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These Birds are Flash Vector Animation. It can be resize and it is only 7kb each. You can also embed it to your html page.

The best part about these button is that you dont have to open the FLA to specify your Twitter Link. You can do this through the embedding code. This is explained in the documentation file.

You just need to open the FLA file if you want to change the color of the bird and remove the background

This appears to be a flash file, you cannot see a preview because flash player is no longer supoorted in browsers, but you can still download the file to study it's source code.
You can open/play the swf file with Flash Player exe