"The red crab and the black crow."
Translation:A' chrùbag dhearg agus an fheannag dhubh.
"am" is used before labial consonants (i.e. b, f, m and p). This is rather like the way the English prefix "in-" becomes "im-" before labials (e.g. inelegant, intangible BUT impossible).
The thing about initial 'f' is that the lenited version, 'fh' is actually silent. So "fheannag" is pronounced as a vowel-initial word, not one with a labial consonant; as such the default form of the article ("an") needs to be used. Note that the lenition happens only because "feannag" is feminine; a masculine noun like "falt" (hair) becomes "am falt" because masculine nouns are not lenited by the article.
As I understand it, the form "a' " is applicable only before FEMININE nouns whose initial consonant is a lenitable labial (b, p, m) or velar (c, g). It is not a question of shortening as such, and it is not an optional process. The form "a' " accompanies lenition and only happens with a feminine noun.