Alive Or Dead

Rambam Maacalos Assuros (2:21) says אֲבָל הָאוֹכֵל בְּרִיָּה טְמֵאָה בִּפְנֵי עַצְמָהּ כֻּלָּהּ הֲרֵי זֶה לוֹקֶה מִן הַתּוֹרָה. וַאֲפִלּוּ הָיְתָה פְּחוּתָה מִן הַחַרְדָּל. בֵּין שֶׁאֲכָלָהּ מֵתָה בֵּין שֶׁאֲכָלָהּ חַיָּה.  This is the rule of בריה, if an entire creature is eaten one is obligated even for less than a כזית,  In Ch. 4 law 3 he says הָאוֹכֵל עוֹף טָהוֹר חַי כָּל שֶׁהוּא לוֹקֶה מִשּׁוּם אוֹכֵל נְבֵלָה וְאַף עַל פִּי שֶׁאֵין בּוֹ כְּזַיִת הוֹאִיל וַאֲכָלוֹ כֻּלּוֹ. וְאִם אֲכָלוֹ אַחַר שֶׁמֵּת עַד שֶׁיִּהְיֶה בּוֹ כְּזַיִת.  Here he defines a "creature" only as a live being whereas before even a dead creature was defined as a "creature" to make one obligated.  What's the difference?  The Briskor Rav (Nazir 51b) says that these are two different dinim.  When dealing with a בְּרִיָּה טְמֵאָה, then any creature fits into the possuk that prohibits it from being eaten, whether alive or dead.  However, in Ch. 4 the obligation is for נבלה (a situation of נבלה מחיים, see Rav Chayim there,) then the law of בריה says that it equals a כזית because of its חשיבות and that is only if the creature is still alive.  Comes out according to the Rav that when dealing with a בְּרִיָּה טְמֵאָה, its not that the שיעור is a כזית, rather the שיעור is the בריה but if one doesn't consume a בריה then they are obligated for a כזית.