The 2013 NES Angry Birds is a surprisingly good port of the homebrew Genesis demake, from the same people, of the mobile phone game, decidedly blowing the low-quality 2012 Nice Code port out of the water. Rare for an NES game, nearly all of the audio (the sole exception being the sound for selecting menu items) is stored as DPCM samples, all taken from the original game.
There's a few missing things (see below) and the physics are slightly off, but otherwise it's pretty faithful.
Unused Graphics#
Buttons used on the end-of-level screen shown after losing or winning in the original game. This version scraps that screen in favor of writing "WIN" or "LOSE" across the screen in white 8×8 tiles, likely to save space.
Unused Birds#
The Blues are never used in any level of this port. However, they do exist... somewhat. Changing the byte at ROM address $03A177 on a headered ROM from $01 to $00 will replace the first Red with the Blues in Level 1 of Poached Eggs.
They don't appear to be finished; they're missing their "separating" ability and lack any collision data, making them pass through blocks and Pigs without destroying/killing them.