Birds on the trees in the background

How do these birds pop up behind the trees on the bottom?

http://grabaperch.com/about

Are they开发者_开发问答 using css with php or what is it?

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They use jQuery:

<script type="text/javascript" src="http://ajax.googleapis.com/ajax/libs/jquery/1.3.2/jquery.min.js"></script> 

They have some JavaScript code in perch.js that animates the birds and the trees.

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They are using jQuery to animate the birds and trees. There is an animate function that pops the birds out from behind the trees - adjusting the z-index will properly layer elements in that fashion.

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jQuery

.animate

http://api.jquery.com/animate/

The reason it's behind is z-index

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It is javascript, the code is here