Birds usually do other damages to crops: for example blackbirds will sometimes pull freshly planted seedlings from the ground to get at the fat earthworms disturbed by our planting activities. And because they are 'ooligans.
Caterpillars however can defoliate a basil, chard, lettuce etc plant in a single sitting.
Noctuid caterpillars in particular will feed at night and since they are extremely mobile (for a caterpillar) they will often be completely gone by daybreak, leaving you with nothing but the measly remains of your vegetables.
If you have seen birds around your plants chances are they were actually feeding on the caterpillars eating your plants.
What to do? if the damage's very serious the plants are most likely goners. You had better replant them. You can use Bacillus thuringensis kurstaaki BTK) to kill off any caterpillar in a safe way for other organism. BT is a pretty good selective insecticide, and it's a compliment coming from me that don't like all that hippie stuff.
You can also apply collars around your plants to prevent caterpillars from climbing up the stem and starting to feast: aluminum foil works pretty well.
