The free flying birds sample the natural world for us. Birds dropping from the sky have, in the past, warned of heavy metals like lead blowing free in dust from ore piles or mercury discharged into rivers. Then the raptors (hawks, kites and eagles) share our position at the top of the food chain. Whatever toxins are entering fish, land animal or plants that might also accumulate in our tissues will be found in these hunting birds.

While humans have viewed the soaring eagle as a symbol of might and power since the time of ancient Rome, they also do a much humbler job in nature. Anyone who has been around the northwestern rivers when the Pacific salmon make their run from the ocean to spawn and then die, will understand that a primary role of the iconic bald eagle is to serve as a sanitation worker. It’s a spectacular sight, though the smell is not so great if you get too close.

Bald eagle numbers crashed in the first half of the 20th century due to lead poisoning from then current mining practices and, more recently, from the ingestion of lead fragments in discarded offal from shot deer. Responsible hunters now bury their leavings to protect both the eagles and the threatened California condor, the largest flying bird in North America. The impressive condor is also in the sanitation business and is, in fact, a species of vulture. Vultures are enormously important for environmental health.

A friend has an old photograph of her young father, an airline pilot, standing in front of a tree laden ominously with Indian gyps vultures. When I visited India a couple of years ago, I was aware of the occasional, circling kite but there were no vultures to be seen. Vulture populations in India and Pakistan have decreased more than 98% since the turn of the millennium. The problem hit suddenly like an epidemic infection, with masses of vultures drooping from the sky to lie dead on the ground. When local and international, from the USA and the UK, veterinary pathologists got onto the case, the insides of these dead birds were spectacularly white. The scientists concluded that the birds were dying of “visceral gout.”

Gout! Most of us will envisage an old man who has drunk far too much red wine over a lifetime and is suffering a painful foot that is carefully propped up on a cushion.  The cause: the accumulation of insoluble white uric acid crystals in his tissues. While mammals don’t normally make much uric acid and get rid of excess nitrogen as soluble urea in urine, birds excrete uric acid to give the white color of their droppings. What the veterinarians concluded from the visceral gout diagnosis was that something had destroyed vulture kidneys to allow the accumulation of massive amounts of uric acid throughout the body.  They looked for a virus or some other infectious agent but found nothing.

Then they reasoned that, as the primary job of vultures is to strip the meat from dead animals, it could be something they were ingesting. Veterinary clinicians had recently started to use the non-steroidal anti-inflammatory Diclofenac to treat cows with sore muscles or joints. These are perfectly good drugs for us and for cattle, but even minute amounts of Diclofenac in the tissues of one dead cow can destroy the kidneys of hundreds of vultures.

With fewer vultures to clear fallen animals from the environment, the numbers of wild dogs grew rapidly and there was a major increase in the number of human rabies deaths. The Parsee, who dismember the bodies of their dead and leave them on stone “tables of silence” for the bones to be picked clean by vultures could no longer follow their tradition and have, in fact, been breeding birds for this purpose.

These are grim stories, but they tell us that putting masses of chemicals into the environment is not without consequences. The effects may not be as obvious and immediate as the two cases discussed here, but we should be in no doubt concerning the importance of monitoring the birds, the most accessible of all wildlife.  Watching the birds lifts our spirits and delights the senses. We have a “duty of care” to ensure their, and our, long-term well being.

Though Peter Doherty is principally know for contributions to biomedical research, he started his professional life working as a veterinary pathologist and is the only veterinarian to have been awarded a Nobel Prize. He is the author of Their Fate is our Fate: How Birds Foretell Threats to our Health and our World and Pandemics: what Everyone Needs to Know.