Birds can be difficult to conflict check. Any conflict check must take into account the bird category and the default postures of the relevant birds, as well as number, color, and arrangement.
Generally speaking, birds are considered different when: (a) they were considered different in period, (b) they are displayed in a period posture, (c) they are drawn correctly, and (d) there is some visual difference between them (i.e., there is really no visual difference between a popinjay and a hawk). If the posture is changed from period or not drawn correctly, conflict may be introduced even to birds in different categories. (http://heraldry.sca.org/loar/2003/11/03-11brd.htm, Appendix M of SENA.)
There are five general bird categories:
Not all period birds fall into the categories above - popinjays (parrots) are drawn heraldically as "regular shaped" birds in period (similar to a crow), but a natural popinjay (parrot) has a pronounced tail that would require comparison with "poultry shaped" birds. Birds not mentioned above must be compared on a case-by case basis with very similar birds (SCA-compatible birds not found in period heraldry, for example, New World birds.)
Any bird known to period Europeans may be used in Society armory – though, if the bird is not itself European, its use may be considered a step from period practice. (An exception is made for non-European birds actually used in period European armory. The turkeycock is perhaps the prime example of this.) (Birds, Online Pictorial Dictionary of Heraldry)
Birds within the same category are not substantially different from one another. However, some birds in the same category are significantly different enough for a CD. Therefore, birds within the same group do not automatically conflict - the default or period postures of each helps determine the conflict.
Birds from different categories, when in period postures for those birds, will usually be considered completely different for purposes of conflict.
There are some bird postures unique to Society heraldry:
SENA begins to categorize postures for conflict checking purposes in Appendix L, as follows:
B. Birds: The postures listed within each group generally conflict, though a distinct change may be given for facing to dexter or to sinister.
* 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 [DC] - unless documentation is provided showing that the particular type of (non-eagle) bird is frequently found in the displayed posture in period.
To summarize the November 2003 LoaR information about bird postures:
This compilation of precedents begins with the first tenure of Francois la Flamme (August 2001 - April 2004), during which a considerable number of decisions were handed down regarding birds, and moves forward in time from that point.