The Post-Mortem was performed and it was found that the cause of death were:
1. Antibiotic Toxicity.
2. Change of Feed (Diet Changes).
3. Mismatch of Pairs.
It was surprising that he was a SHE,
The root cause were:
1. Ulcers in Gizzard. (I physically looked her Gizzard there were lot of redness in the gizzard inner skin)
2. Kidney Inflamed.
1. Undigested food in stomach.
2. Intestine almost empty.
3. Lungs Normal.
4. Liver Slightly Darken.
The doctor recommended not to perform culture after death because body started to decompose and the bacteria formation will increase rapidly so the main bacteria causing the problem will be almost very difficult to find.
I think, I killed my cockatiel by giving her a dose of tetracycline, which actually caused her death
I went to the shop-owner and he told me that he frequently uses very small doses of tetracycline to all the birds, even the main dealer in the main city uses tetracycline before handing the birds to the re-sellers and uses tetracycline in very small dose and he also suggests them to use it.
I will never experiment any antibiotic on these exotic birds in future.
I was very disappointed after coming to home, but soon I am happy also..
...Continued in the New Thread.. Death is not an end but Life is a new beginning.
Before ending this post I want to thank EllenD, tielfan & other poster for helping me and educating me the proper process for keeping these exotic birds healthy.