Thanks for the responses, everyone. Unfortunately, I can't get helpful photo evidence right now, since the seedlings which were damaged pretty much all died. (I'm hoping that when I put out the replacement seedlings, they don't go the same way, but if they start to decline, I'll make sure to document it right away.) But checking at night for slug activity sounds like a good plan. I saw no evidence of slug trails, I don't think, but I have lost seedlings to slugs in the past.

Leafcutter bees (Megachile spp.) are using the missing pieces for nesting. Don't worry, it shouldn't hurt the tree at all.

If you see no slime trails it's mostly likely the earwigs. Sowbugs and slugs don't normally take up residence under gravel, but earwigs will. Yes, I would move them up from the gravel....it supplies enough crevices for them to find daytime shelter. However before you do, I suggest you leave them as they are today and run out tonight with flashlight in hand. That way you can be sure what is in fact doing the damage. IF you find that is is earwigs, try putting out unwashed tuna cans filled with veggie oil as a trap. Also Rotting pieces of fruit/bacon works well too in a small paper sack turned on the side or in dishes...gather it up and toss in the morning; crumbled up moist newspaper tucked here and there, ect. will attract them also. The messed up thing is that they voraciously go after aphids and other soft bodies pests, but when the population is low they turn to tasty seedlings :(

🤔 hmmm?! Didn’t know birds would eat leaves too. I’ve seen them dig up seeds I’ve just sowed and mess with my seed tray soil but not eat leaves. I learned something new. Your culprit could be cut worms. They will eat off most of the leaves and can do it all over night. I’ve heard you have to go out with a flashlight at night to catch em and pick em off if you want. I can see them at sundown, but can be hard to miss they look just like the soil and blend in with some seedling stems. I use diatomaceous earth...and that seems to work. They crawl on the surface of the soil. I just spread some over the seedlings and soil surrounding. I’ve seen a higher survival rate, but I feel like I’m always laying down new seeds cause of this. Good luck!