Hummingbird was an update that changed the way google understands queries and was a new engine for Google. It improved the understanding of synonyms, search intent, and conversational search. There were NOT widespread reports of ranking drops from this update, so if you dropped around this update, see if something else happened with your site.
Other Update Guides
Google Algorithm History | Panda | Penguin | Hummingbird | Webmaster Tools Warnings
Hummingbird Update History
Hummingbird came out around August 20, 2013