I think the culprit is a.... bird.

Look for cutworms.

Dave

When the main stem is cut off, you can insert it in the ground, water it, and maybe give it partial shade if the sun is hot. It should come back, though of course you'll lose some time. (Since tomatoes are such survivors, if I ever lost a tomato to a cutworm, I'd wonder if the base of the plant might resprout.)

I plant my tomatoes with half a toilet paper roll wrapped around the stem and have never lost a seedling; others use other tactics.

You can also take preventive action re. cutworms:
http://www.uri.edu/ce/factsheets/sheets/cutworms.html

I was wondering about birds. We have many. I'll look for cutworms, but as yet, haven't seen any insects on them. Any idea how to stop the birds?

3/4-Inch by 100-Foot Holographic Bird Scare Tape (Dalen)
Scare Eye Diverter (Bird B Gone)
Scare Eye Balloon (Bird-X)
Hawk Decoy (Bird B Gone)

This post was edited by Daniel_NY on Sun, Apr 28, 13 at 11:31

Cut worms come up out of the soil to do their damage. You see them when digging for planting. They aren't an insect. There are all sorts of cutworm protections available - stick, plastic straw, TP roll, cornmeal, big nail, alum. foil, etc. etc.

Bird damage is usually only a problem in early spring and is stopped/prevented by blocking access to the plants - covering the plants with something until they reach size enough that the damage doesn't bother them.

Down here one often sees rows of young tomato plants each with a gallon milk jug over it for early spring protection from all sorts of issues.

Dave

I'll look for cutworms, but as yet, haven't seen any insects on them.

Cutworms eat at night and hide underground during the day. You may notice them when digging a hole to plant seedlings....

Here is some information, including photos, on cutworms, their damage, and some suggestions on control.

I hope that helps.

Betsy

Here is a link that might be useful: Cutworm Information

Cutworms are moth larvae -- true caterpillars -- so technically insects. As true caterpillars, they can be killed by BT kurstaki (though they do not die immediately, and can cause more damage before dying).