I do find them regularly on mossy coppiced willows in my area, however, nearly none of them on e.g. regular Sallows (fraxini instead 
). However, here in the north they are much rarer than fraxini. At least, I know from southern Germany that there they are on most willows and can easily be found, similar to fraxini in my area.
Ah ok, I thought you had found them on Populus tremula as well. So maybe they are more on willows and not so much on poplars, but that is just speculation without better data. I did search some trees of Salix caprea in my area for Catocala eggs, but had no luck so far. I'm also unsure if C. nupta likes trees deep in the forest so much. But I have to search on willows more. I focused only on P. tremula this year. At which height did you find most eggs of C. nupta? I remember them being higher and not so much at the stem base, but maybe I misremember that. I found C. fraxini mostly below 30 cm above ground level.
Thanks a lot for your replies! So far, I have found two eggs of (presumably) C. nupta: one on Salix alba and one on Populus x canadensis. On the exact same places as where you would expect C. fraxini eggs. Like you both have described, the eggs of C. nupta seem to have a finer structure. To me the ridges seem more apparent. Maybe this is because the ridges in the chorion of C. nupta eggs are actually folds, contrary to the ridges on the eggs of C. fraxini. Like described by Kolesnichenko & Sidorov (2019; 2021).
In the Netherlands, fraxini and nupta are the only Catocala species on Populus. As far as I know, C. electa went extinct a long time ago. Nupta is far more common than the very rare fraxini. Hence, when I posted my egg findings on de Dutch observation.org, a couple of admins started asking me questions. How do I know with absoluty certainty that these are the eggs of fraxini? A right question, when taking into account at the limited available comparitive material on the internet or in literature. When I look up 'Catocala nupta eggs' on Google, some eggs -like shown here- have striking similarities with the fraxini eggs I found.
For now I stand by my opinion that I really did find live eggs of Catocala fraxini 
. I'm glad that you think the same!
Best regards, Silvio
they can be found on nearly all willows and even Poplar hybrid trees.
I usually find them between ~1-1,5m high, but also lower ones.
Interesting, well I definitely will give it another go on willows in my area. We have mostly older Salix caprea in closed forests and younger ones in open more ruderal areas. But of course in the floodplains and along small creeks there are also Salix alba, Salix fragilis and some other species that I didn't really determine closer.
I have found two eggs of (presumably) C. nupta: one on Salix alba and one on Populus x canadensis.
Yes, the egg you've shown as C. nupta does indeed look like C.nupta to me. Perfect that we can compare it now, even with eggs found in the wild. It's also interesting that you found them only on Salix and Populus x canadensis which would support my hypothesis that C. nupta does not go on P. tremula (at least more rarely).
In the Netherlands, fraxini and nupta are the only Catocala species on Populus. As far as I know, C. electa went extinct a long time ago.
C. electa also mostly feeds on Salix species. At least in parts of Germany they were exclusively found on Willows and if they occasionally take Poplars their distribution suggests that this would likely be P. nigra or P. x canadensis. It would certainly be only a problem in the floodplains of large rivers.
When I look up 'Catocala nupta eggs' on Google, some eggs -like shown here- have striking similarities with the fraxini eggs I found.
Honestly, I don't think these are C. nupta eggs. They look like C. elocata to me, which is plausible because the adults are somewhat similar to C. nupta and can be confused. I could be wrong, maybe the pictured caterpillar hatched from these eggs. But the image quality is also way too bad to see the structure of the eggs. I certainly wouldn't trust every image on google to be correctly determined or even correctly labeled. Right now I think C. elocata is the only egg that might be easily confused with C. fraxini. I might be wrong and I can't really tell for sure because I haven't seen an egg of C. elocata yet. It is however also grouped with C. fraxini in the studies of Kolesnichenko & Sidorov you mentioned.
Anyway, I would be very certain that your determination is correct.
Greetings Dennis
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