Actually I have had this happen. My first silver chick had fledged and was well independent. He was still with his parents as he was the only chick of the clutch of eggs that hatched. He was a picture of health as you can see in this post...he was a whopper of a bird
http://finchforum.com/viewtopic.php?f=7 ... rst+silver
He was fine one day and the next afternoon I saw him sitting on the bottom of the cage looking miserable. I thought he must of somehow injured himself. I reached in the cage and he didn't even try to get away from me.Then I knew it was dismal. I picked him up and and removed him from the cage . I begin to examine him to try and figure out what was wrong when he regurgitated fresh blood. I was shocked! he did this a couple of more times and past away quickly. I went back to the cage and his parents were fine. There was a spot of blood on the cage bottom where he was sitting.
I have no idea what happened. If it had been a bacteria infection there would of been a gradual decline in health before it reached this point. Any contagious disease and other birds in the bird room would of been affected I believe. The parents went on to hatch out the next clutch with no problems. So I wrote this mysterious death off as a freak occurance, a genetic fault, a birth defect, or terminal cancer. I went through many possibilities...
I assumed the bird either gorged himself on oystershell ? Once I had a bird get pierced with the dried sharp edge of a large herb leaf in the product 'herb salad' so I thought maybe he had swallowed a sharp piece? Maybe a piece of bark/splinter off the perch? Maybe a ruptured blood vessel that did indeed fill his lungs with blood. A tumor. A defority that proved fatal as the bird matured. I will never know but it was an extremely strange occurance. I will never know what happened.
Sorry for your loss, possibly one of those freak incidences that happens to us all but keep a close eye for any developing listlessness, puffiness amongst the rest of the flock you may want to seek the help of a veterinary.
When birds are housed outdoors and not screened in there are many factors that can lead to problems over the inside counterparts, like west nile, worms, etc. Not sure if you had them inside or outside.