A nice checklist on assessing colony loss in Spring & another on mites. Make note that it is from ONTARIO, CANDADA so expect some regional differences such as wrapping hives which is not recommended for this area (mid-Atlantic).
Also of note – before some of you jump to any conclusions about bees dying of cold- they say extreme cold and again, this is coming from a CANADIAN perspective- the comments about small dead clusters that may have died from other causes is an important distinction. Look to see if there are chewed up wax cappings (evidence of robbing) and consider was your Cluster/Queen weak, sick, and/or dying & then was robbing out by other bees? Or were they really unable to access honey or did they not have enough stores going in to winter.
http://www.ontariobee.com/sites/ontariobee.com/files/document/spring-checklist.pdf
A lot of dead outs in this area when you find honey left in the colony & no signs of robbing can be attributed to PMS and other mite related viruses. Take a look at this to help: