why my paraket is siiting on the bottom of the cage with the beak hidden in the feathers?

by lisa k
(rialto, ca, usa)

i don' t know what happen my bird is sitting on the bottom of the cage for the last 3 days eat and drinking fine i have all together 4 paraketts 3 are fine sitting on the top except this one i dont' know what can i do to help this paraket. since there are 4 they need alot of food , funny thing when this particular started sitting on the bottom of the cage i used a sour cream empty container put alot of food so this sick one can eat more since this one is sitting on the bottom he or she sttarted stting on the bowl i don't know what's wrong please let me know