It's quite possible you'll be seeing baby birds soon. Congratulations to your birdie family!
Do: Increase the calcium supplements for your birds. Provide cuttlebones, or oyster shells, or even chicken egg shells will work. Just be sure to bake the rinsed shells in a 350F oven for a half hour to 45 minutes. Keep the cage clean. Keep the water clean!!! Their cup should be as clean as yours. Often, illness is from dirty water dishes.
Don't: Disturb the mother and her eggs, unless she abandons them. Sometimes they lose interest. Only they know why.
Prepare for hatching by talking to your vet, or at least the pet store. You didn't say what kind of birds, but that's okay. They know the needs of various species, and they'll let you know what to provide.
Be patient and speak softly to them both. Being protective is natural, and the sign of a good parent. These bites and nips are how he's communicating their need for peace and a sense of safety.