As a final refinement I would group them by folders. Each bird type in one separate folder. Because you have 3 animations per bird that it is 6 files...
You have only two files per animation so it isn't necessary to group gif file and corresponding spritesheet to the folder but if you'd have many animation per one object (for example 7), it could be useful
Also very common and comfortable to use spritesheets with all the animations of one object (birds in your case). It is when each animation uses its own row... It isn't necessary but easy to use in many engines....
Example: