First, i feel for your lucy and sorry for you, i'm pretty sure Lucy may of died of sickness, many birds usually fluff up their feathers when they have sickness to keep warm.
Many symptoms like:
fluffed up feathers
always at the bottom of the cage because too lightheaded to grasp the perches
cranky
sleepy
a change in regular voice
not eating much
inactive
lighter or smaller when you carry it
watery droppings
black or blood red covered droppings
soiled vent (meaning if their the place where their droppings come out is stained)
drinking water less or more
vomiting or regurgitation
sticky or wet facial feathers
inflamed or crusty eyes or nostrils
coughing, sneezing, irregular breathing, breathing out of mouth
increased blinking, squinting, or cloudy eyes
hunching
drooping head, wings, or tail
bright yellow, or green urine
swelling in the eyes feet, legs or other body parts
panting, wheezing, or noisy breathing with clicking
foul or sour odors coming from breath
loss of balance
plucking or chewing feathers, molting
bleeding
diarrhea
and many more, but i would check the other bird before to dies as well, ASAP!
one of the most common sicknesses for birds would be bacteria infections, i know you clean the cage all the time and keep it clean, but have you ever thought about the things i the room that may have caused bacterial infections? birds are supposed to be in fresh air and the bacterial infections are commonly found somewhere in the home.
bacterial infections in home:
Unwashed fruit or veggies
wild rodents
mouth feeding a bird
stress or lack of sleep
hanging out in the bathroom (where a LOT of bateria is!)
dirty hands
owners or other people having sicknesses
dirty supplies such as sponges, rags,
water is sink filtered or dirty
seeds diets, espically if sprouted
Direct contact with another bird or cage mate
so get your other bird checked ASAP!!!
(Your bird should be fine but may be depressed and lonely for a while)