It really doesn't matter. Keep the sound going with a good wind. Bear hear distress, whether that be fawn, rabbit, calf or whatever. It's just something hurt they might can eat. I called in a jackal in South Africa with a jack rabbit hand call, lots of eastern coyotes and bobcats with the same call. None of them ever heard a jack rabbit before. Put some emotion into it and keep the sound playing. Bears can lose interest pretty quick. I called a brown bear in from 500 yds with a cotton tail call only to have the wind give us away in the last 40 yds. Bowhunting has it's challenges. I called a black bear about 200 yds with a blade of grass because I had switched jackets to a dry one and forgot to get my calls out of the wet one. It ain't rocket science, it's a free meal.
used fawn in the spring and rabiit fall in and spring ... found out so far the rabbit working way better here in the yukon but as i discovered you better watch out for grizzly and here the call is not distinctive but luckily we still have OTC tag for grizzly here ...
If you want to see an incredible video, find the Stone Glacier URSIDAE an Alaska bear hunt, on you tube. They have some nice examples of calls in the spring. 2/3 of the way through is an epic scene.
I try calling every year here on Vancouver Island and it's very hit or miss. I really think most of it depends on the individual bear and less on the particular call.