Birds Ripple-like lines in bird image backgrounds

[LMB49]

I have a Sony a6600 and a Sigma 100-400 mm 5/6.3 DG DN OS Contemporary lens and I use DxO Photolab 4 for processing my RAW images. I photograph birds almost exclusively, primarily in aperture priority. I’ve noticed that in images with backgrounds that can be described as “complex” (grasses, reeds, tangles of leafless vines/thin branches and apparently also beaches) I get these very close-together, more or less vertical lines (typically in the upper part of the image) that look totally unnatural and like artifact or distortion. Here is an image that shows this phenomenon. This does NOT happen when the background is water, sky or leafy trees. What is this phenomenon And what causes it? Is there anything I can do to prevent it?

 

[FowlersFreeTime]

I think there was another forum member who had issues with that lens, but I cannot find the thread now. If I come across it, I will share here.
EDIT: nope, I cannot find it, but I am sure someone else had a similar issue with the 100-400 lens. Maybe I saw it on another forum.
Is the lens on the latest firmware?

 

[JDinTulsa]

Interesting! Have you noticed if it's only at certain focal lengths, ISO setting, or shutter speeds?

 

[Jeff A]

We have a few frequent Posters on this Forum with this lens, and I used to be one of them. I never had the problem and I have not noticed others with this lens and problem. From memory (a very dangerous thing) the Users Aussie, J. Duffy and Charlyee might see this post and comment. I believe those people have this lens. My Sigma 100-400 produced excellent images.

 

[Uncle Kevriano]

It's known as Nervous Bokeh and is something that is very hard to avoid with this kind of background. It happens with all lenses, and can be made worse if you shoot through a window or if you have a filter attached. Have a google of that term.