Cutworms?

tj

Sounds like cut worms to me based on your description. You should be able to dig around the recently nipped plant and find the culprit(s).

You can put a couple toothpicks next to the stems to stop it.

A third vote for cutworms. How annoying! I always put small sticks against the stems of things like tomatoes and peppers, just in case........

Haven't had to do the toothpick thing in a long time, but one on each side, touching the stem and pushed down about and inch or so into the soil is what worked for me. Little twigs work too.

get some food grade diatomaceous earth which is available at just about any retail chain or garden store. sprinkle moderately about 8-10 inches around the base of each plant, including on the lower part of the stem/plant. works wonders for almost all types of bugs and keeping them from getting anywhere near the plant stem, but keep in mind it can also do harm to bees, which shouldn't be a problem unless you're applying all over the plant (it's also very effective at keeping critters off the rest of the plant as well).

Large seeds like cucumbers and sunflowers are often attacked by birds and not necessarily eaten. Sunflower sprouts are especially attractive. A bird hopping around in a vegetable garden most likely cannot resist pecking at sprouting curcubit seeds. I cannot get sunflowers to grow unless I put some protection over them until their first leaves fully form. After that it doesn't seem so attractive to curious birds.

I've experienced something similar, where sprouting beans were pulled out. I wondered whether the bird was pecking at a pest and the sprout was just a casualty.