I used the Google while I was on the Internets and came up with the following http://www.takeourword.com/TOW173/page2.html

The phrase, according to James Lipton* in his An Exaltation of Larks, dates from 1450 in the form a mursher of crowys. It was a murther of crowes by 1476. Whether it arose because murdering was thought to be a characteristic of crows or simply as a negative comment upon flocks of crows is not known. The mursher form is problematic, however, as we must wonder if it was not intended as murder but was mistakenly interpreted as such. We could find no instances of murder with a similar spelling.

*This James Lipton is the same James Lipton who hosts Inside the Actors' Studio (on U.S. cable television) and is a screenwriter, among other things.

This one is also kind of fun: http://www.field-notes.net/2002_11_17_archives.html

And according to Wikipedia: Groups of ravens have historically been called an "unkindness."

So basically, large black birds = evil