Big Birds are the bigger, more powerful version of birds. The only canon example is Bigger Brother Bird, but some fanon versions were made.
“The Big Blue Bird is the blue bird equalient of the Big Brother Bird. He is the Big form of the...Big Blue Bird
“It's ability to crush is very helpful on most levels with it! It crushes any tipe of material...Big Brother Bird
“The big black bird is the big version of the black bird. The Big Black Bird is the most powerful...Big Black Bird
“Big Green Bird is the big version of the green bird. The Green Bird is slightly bigger than the...Big Green Bird
“Big Yellow Bird is the big version of the yellow bird. The bird is about the size of the big red...Big Yellow Bird