Okay, since I didn't go with my name, and there is only one bird I absolutely *adore*, I can assure you that I only have one nick on these boards, so I wouldn't know for sure why other people feel the need to have more than one. But here are a few ideas:
1) They couldn't decide on which name they liked better, so they went with both, plus the nick the spouse, girlfriend/boyfriend or their own mother use on them...
(quoting Terminator: "I'll be back!" The Phoenix Rising has not won yet...