I've just finished making this. The bird folds flat inside the card, and pops up automatically when the tab is pulled out. Here is a youtube video of the bird popping out.
The instructions are online here. Please link to this blog post rather than the file itself! I'll upload a PDF for building the "egg" as soon as I've made one!
You can use the file to print and make as many birds as you like for personal use. Commercial use (including selling the PDF or birds once constructed) is forbidden.