I have been looking and have found several articles about how the Ravens came about.
The oldest article stated that Baden-Powells first group of boys were "were arranged into four patrols: Wolves, Ravens, Bulls and Curlews". Information taken from- http://en.wikipedia.org/wiki/Brownsea_Island_Scout_camp
and More Recently i found this link.http://ravenlinks.com/new_page_1.htm
I had found a few more but forgot to book mark them so I am having trouble finding them again.
However I am still looking for more.and will share them when I do find them.