Nice entry! You succeded on making the game feel like it came from the NES era, from the sound to the number of colors per sprite, and the metroidvania aspect was cool, altought I almost tought that the game was linear at the beginning.
The only aspect that perhaps could be improved are the Barn backgrounds, they are maybe a little too monochromatic, and I feel that maybe there is a bug when you dive bomb into enemies, sometimes the main duck takes damage anyway, so its usually not a safe option to use it. And cool Boss, altought maybe at the third fight he could have used a new special attack, he was fun to fight.
Nice little game! And also good map, it helped finding non explored areas, congrats!