Lemon liked to puff up. We did not know why he did it, but he often could be found with his feathers puffed out.
One day, I put the bird cage near a window, because I'd read that birds like lots of light. Unfortunately, we live in an old, drafty house in Idaho. Lemon was found the next day by distressed children in the bottom of the cage, deceased.
Back at Pets Are People Too, I asked the clerk, who wandered around the store with a parrot on her shoulder, why our bird did not make it.
She explained that puffy birds were sick birds, and the cold draft probably finished him off. She said to keep the birds warm. She also gave Little Mister another parakeet, this one blue and with a wing that sticks out at an odd angle. Mister felt consoled. He named the new bird Melody, and we brought him home and put him in the cage.
We moved the cage to a less-drafty spot. After a few days, when one of the birds looked a bit puffy, I moved a heater right under the cage.
Since then, both birds have been sleek and singing.
Warm birds are happy birds.