What are these marks on my washing?

[PeterPanandWendy]

Been finding marks on my white washing for a while now- lin dried. No trees too near. Marks are about 2-3 mms across- small- and a tea/coffee colour. They don't come out easily and DH's best white cotton shirts have been marked.
Is it insect poo- or what? Washing line has been in same place for over 15 years and never had this problem. Could it be from birds, or possibly shrubs a few yards away when it's been raining and windy?

we had loads of these and i changed the indoor clothes horse because there was some exposed metal, and also started using purified water in the iron - they looked like rust marks.
whichever it was, we haven't had any since (we also changed nanny at about the same time so who knows if that has anything to do with it?)

o & also be vigilant about running an empty 90degree wash through the machine with a load of Bicarb of soda every month or so.

But the problem is they are not there until I put them on the line outside to dry!

Yesterday I washed a brand new white duvet cover, to soften it before use, and today when I am ironing it, one of these blasted marks is there.

look for cracks in the plastic coating of your line if it's one of those. if it's nylon, got no idea.

We've had these - outside line too. I wondered if was insect poo?! yuk - but they are proper stains that don't come out.

Yes I've had them too on a couple of tee shirts. They don't come out at all :( No idea what they are caused by though!!

What is weird is that it's only happened this year in all my years of Good Housekeeping.

Have now had to stop putting DH's white shirts out and dry indoors. Deffo not coming off the line- they marks are in the middle of shirts and duvet cover.

Could it be pollen carried on the wind?
I know that flower pollen doesn't wash out.
Just a thought.

Could it be the pegs?
Sorry if that's completely wrong.

I read somewhere that it could be caused by too much washing powder, if that's any help?