Growing feathers take a huge amount of hit on the bird’s overall energy expenditure. They are grown so fast, and sometimes many grow in at once (like during a heavy moult, or the young bird’s first set). If there is anything that may interrupt that bird’s supply of nutrients to that growing feather, the feather will show it. Maybe not now, but in the future.
The most mild form of this is premature wearing. Just like clothes sewed from cheap, poorly made cloth will tend to wear out and wrinkle really easily, feathers grown from a substandard set of supplies provided by the bird’s system will start to look awful before their due date. This can be due to nutritional imbalances, underlying illnesses, stressful events, lack of foodâ€¦ the list goes on. But, feathering in poor shape is an important symptom we look for to indicate a possible problem that needs further addressing.
For further information on this, look at the recent publication Birds USA 2008; Dr. Rolfe has an article in that edition which will further explain the health-feather connection.