function getElementsByClassName(node, classname)
{
    var a = [];
    var re = new RegExp('\\b' + classname + '\\b');
    var els = node.getElementsByTagName("*");
    for(var i=0,j=els.length; i<j; i++)
        if(re.test(els[i].className))a.push(els[i]);
    return a;
}

function jailBirds()
{	_birds = getElementsByClassName(document,'jail');
	
	for( i=0; i<_birds.length; i++)
	{	_birds[i].style.backgroundImage = 'url('+_birds[i].src+')';
		_birds[i].src = 'imgs/jailBars.gif';
	}
}