Birds are bipedal, but their trunks/spine are closer to perpendicular to their legs (while ours are parallel and stacked). Birds also have a non trivial amount of mass hanging down from their trunk between their legs, and their hips tend to extend significantly past their legs.
So while birds actually need significant posterior chain muscles to keep the femur extended, those muscles get to attach to an extended hip and so form more of a triangular/trapezoid shape compared to human butts. If you ever noticed that chicken thighs from the store seem a little triangular, and the bones are 'off center', then the the larger triangularish half of the chicken thigh is the chicken analogy to the glut complex.
EDIT: I think people would generally define the depth of the human intergluteal cleft ("butt crack") as one of the defining features of the human butt. In that context, I think humans have this feature at least partly because the human hip is... suboptimal in many ways. When considering the gluts as a hip/femur extensor, you have some pretty bad leverage, and so you're already biased towards having a very bulgey muscle there. As opposed to something like our hamstrings which has a much better leverage scenario, and so you see much less of an obvious bulge. By analogy, since birds tend to have an extended hip to attach to, their glut max analogs are no where as bulgey.
So while birds actually need significant posterior chain muscles to keep the femur extended, those muscles get to attach to an extended hip and so form more of a triangular/trapezoid shape compared to human butts. If you ever noticed that chicken thighs from the store seem a little triangular, and the bones are 'off center', then the the larger triangularish half of the chicken thigh is the chicken analogy to the glut complex.
EDIT: I think people would generally define the depth of the human intergluteal cleft ("butt crack") as one of the defining features of the human butt. In that context, I think humans have this feature at least partly because the human hip is... suboptimal in many ways. When considering the gluts as a hip/femur extensor, you have some pretty bad leverage, and so you're already biased towards having a very bulgey muscle there. As opposed to something like our hamstrings which has a much better leverage scenario, and so you see much less of an obvious bulge. By analogy, since birds tend to have an extended hip to attach to, their glut max analogs are no where as bulgey.