Questions on 1902 college slang article: "birdies", "barkers"

[Cohen, Gerald Leonard]

The 1902 college slang item spotted by Benjamin Zimmer contains the following two sentences pertaining to Yale:

 'The Skull and Bones men are called "rattlers," and the Skull and Key men "birdies," and the Wolf Head men "barkers." There are three senior societies, and juniors working to get into these are called "heelers." -- (parag. 15).

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I'm now wondering: Why are the Skull and Key men called "birdies"?  And why are the Wolf Head men called "barkers"?

Any help would be much appreciated.

Gerald Cohen
P.S. for "heelers" I assume we deal with HDAS "heeler" #1: "Orig. Underworld, a usually unskilled accompllice of a swindler, thief or other criminal; a hired thug; a servile supporter or backer; hanger-on."

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