Or a dozen. Or a whole flock.
jsbat.js  (v1.2 just 1.9 kB)
Download the file to your website or feel free to link it here. Then add as many
as you want bats flying around.
It’s all inclusive, relies on base64 images and CSS animation, so it should be hardware-accelerated on all decently modern browsers (ie. anything but IE9 or older).
It’s free and/but unsupported, so use it at your own risks. If it bites your visitors, not my problem. Stake the buggers on your own! But feel free to link back or show your appreciation \/**\/
For WordPress sites
You can insert the bats in a particular post by
- downloading jsbat.js 
- go to Media, then Add New and upload
- in a Post source, go to the text tab and insert something like
<script async src="/wp-content/uploads/2013/10/jsbat.js"></script>
It’s important to have the js locally, in your content, and the path without “http” or site name, or WordPress will strip the src tag.