Answer 1: This is one of those items of common knowledge that may have taken a small tidbit of info and exaggerated it. Some people will insist pigeons never poop while flying because feral pigeons (Columba livia) hold their feet against their body while flying. To poop while flying, they would have to poop on their feet, so they dont. Others insist that pigeons have to back up to poop and they cant do that while flying. Even people who keep racing pigeons, and therefore have quite a bit of experience with them, will insist that the birds dont poop while flying.
On the other hand, its not hard to find eye-witness reports of pigeons defacing cars as they fly overhead.
Dr. Daniel Haag-Wackernagel at the University of Basel in Switzerland, one of the worlds most knowledgeable experts on feral pigeons, told me that Normally pigeons discard feces not during flight, mostly when roosting or overnight. However, Dr. Haag-Wackernagel, who studies how cities can control pigeon populations, also said that Pigeons can excrete an evil-smelling scare-feces (undigested feces) when attacked by a bird of prey.
Also, because pigeons can fly for hours at a time without stopping, I suspect they must poop at least sometimes on long flights. The fact that they normally hold their feet in the way doesnt mean they cant move them long enough to poop while flying. So, it may very well be that pigeons prefer to poop on the ground, but they do sometimes poop in flight. And if a flock of 50 pigeons flies over your car it only takes a fraction of them to make a mess of your windshield.
Answer 2: Ricks observation is one that many city-dwellers can confirm. Look closely at the feet of a flock of feral pigeons and sometimes it seems that every other bird is missing toes or sometimes even missing a foot. Others have feet that are swollen and infected. What causes this? When I put this question to Dr. Haag-Wackernagel and to Dr. Lisa Jaquin, of McGill University in Montreal, who also studies feral pigeons (among other things), I got the same answer: fibers. Dr. Haag-Wackernagel explained what happens, Filaments twist around the feet and toes leading to insufficient blood supply, necrosis and loss of the toe. Sometimes the necrosis shows infections. In cities, a large proportion of the feral pigeons show such mutilation.
Dr. Jaquin said, We do pigeon captures regularly in Paris and we often see pigeons with tangled strings or hair in their swollen feet. The fibers can constrain blood circulation and result in necrosis and amputation. Bacterial infections can also cause leg deformity and abscess, but most of the amputations we see in cities have a physical cause (strings or hair caught in the toes).
I can attest to the ability of fibers to do damage to the feet of birds. I once kept pet zebra finches and when they wanted nesting material, I made the mistake of giving them strips of nylon yarn. A few weeks later, I noticed several had swollen feet and toes. When I got them in hand and looked closely, I realized, to my horror, that tiny fibers of nylon were completely tangled in the scales of their feet and as the poor birds tugged to try and remove the fibers, they just tightened the little tourniquets. It took me quite a long time with tweezers, tiny scissors and a magnifying glass to get all the fibers out, 
but once I did, the birds feet healed up back to normal in a couple of days. Needless to say, I never gave the birds that kind of yarn again.
Im not sure theres any practical way of reducing the toll that fibers in the environment take on the feet of feral pigeons aside from the efforts of people like Dr. Haag-Wackernagel to reduce the number of feral pigeons. But if you see a pigeon with damaged feet and can catch it, this website explains how to help remove the fibers. http://www.pigeonrescue.co.uk/feet.htm
As to the question of whether they are negatively affected by missing toes and feet
Missing a toe or two doesnt seem to stop them from foraging, flying or mating normally. Missing an entire foot seems more likely to affect a birds ability to walk efficiently and certainly to perch well. The fact that there are so many city pigeons missing feet, however, suggests that it isnt a fatal problem. City life with very few bird predators is not as much of a desperate struggle for survival. But I wouldnt be surprised if missing a foot affects the ability of males to compete for mates and to perform the physical act of standing on a females back to copulate.
More Information: All domestic, racing and feral pigeons are descendants of the rock dove, a bird native to a band across North Africa, Southern Europe, the Middle East and India. In its natural habitat, they nest on cliff faces, where presumably they rarely encounter man-made fibers and therefore have all their toes. APAMLAHarvardVancouverChicagoIEEEAsk a Naturalist® (March 28, 2024)
Dr. “We do pigeon captures in Paris on a regular basis, and we frequently see pigeons with tangled strings or hair in their swollen feet,” Jaquin stated. The fibers have the potential to impede blood flow, leading to necrosis and amputation. Although bacterial infections can also result in leg deformities and abscesses, the majority of amputations that occur in cities are caused by physical trauma (e.g., hair or strings stuck in the toes). .
Response 1: This is one of those common knowledge items that could have taken a little fact and overstated it. Due to the fact that feral pigeons (Columba livia) press their feet against their bodies when flying, some people will insist that pigeons never poop. They wouldn’t poop while flying because they would have to poop on their feet. Some contend that pigeons cannot poop while in flight because they must back up. Even those with extensive experience keeping racing pigeons will attest that the birds do not urinate while in flight.
Im not sure theres any practical way of reducing the toll that fibers in the environment take on the feet of feral pigeons aside from the efforts of people like Dr. Haag-Wackernagel to reduce the number of feral pigeons. But if you see a pigeon with damaged feet and can catch it, this website explains how to help remove the fibers. http://www.pigeonrescue.co.uk/feet.htm
They appear to be able to forage, fly, and mate normally even though they are missing one or two toes. A bird’s capacity to walk effectively and, most definitely, to perch well appears to be more likely to be impacted by losing one whole foot. However, the sheer number of city pigeons without feet indicates that the issue isn’t lethal. With so few avian predators, city life is less of a desperate struggle for survival. However, I wouldn’t be shocked if a male’s capacity to mate and carry out the physical act of standing on a female’s back to copulate is hampered by the loss of a foot.
Further Details: The rock dove, a bird that is indigenous to a region that includes North Africa, Southern Europe, the Middle East, and India, is the ancestor of all pigeons, domestic, racing, and feral. Since they build their nests on cliff faces, where they most likely don’t come into contact with synthetic fibers very often, all of their toes APAMLAHarvardVancouverChicagoIEEEAsk a Naturalist® (March 28, 2024).
According to what I’ve read, a city in California is going to start killing geese because they deposit roughly three pounds of manure every day, which is becoming a health risk. Man, thats a lot of poop from a small animal. Many of these birds would be saved if it were eating healthily and receiving some toilet training, wouldn’t that?
and many, many birds poop in flight. I catch birds, band them, and release them. I would say that about 80% of them can be let go after they take off. (Too many, who appear to have an endless supply, also do it while I’m holding them.) Maybe it’s a defense mechanism “aimed” at any possible predator that might be following (Odie, were you chasing those innocent gulls? Luckily, they weren’t cormorants). Maybe it’s to lighten the load. Theyd have barfed on you. Yuck). In any case, I’ve witnessed almost everyone doing it, including hawks, wrens, warblers, thrushes, and chickadees.
I discovered a fascinating (?) aspect of bird behavior: if a bird is perched and poops, he will quickly take off. I’ve seen more different species of birds urinate than I can recall; the worst was getting stuck beneath a group of gulls that had a large excavation. Their depth charges striking the pavement are still audible to me.
In a strange way, their ability to produce manure is impressive; wild geese eat mostly weeds, which pass through them like, uh, grass through a goose. Penalty of a high-fiber, low-nutrition diet. A domestic goose would probably have less need to scatter Toostie Roll-sized logs across the parking lot, er, barnyard, if it were fed cracked corn, game bird crumbles, and Purina Goose Chow.
By any accepted definition, a gooseI assume a Canada goose because I haven’t read the story in questionis not a “small animal.” The geese at my office complex are wandering the campus for unknown reasons, most likely related to the goslings growing up. It’s a rare morning when I don’t see six or seven adults and several goslings obstructing my path and growling at me (I hiss back; even though geese are not small, I do outweigh them by a significant margin).
FAQ
Can birds poop mid flight?
Can birds control when they poop?
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Does bird poop wash off in rain?