I noticed this regular pattern of marks on an old desk out in the garden (it was supposed to be in the bin, but I keep forgetting). The surface has a plastic "wood-effect" finish and the algae film has built up over a longish (!) period. Recently I noticed these pear-shaped marks in the algae. Each pear is 4mm at its longest point, they're very regular and they appear to follow wandering tracks. I hope the pics show you what you need.
Location: Derry Northern Ireland.
Slugs? Snails? Mouses? Not birds, I think??
