I seriously doubt anyone *laced the grapes wih poison.....
What happened to these birds isn't all that un-heard of....especially with wild birds.Diagnosis
As postmortem findings are usually not revealing, diagnosis is usually made by laboratory analysis. The most reliable diagnostic test is the determination of the acetylcholinesterase level in red blood cells, but it must be performed on fresh samples. Acetylcholinesterase levels can be determined on red blood cells, whole blood or plasma. The analysis which is usually used is the detection of organophosphate degradation products in the stomach contents and liver and kidney tissue. Analysis of brain tissue for decreased acetylcholinesterase levels is also good if done within a few days following death.
You can read the entire article here.... organophosphate poisoning