A cause is now known for bird deaths that eluded scientists for more than 25 years.

It is beyond dispute that there are human activities that have a known negative impact on the natural environment, but sometimes events come together, only in part arising from human activities, to create unknown environmental consequences.

During the winter of 1994, 29 bald eagles died at DeGray Lake in Arkansas, the largest undiagnosed mass mortality of bald eagles in the United States. More than 70 dead eagles were found over the next 2 years. There were no obvious signs that humans had played a role.

By 1998, the emerging cause was a disease that had a name, avian vacuolar myelinopathy (AVM) and had been confirmed at 10 sites across 6 states, a neurological disease, having been implicated in the deaths of more than 100 eagles and thousands of other birds.

But the source of AVM was not identified until a study published last Friday in Science.

After considerable effort, a team of scientists identified the cause of these bird deaths as an insidious combination of factors: An invasive plant introduced into lakes and reservoirs, an opportunistic previously unidentified cyanobacterium, exposed to bromide in those lakes and reservoirs, often (but not always) anthropogenic in origin, resulted in what scientists have now termed “aetokthonotoxin” (AETX), that bioaccumulates to kill bald eagles and other birds.

So, the scientists confirmed that AETX is the causative agent of AVM.

But knowing what killed the birds doesn’t really answer the question? It certainly does not rise to the level of a proximate cause. First, what was the source of the invasive water plant, hydrilla? Was it a discarded aquarium plant from a person emptying a household aquarium, or ..? Second did the cyanobacteria arrive in the leaves and stems of the hydrilla or was it already in the water bodies? Third, and possibly most significant because bromide availability is necessary to promote the toxin production, was the bromide naturally occurring, or could it be coming from human activities (e.g., government water treatment plants)?

So, now we know what is killing the bald eagles and other birds. The study published in Science is fascinating, but the environmental issue is complex and there appears no good solution to forestalling future avian deaths.

Of course the total numbers of AVM deaths pale by comparison to the numbers of birds killed each by housecats, the greatest threat to birds.

Public policy could discourage people from discarding home aquarium plants or for that matter having housecats, but each sound like a solution chasing a problem.