The applications stored on your Android device are stored as APK-files. If your devices is rooted, you should be able to simply connect to it and grab the files.
A much easier way is to simply install ES File Explorer from the Marketplace, run it, click the AppMgr button, long-press the app you want and select “backup”. The apk file will now be stored on your SD-card in the folder “backup”.
Copy the file to your computer, and open the apk-file with any zip-capable extraction program (apk is just a fancy zip-file). Copy the classes.dex file, as this is the one that holds all the stuff you need.
Before you can get at the code, you need to convert the dex-file to a regular jar-file. This is easily done with the dex2jar tool, downloadable here: http://code.google.com/p/dex2jar/downloads/detail?name=dex2jar-0.0.9.9.zip&can=2&q=
Unpack it, and from the command prompt run “dex2jar.bat classes.dex” and it will produce a classes_dex2jar.jar. This jar-file can now be opened with any Java decompiler, such as JD-GUI, which can be downloaded from here: http://code.google.com/p/innlab/downloads/detail?name=jd-gui-0.3.3.windows.zip&can=2&q=
And there you have it, the source of your angry birds.