Last year I got Audobon Apps on sale for $5 each to use as our main field guides. You can look up the usual information from field guides plus multiple pictures, and often times sounds, keep life lists, share sightings, etc. I downloaded them to my tablet and they have worked wonderfully for our nature study. It was an investment for all the apps, but well worth it. I got an email today- these apps are on sale right now for $1.99 on Anazon and the Google store. The apps run on an Android platform.
http://www.amazon.com/s/ref=nb_sb_ss_sc_1_4?url=search-alias%3Dmobile-apps&field-keywords=audubon&sprefix=audo%2Cmobile-apps%2C463