Mysterious Hair Loss on Lamb's Ears

I seldom blog about health worries, I once had a lamb blow up to the size of a balloon with bloat, I was too scared to take a picture thinking that if she died it would be too sad to look at the picture (yes I was not born a farmer and my sheep are more like pets than livestock)... That lamb did live and in fact is Blackie sheep whom you may have read about.

I have an odd health worry now, but not one that is too serious, it is mostly just puzzling. 

As you recall I had 7 lambs born in October. It is now January. A week or so ago I noticed two lambs (different moms) had some hair loss on their ears, just about where their ears attach to their heads. No other lambs had any such problems. The only thing the two lambs had in common was that they were white and black;  I also have 3 other lambs that are all black, one white and brown, and another black and white (unaffected at that point).

I thought.. hmm, maybe they need a new mineral block, so got that, but nothing seemed to change. I do not know if the lambs even lick it.

Well a couple of days ago the other black and white lamb also is showing some signs of hair loss on the ears, and he is also showing small areas of hair thinning on the sides of his nose.

The other 4 lambs are fine and show no hair missing.  There is no hair loss on the ewes either..  They are all wool sheep x hair sheep.

The areas of hair loss are too even and symmetrical to be anything such as ringworm or mange. 

I have found nothing online except one post that suggested an allergy to food combined with sunlight.

I should add that it is winter here, temperatures have been below freezing, we have snow and the sheep are getting hay and grain (oats). 

If anyone has any ideas I would love to hear them. My vet is not as familiar with sheep as he is with cattle and nobody else here has a clue. 

I have not been out with the camera (when it is cold the battery freezes too quick and I have just not thought about it), but I have used a picture of a different sheep and drawn in the places of hair loss.

Update - Below is a picture I took today, it's pretty hard to get a good picture of a goofy lamb with a crappy camera, but I gave it my best shot - note that the lamb's are not itching.  It actually seems like maybe the hair is growing back in, it's hard to tell.

Solutions to Common Skin Problems in Dogs