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Last week I noticed this odd track made of "squiggly triangle footprints" on the outside surface of a dirty skylight in my home in the northern Netherlands, and I found it odd that I can't imagine what kind of animal this track would have come from. I assume it's animal since the movement seems very intentional and the triangle shapes are periodic, rather than being a continuous smudge that might be caused by a tiny leaf blown in the wind for example. Each tiny triangular "footprint" is around 3mm wide. Any pointers on how to identify what caused this track?

enter image description here

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    $\begingroup$ My guess is a rocking and rolling pinecone. $\endgroup$
    – DKNguyen
    Commented Aug 14, 2021 at 16:11
  • $\begingroup$ any kind of close up would be a huge help, I can't see any detail. $\endgroup$
    – John
    Commented Aug 14, 2021 at 18:38
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    $\begingroup$ Yeah I thought of a pinecone-like object being blown around, but note that it appears to all be a single track which moves up and down and around in a very consistent pattern, no skips. Also note that this is a pretty steep skylight; I wouldn't expect even a very fortuitous wind to keep a roundish object in place long enough to do such an intricate pattern. @John sorry that's the only image I grabbed before I washed the offending window :-) but a reasonable amount of detail is visible if you zoom in. $\endgroup$ Commented Aug 14, 2021 at 19:00
  • $\begingroup$ @TopherHunt Is there a coniferous plant above the window? Have you tried taking one of the cones and reproducing the pattern? Perhaps if the cone is not roundish, but more cylindrical or cone-like shaped, this could explain the left-to-right wiggly pattern of its descent ... $\endgroup$
    – Domen
    Commented Aug 14, 2021 at 19:11

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Snail feeding.

It is really obvious if you have seen it before. It is the track left by a feeding snail.

Here is the grazing pattern of the common snail.

enter image description here

https://commons.wikimedia.org/wiki/File:Land_Snail_radula_tracks.jpg

https://alexhyde.photoshelter.com/image/I0000oboo93ZzSEI

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    $\begingroup$ YES thank you. And that fits because Friesland is teeming with snails and slugs. Allegedly they don't have many predators here. $\endgroup$ Commented Aug 16, 2021 at 6:59

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