Julie, This one is tough, since the symptoms listed could be any number of things, but yes most all would or should have showed up in the necropsies performed by the labs. There are three things that come to mind
1- Ornithosis - it kills but does it slowly, and usually birds live with it but start dying under stress, like breeding, feeding young or in young their molt. Not really the number one choice for the cause but thought I'd state it, since it rarely shows up on lab tests or is even looked for.
number 2 and 3 on my guesses would be more plausible
2 and 3 - poisoning - and I state 2 and 3 because the two things that come to mind are some kind of agent, chemical or otherwise or a gas leak
I say this because I have seen both happen. I have seen people who treat for fleas using flea busters or dia/earth products for lice or mites, and they simply dehydrate the birds so far they die. and they react like you described. It and other products dry out things so extensively the birds simply suffocate over time and lose fluids. Other chemicals can have a similar affect. Next is gas. A very good friend had his master bath replaced. The plumber nicked the gas pipe in the wall. They never smelled it nor felt ill, and then he lost all the bird on the wall closest to the adjoining door. When the birds started to die on the other wall he noticed a strange smell. When the house was inspected, a testing company tested the house and found the gas levels at a point of sever danger to the family. Strong enough to kill if it continued or definitely strong enough to have blown the house down if any one had lit a flame of any sort. Now I have heard of other strange or similar events from different types of candles, air fresheners, plug ins etc. I've never seen it personally but have heard second hand of certain ones being killers of birds. I don't know if the labs would be testing for levels in a birds system from such things but maybe ask. Maybe also find a company to test the air quality of your house. Even certain methane gases can be present from sewer leaks, sometimes a toilet pipe, you wouldn't smell.
These are outside the box but all causes I've seen or heard about, and again all something a lab generally wouldn't test for unless directed to or informed to look in that direction. They tend to eliminate the most obvious and don't go much further unless prompted. Remember the canary in the coal mine, we wouldn't smell it, our alarm was the canary died which meant danger danger.
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