Why do my pet birds keep dying!?! HELP?

Iv had a atiel who then flew away on my and it was terrible! so i decided to get another bird and he died mysteriously. I have good experience with birds and know how to tame and take care of them. Once he came into the house a day after he would show signs of sickness like sitting on the bottom of the cage and then would eventually die. So i got another one and one more and they all died on me. Could this be from cold weather its usually 60 degrees in my room. Could it be something in the air. I clean the cage so i just don't know!