It could very well have been taking a nap. Birds have two sets of eyelids and will close one set and take a short nap. I observed this with my BB Red chickens.

I took my Chihuahua out to do his business and discovered a bird on the back of my plastic lawn chair. I walked up to him, talking to him and he never moved. I called the dog over to see, I went in and got my camera, still never moved. I took 12 pictures, some very close up with Macro and he never moved. I went and got my husband and he never moved. I thought he was dead and was going to put him in a plastic bag but decided to leave him for my daughter to see when she came in from work. She was due in shortly. After downloading my pictures I went back out to check on him and he was gone. Thank you for the explanation.