An owl close guardant and a dove close guardant have a substantial difference in the "regular birds" category, based on their shape and default postures.
But there is no difference between a falcon rising and an eagle rising, based just on shape (both are raptors.)
Sable, a duck close argent and Sable, a stork close argent are clear of each other because they are in different categories and are both in their default posture. Both of them are also clear of Sable, a falcon close argent.
Sable, an owl close argent does not conflict with Sable, an eagle displayed argent. They are in the same bird category, but their default postures are significantly different enough to clear them of conflict.
Sable, an owl displayed argent does conflict with Sable, an eagle displayed argent. They are in the same category, and the owl is in a non-default posture that does not make it significanty visually different than the eagle in the same posture.
A correctly drawn dunghill cock close is substantially different from a heron close, a swan close, or a martlet close - they are all in different bird categories and in period postures.
However, a dunghill cock volant is not substantially different from a dove volant, because, while the dove is found in the volant posture in period heraldry, the dunghill cock is not.