Thank you all for your suggestions and concern! I totally freaked out and moved all the birds into my living room, which now looks like a pet store!!! To clarify....the birds were in a "game room" built directly above our detached garage. It is a very nice furnished room. They were all in there because we are in the middle of painting and remodeling our home and we didn't want the fumes to hurt the birds.
I have lived in the woods for 30 years, and I am very familiar with the wildlife and the problems associated with living in "their" territory. This was DEFINITELY a black snake, approx 20" in length. I know they are not venomous, but they do bite when provoked. I have noticed a few mouse droppings in that room.
Now...to get rid of it. I haven't seen it again. I have a feeling that it got in through the bathroom, where the water line comes in from the garage below. Hopefully it got scared and went back down into the garage. We have re-sealed all the openings with caulk. We also placed a new thing under the door to seal it tighter. I have literally torn that room apart, and can't find the snake, but the birds are out, so if he shows up we can deal with catching him. I have once caught a snake inside by placing a large salad bowl over it, and then sliding cardboard under the bowl. Then I could pick it up and take it outside without being bit.
I will let you know if he shows up again. While the glue trap suggestion has merit, I would prefer to catch him and set him free. They are "good" snakes, and they definitely help keep the rodent population under control.
In the meantime, all of the birds are doing quite well in the living room where I know they are safe!