipin/JJ8Jc/

Here a CSS animation fork of jezzipin's Solution, to seperate code from styling.

JS:

$(window).on("scroll touchmove", function () {
  $('#header_nav').toggleClass('tiny', $(document).scrollTop() > 0);
});

CSS:

.header {
  width:100%;
  height:100px;
  background: #26b;
  color: #fff;
  position:fixed;
  top:0;
  left:0;
  transition: height 500ms, background 500ms;
}
.header.tiny {
  height:40px;
  background: #aaa;
}

http://jsfiddle.net/sinky/S8Fnq/

On scroll/touchmove the css class "tiny" is set to "#header_nav" if "$(document).scrollTop()" is greater than 0.

CSS transition attribute animates the "height" and "background" attribute nicely.

http://callmenick.com/2014/02/18/create-an-animated-resizing-header-on-scroll/

This link has a great tutorial with source code that you can play with, showing how to make elements within the header smaller as well as the header itself.

Based on twitter scroll trouble (http://ejohn.org/blog/learning-from-twitter/).

Here is my solution, throttling the js scroll event (usefull for mobile devices)

JS:

$(function() {
    var $document, didScroll, offset;
    offset = $('.menu').position().top;
    $document = $(document);
    didScroll = false;
    $(window).on('scroll touchmove', function() {
      return didScroll = true;
    });
    return setInterval(function() {
      if (didScroll) {
        $('.menu').toggleClass('fixed', $document.scrollTop() > offset);
        return didScroll = false;
      }
    }, 250);
  });

CSS:

.menu {
  background: pink;
  top: 5px;
}

.fixed {
  width: 100%;
  position: fixed;
  top: 0;
}

HTML:

<div class="menu">MENU FIXED ON TOP</div>

http://codepen.io/anon/pen/BgqHw