And maybe determining whether a creature is common, rare, or unknown would depend on a Knowledge[nature] roll when in doubt.
The DM determines that given the PC's background, recognizing the Flying Squirrel tracks is DC 18. If the check is made, the ranger can treat the creature as common (and this should probably be noted somewhere). If he fails the check, its rare. If he rolls a 1, its unknown.
This looks pretty good. Could a ranger determine how long ago a creature passed without learning the number? For example, Faragorn comes across a muddy road recently passed over by gobblers. Bending over the first clear track he comes across, he examines the track and determines it is 8 hours old.
He then stands up and looks around, determining that there were 12 gobblers in the band. This means instead of a DC +3 check, he would have (I guess) a DC +1 and a DC +2, respectively?