Under the polymorph rules
The DC for any of these abilities equals your DC for the polymorph spell used to change you into that form.
The druids wild shape ability
At 4th level, a druid gains the ability to turn herself into any small or Medium animal and back again once per day. Her options for new forms include all creatures with the animal type. This ability functions like the beast shape I spell, except as noted here. The effect lasts for 1 hour per druid level, or until she changes back. Changing form (to animal or back) is a standard action and doesn’t provoke an attack of opportunity. The form chosen must be that of an animal the druid is familiar with.
And as a druid progresses in levels, more options are added than just beast shape 1.
However no where in the wild shape description does it mention what I should be using for the DC's of abilities. As a clear example lets take elemental body 1. I become a small air elemental which has the whirlwind ability. Now the small air elemental whirlwind is based on 10 + 1/2 HD + str mod, so in this case is DC 12.
However, the polymorph rules indicate that it should be based off the spell, except that its supernatural (SU). So if it was being based off of a spell, BS1 is a 3rd level spell, so DC should be 10+3+4=17 (I assumed our druid in question has an 18 for wis). Now DC 17 is pretty good.
However, what happens at higher levels (assuming I am staying as a small air elemental)? Does the DC change each time I gain access to a more powerful version? Thankfully it seems that Beast Shape, Elemental Body, and Plant Shape are all accessed at the same level, so BS1 is 3rd and accessed at 4th druid level, BS2 EB1 are 4th and accessed at 6th druid level, and so on so the DC's are equal at each level and dont encounter mismatches.
- Level 4 - DC 17
- Level 6 - DC 18
- Level 8 - DC 19