"Storm clouds fly, and ravens gather."
-SM Stirling, The Protector's War (New York, 2006), Chapter Thirteen, p. 359.
I see that ravens have already had good coverage on the blog. Poul Anderson writes about Odin who is accompanied by ravens. Odin's descendants display his raven emblem on their sails.
Ravens are important at the Tower of London. See here. For this reason, Bryan Talbot, writing and drawing an anthropomorphic animals alternative history, Grandville (and see here) has the Tower guarded by large, armored ravens.