I used to breed parakeets. The budgie as they are called in Australia tend to fluff up & sit still or on the bottom of the cage as has been mentioned. A later stage of being sick is when the front of their chest & around their beak gets wet & sticky. This can be because they have a chill. They need to be kept in a warm quiet room & cover them up at night. Bright lights & lots of noise stops them sleeping at night & allows the cooler air to chill them. If they self pluck it then becomes a habit as they get the taste of blood from the ends of their feather quills. They do this from stress. If they are lonely they can do this also so either give them a mirror so they think their reflection is another bird, or give them another budgie so they have a mate, or you can buy a bird buddie which is made of soft material that they cuddle up to for warmth & company. You can also try putting a toy bird in their cage for them to play with. You can buy these on a bendy spring so they move when your bird touches them. To relieve boredom put some natural unsprayed tree branches in their cage for them to play & chew on. I would also give them a long scewer with a corn cob, a head of brocerlie, a carrot & a strawberry on each end. Make sure its threaded through their cage bars so they cant impale themself on the end of them. This will entertain them & they will climb on it & eat it for hours. They may be afraid of it at first but (keep changing it after a couple of days as spoilt food can make them sick) & usually within a week they will be use to it being part of their cage. The veggies raw are good for them. Never leave a sick bird in with other healthy birds. They may pass on what they have. Separate the sick one to a small cage or box with water & seed & you can add a tiny amount of red cordial to their water to give them energy if they are listless. If the red coloring shows on their wet patch on their chest then you know they are not able to keep the water down. They don't have to have a perch if they are sick as it uses up energy that they don't have climbing on & off it . Check their bottom for runny green diarrhea that sticks their feathers together. You will need to wash their bottoms gently with warm water but have them in a warm place to dry off so you don't make their chill worse before they go to bed for the night. Better to wash them in the warmer part of the day. Hope this has helped.