Egg shell spots suddenly

[bwood002]

Over the last month or so, I've noticed one or two hens developing white speckles on egg shells. I've read these are likely signs of calcium excess.

This past week, I'm now seeing brown speckles that are not customary to the breeds. With all the excess heat and humidity in the south this summer we are concerned about mites. We use prevention and check the birds regularly but haven't seen any signs. Except this last week. These are new on the shells and seem to wash off but not completely.
And, they appear on eggs that are collected almost immediately after laying. so if mites, how could they coat the egg so quickly?? not sure unless internal or a bad infestation.

Looking for any help please. what could this be?

 

[ChickyCh00k]

Those speckles aren't mites or caused by mites. It's likely something the hen's gland is doing prior to laying the egg and likely nothing to worry about.

If you are concerned for mites, give the birds a good once over, looking closely at the skin near and around the vent for any tiny moving bugs. That's a better indicator of mites. If mites DID appear on the eggs they would tiny black specks (all about the same size) moving around on the egg.