Greenster:

I find that the best source of this kind of information is the Oxford English Dictionary.  It is available online, but to get most of the "good stuff" you have to subscribe.  Any half-decent library near you probably has a subscription you can access.

In looking into "Murder" in the sense of "flock of crows," I see that the lexicographers express doubt about the word, calling it "one of many alleged group names found in late Middle English (ME) glossorial sources; apparantly revived in the 20th Century."  (italics mine)

The entry provides three such sources (for which I'll only give dates and spellings):

1475:  "Morther"

1475:  "Mursher" (possibly a transfer error)

1478:  "Murther"

(Note:  A shift in pronounciation occured between ME and the more modern form of English, leading to the shift in spelling.)

And then the next dated source for the word appears in 1939.

Further reading on the word's etymology:  "Origin uncertain; probably same as "murder" (the other one), perhaps alluding to the crow's traditional association with violent death, or, as suggested in the 1939 quote, to its harsh and raucous cry."

So there you have it.  Most likely, some Middle English scholars living in the early 20th century liked the word & decided to re-inject it into the lexicon.  That is, of course, conjecture, but until the word shows up in this specific context in a more modern text (yet pre-20th century), it's  as good a guess as any.

This has been a fun li'l project on a slow afternoon.

Herring405