Birds in different categories can be considered substantially different from one another when displayed in period posture and drawn correctly. Once the posture is changed from period, conflict may be introduced based on the posture comparison (http://heraldry.sca.org/loar/2003/11/03-11brd.htm, Appendix M of SENA.)
SENA categorizes postures for conflict checking purposes in Appendix L, as follows:
* May 2014 LoAR adds this about the volant posture: This month we ruled that a bird volant had substantially different posture from a bird close. SENA A5E5a does not currently list the volant posture in one of its groups of postures that receive substantial difference from other groups of postures. Look for a Palimpsest rules letter to further discuss whether or not volant should be added in general to SENA A5E5a, and if so, under what circumstances.
* November 2003 LoAR adds this about the diplayed posture: All birds (other than eagles) in the displayed posture are considered [a SFPP] and are not eligible for substantial difference [SC] - unless documentation is provided showing that the particular type of (non-eagle) bird is frequently found in the displayed posture in period.
The November 2003 LoAR has additional information about postures:
Here are a few generalizations concerning bird posture...
There is a precedent in February 2007 that re-states the definitions for the postures of "stooping" and striking":
As defined for SCA use (in the LoAR Cover Letters of July and Aug 1986), stooping is reserved for when the raptor is "dive-bombing", falling on its prey: wings swept back, body vertical or diagonal with head down. Striking is the moment when the raptor comes out of its stoop, its feet ready to grab or punch the prey: body diagonal with head up, heraldically equivalent to rising." [Balin Kendrick, 02/2007 Calontir-A]
If a bird is not in the standard posture, you can use this table to determine if the posture visually conflicts. Sable, a martlet addorsed argent would be clear of Sable, a martlet displayed argent, for example.
Images from the SCA Heraldic Primer, or from LoI archives as appropriate.