The myth of pigeon s*&t

I’m always astonished by the number of people who don’t like pigeons. I wonder if they’ve ever looked closely at one. Past the seeming grey of their wings and to the lovely iridescence of their throats. The delicate feathers that run up their necks, their intelligent eyes. Their beaks are often a beautiful, rosy red.

But no, people hate them. They come in numbers. They are where they are not wanted (not that anyone “wanted” particular animals or not in a particular place). And perhaps most importantly, they do the one thing that makes many humans feel out of control, and generally at the butt-end of the universe’s displeasure:

They poop.

From the air. On buildings. On you if you stand still in the wrong place.

When I started studying pests, and why people hate them, I immediately came across many tales of the harms that animals we hate do to us. They spread disease, they eat your cat, they spoil your food and bite your children.

If they are pigeons, their poop is apparently so fowl (eh?) that it can eat away at STONE. Bye bye Sacre Coeur, eroded under a mountain of pigeon poo! No more Notre Dame, all those flying butts will be the end of flying buttresses!

I immediately envisioned beautiful sculptures, pitted and worn, faces wearing away as the pigeons pooed, triumphant, on their heads.

Then I remembered the Piazza San Marco in Venice. The Coliseum, the Vatican, Notre Dame, St. Pauls. They are not just sad, melted lumps of stone, even though pigeons flock there by the thousands.

Is it really true? The answer is no, but there’s lots of science behind both the origin of the myth, and its potential downfall.

The s*&t science (noun, not verb)

As I note in my book* pigeons excrete 9 and 28 pounds of waste per year . “Pigeons and a lot of birds have very large crops,” says Maggie Watson. She’s a conservation biologist who has studied the depredations of pigeons at Charles Sturt University in Australia. "They can stuff themselves and work through it, so it depends on where they’re going for their food.” But no matter what, a pigeon weighs less than a pound, so it could definitely end up buried in its own droppings. On the other hand…given the volume of our own species’ shits, so could we!

We of course, poop in buildings, not on them. And people preserving historical buildings can get very mad about it.

But scientists have long worried the effects could be more than ugly. Many historic and elaborately carved buildings are made of sandstone and limestone. These rocks are really piles of sand or the skeletons of coral and mollusks that have been piled up and smushed together over millions of years. Both limestone and sandstone have a lot of calcium carbonate (CaCO3). It makes up the shells of the coral and mollusks that died in service of limestone, and it’s also the chemical that— in smaller amounts— holds the tiny grains of sand in sandstone together.

Calcium carbonate seems tough, but it’s a wuss for a weak acid. This is why so many ancient buildings ended up sad and streaked from the effects of pollution and acid rain. But those aren’t the only sources of acid from the sky. Pigeons poop acid too.

Well, birds don’t poop or pee, really. They squirt them both out together in one single, unholy mix through their all-purpose rear cloaca. The result is slightly acidic. But this can be balanced out by other things in the bird’s diet, depending on the bird. So bird crap can be anywhere from pH 5 (slightly acidic) to pH 8 (slightly basic!).

And yet, it was a story I’d come across over and over in the historical literature on pigeons, especially with worries about historical building preservation. The pigeon poo might eat away at the stone! You had to do whatever you could to keep the stone clean and the pigeons off.

I’m not the only one to question this. Watson did too, and began to hunt in the literature….to find the good shit, as it were.

They found only three papers that looked at pigeon poop and building materials, and, Watson says, “if we looked at the methods they’re pretty dodgy.”

Thomas Adam and Peter Grübl, a pair of Germans with an animal rights group, filled silicone cups with pigeon poop and popped them on sandstone, granite, concrete, two kinds of brick, wood and metals. The samples were stored at close to 100% humidity and 89 degrees Fahrenheit to speed up the process. After 70 days, the scientists found some pretty disgusting-looking decay on the copper, steel and bronze.

Unfortunately, the study didn’t have a control. There was no sample where they balanced out the pH of the poo first, for example. All of the samples were kept without oxygen — a condition most pigeons won’t be pooping, or doing anything else, in.

In 2009, Elena Bernardi and a group of colleagues in Italy and England collected bird poop from a back yard, temptingly spread with birdseed. They blended up the results with water, and dropped it on to copper and bronze statuary, and again kept the samples in 100 percent humidity, this time at 77 degrees Fahrenheit. 15 days later, the metals were looking distinctly tarnished, But they also looked really moldy.

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Bernardi and colleagues even examined some real statuary, just in case. Two of the bronzes from Anglesey Abbey (Silenus and young Bacchus and the Versailles Diana) showed the presence of copper urates. This suggested that the uric acid in the environment might be reacting with the metal on the statures. But Watson notes that again, there were no controls, and spreading out birdseed and hoping doesn’t mean you’ll get pigeon poo specifically. Bernardi and her colleagues published their results in 2009 in Science of the Total Environment.

Finally, Adrian Vasiliu and Daniela Bruin, scientists at the University of Galati in Romania collected 5 kilograms of pigeon poo (proudly displayed in figure 5 of their 2010 paper in the International Journal of Conservation Science). They dried it, and in a true sign of scientific devotion, mixed up into a water slurry — by hand. Vasiliu and Buruiana then dunked in copper, bronze, brass and galvanized plate samples, and left them to sit for up to two months. Afterward, they noted the copper samples lost weight, and concluded that the poop smoothie had corroded the metal.

The proof is in the pooping

Unimpressed with the studies she reviewed. Watson and her colleagues decided to see for themselves. They began by feeding a group of five lucky captive pigeons a set of different diets. These were not pigeons caught from the sky, Watson explains, but rather pigeons she keeps in her back garden. Along with chickens, Watson keeps a few fancy pigeons—an impulse buy she gave into at a chicken sale. She’s got two archangel pigeons, gorgeous slender birds with a metallic purple sheen on their heads and elegant forest green backs. These fancier birds keep company with a few former racing birds that Watson found injured and nursed back to health. Together, they formed a willing set of subjects — eager to eat most of what Watson had to offer.

The birds got various diets, and the scientists waited to test the inevitable results. Usually a pigeon will digest food up and poop it out in between 5 and 8 hours. But Watson said they waited 24 hours before collection, “just to be sure.” The scientists soon found that a “normal” pigeon diet of grass seed produces bird poo with a pH of around 6.4, about the pH of human urine. Not so different after all.

When the pigeons were offered a diet of bread, French fries and other tasty snacks, however, the pH of their waste dropped to 5.58, which is about 10 times more acidic than before. True, 5.58 and 6.4 don’t seem that far apart. But pH is on a logarithmic scale. So a drop of 1 pH unit means a substance is 10 times more acidic. In this case, the bird BMs tanked from the pH of urine to a pH closer to the pH of human sweat.

The pH also changed over time. If the bird doo was allowed to settle for up to 22 days, it got less acidic, as bacteria and fungus fed on the fecal feast. So the good news is that the bird poo gets more basic, and less dangerous to any buildings. The bad news is that a fresh load of crap may be more than just ugly — it could help degrade the stonework.

What makes human food produce such acidic crap? Watson admits she’s just not sure. “When [they’re] eating human food…they’re eating tomatoes and things that have high salt content like chips and breads. Who knows what's actually happening inside. But that just is what came out the other end,” she says.

It didn’t help that no two pigeons pooped the same, even though they all ate the same type of diet at the same time. “Some of them would come out with these…solid green things,” she says. “[Others] would always have like these big, fluffy, gray things. [It’s] like, what are you doing?”

Watson and her colleagues also did their own tests with more poo, spreading it on different types of stone and wood. “We decided to collect some poo and put it on there and see what happens,” she says, “and nothing really happened.” They got some mold, a little tarnish on the right materials. Not much else. Watson and her colleagues excreted their own work in the International Journal of Building Pathology and Adaptation in 2017.

Pigeon crap: not any more acidic than sweat, and becomes basic in just a few days.** So what harm can a pile of pigeon poo really do? Watson says that many people who conserve historic buildings simply assume that the poop is eating up their stonework. But we’ve all heard what happens when you Ass U Me.

Watson blames what she called the “woozle effect, when you get citations of previous publications that lack evidence, but everybody cites it because the person before them cited it.” It’s the scientific equivalent of a game of telephone. Someone wrote they’d read a paper that showed pigeon poop corroded buildings, someone else copied them, and on and on down the line. And sure, leaving some poop on copper long enough does seem to cause corrosion. But how long does it take and under what conditions? Is pigeon poop any more harmful than air pollution or acid rain or people constantly rubbing parts of a statue for luck? There’s just no science for that.

And maybe even the concerns themselves are overblown! Many of the people actually running historic buildings are pretty blasé about the bird poo. Watson and her colleagues ran a survey that asked people working in historic buildings in Australia exactly what they thought of pigeons. “We thought there’d be a lot more hatred,” she says. “If you go into any of those sites those websites trying to sell you all this stuff, rats of the sky and all.”

The scientists asked 59 people who worked with historic buildings how they felt about bird poop. “Some were like ‘birds are bad,’” Watson says, “and others were like 'I’ve been here a while and I don’t know if the birds do anything.’” The historians ranked bird poop about as bad as vandalism, but nothing like as bad as dampness or earthquakes. One participant questioned whether birds were a problem, or just something that pest control thought should be a problem. The overall response was a collective shrug, which Watson and her colleagues published in the Journal of Cultural Heritage Management and Sustainable Development in 2018.

Poop is such an automatic gross out to so many people, especially when it seems to come uncontrollably from the sky. It’s easy to assume the results must be as physically harmful as they are psychologically (large amounts CAN be bad, pigeons do spread diseases to humans…though not with nearly as much frequency as humans do to other humans. We are disgusting creatures). But stone is not subject to human emotions. And it turns out, it probably doesn’t give a shit.

References:

1. Spennemann, Dirk H. R., and Maggie J. Watson. “Experimental Studies on the Impact of Bird Excreta on Architectural Metals.” APT Bulletin: The Journal of Preservation Technology, vol. 49, no. 1, 2018, pp. 19–28. JSTOR, https://www.jstor.org/stable/26452201. Accessed 6 June 2025.

2. Spennemann, Dirk HR, Pike, Melissa and Watson, Maggie J. "Behaviour of Pigeon Excreta on Masonry Surfaces " Restoration of Buildings and Monuments, vol. 23, no. 1, 2017, pp. 15-28. https://doi.org/10.1515/rbm-2017-0004

3. Spennemann, Dirk H.R. ; Pike, Melissa ; Watson, Maggie J. Effects of acid pigeon excreta on building conservation. In: International Journal of Building Pathology and Adaptation. 2017 ; Vol. 35, No. 1. pp. 2-15.

4. Spennemann, Dirk H. R. ; Watson, Maggie J. Dietary habits of urban pigeons (Columba livia) and implications of excreta pH – a review. In: European Journal of Ecology. 2017 ; Vol. 3, No. 1. pp. 27-41.

5. Vasiliu, Adrian and Daniela Laura Buruiană. “ARE BIRDS A MENACE TO OUTDOOR MONUMENTS ?” (2010).

6. Bernardi E, Bowden DJ, Brimblecombe P, Kenneally H, Morselli L. The effect of uric acid on outdoor copper and bronze. Science of the total environment. 2009;407(7):2383-9.

7. Thomas Adam and Peter Grübl, Einfluss von Taubenkot auf die Oberfläche von Baustoffen. Prüfungsbericht Nr. 195.04 vom 26.8.2004

Where have you been?

• This is from Feb but I think I need it running on repeat: "anxious humans often try to think about how other people can be more mature and step up to meet the moment. We become overinvolved with other people’s responses to challenges and less involved with our own."

• In search of “efficiency,” we may end up with salmonella. And E. Coli.

• Yes, your blood might indeed be full of microplastics. But with not much to do about it, is it worth a $150 test to find out?

• We've been telling smart students for YEARS that science is a way to make a difference. To have a good stable job that matters. Those students trained for years, they have deep knowledge that can't be replaced. Knowledge that would lead to cures and solutions. Now those students are out in the cold, and so is their work.

• Love this essay on AI slop: “It's so emblematic of the moment we're in, the Who Cares Era, where completely disposable things are shoddily produced for people to mostly ignore.”

• Extinction is not something you can ctrl+Z, and what Colossal is selling is a fantasy.

• America’s national parks are our glory, and the current administration wants to gut them, even though they make far, far more money than they spend!

• Apparently before latex, people made condoms out of sheep appendix, linen, and….turtle shell? As you might imagine, effective they were not.

• A great set of visualizations showing just how much science funding has been cut.

*Much of this is from a large folder I call my “dead darlings” which is where I trim stuff that didn’t fit or didn’t work from an article or book. And there just wasn’t room for….all this shit.

**I will admit I am now very curious to find out if human sweat can degrade masonry.