Thanks for your replies, everyone!

I agree with Dan Musicant and simy1 that birds are the most likely
culprits. The green parts of the seedlings were cleanly sheared off,
as you might expect if a bird had bitten them with its beak. The soil
was completely undisturbed. I don't see either snails or earwigs
anywhere nearby. I have a few pill bugs hiding under some plants about
forty feet away from the vegetable garden, but I can't imagine them
going so far to forage.

I have made a point to encourage the presence of birds in my yard with
certain plants, but I forgot about providing water. My wife and I are
planning a small backyard fountain. Once that goes in, I hope that
we'll have no further trouble. The winter rains have started, so the
birds should have the moisture they need for several months.

Thankfully, the benign California winter climate means that I can try
to sprout seedlings again. I'll try starting seedlings in containers,
as suggested by sueb and Bill Rose. Row covers as suggested by Dan
would probably also work, but I already have blister packs at hand. It
seems a bit strange, starting annual plants with deep taproots in
containers, but I have a reference which says it should work
(_California_Vegetable_Patch_ by Karen and Duane Newcomb).

Happy gardening to all of you.

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