African grey parrot lying down !

Question
My sister owns a 40 year old African Grey.He has been with her for about 25 years. He has recently had a clean bill of health from the vet, but has started to lie down during the daytime.He sometimes lies over to one side and sometimes just leans forward onto his chest with his head lying on one side.
He has quite a large territory that he roams about in, with plenty of perches and different surfaces to clamber about on. His wings are clipped.  He doesn't appear to be unhappy and doesn't appear to be sleeping when he is lying down. We have been wondering whether he might be suffering from arthritis in his legs, or something similar. Do you have any ideas please ?

Answer
I don't think it's arthritis, but another infection may mimic this.  It could be anything from salmonella to a number of other infections going on internally.

 Birds are among the BEST in the animal kingdom at hiding problems until far advanced. By the time we see them have symptoms, it's been going on for some time.  

This little guy needs an avian vet check quickly (today).

In the meantime, keep a heat source for him in the bottom of the cage by using any clean, cloth sock. Fill it 3/4 with raw, plain rice and knot the end.  Microwave it for 1 - 1 1/2 minutes and then shake it out to distribute the heat and be sure it's not too hot.  It will last an hour or two.  
 An eyedropper with a few drops of unflavored Pedialyte may also be helpful in maintaining his hydration and energy.  Even if he seems better - it's essential to have him checked out by someone who really knows birds (not just any vet).

 Please don't delay.  Let me know what happens ok?