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For an alternate Earth I am trying to develop a type of flying creature which poses an unusual threat model for the unaware explorers wandering in their habitat: when the creature is flying above 10 meters of height and does its business, its droppings can knockout a human who happens to be with their unprotected head on the path of the thing's free fall.

I know that blows to the head can rather easily send someone unconscious, but I am not sure that a creature which can achieve it with its droppings can also be able to fly.

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    $\begingroup$ Apparently many birds swallow stones to aid digestion, check gastroliths, dropping a stone from 10m might knock someone out. $\endgroup$
    – mwarren
    Apr 29 at 12:48
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    $\begingroup$ Does the knockout have to be from concussive force, or could some other mechanism (for example a chemical agent) be responsible? $\endgroup$
    – Joe Bloggs
    Apr 29 at 12:55
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    $\begingroup$ @JoeBloggs, if it can be "instantaneous" like a blow to the head it can be also other mechanism. $\endgroup$
    – L.Dutch
    Apr 29 at 12:56
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    $\begingroup$ Contrary to Hollywood it is actually quite difficult to knock someone unconscious. Especially for longer times (longer than a few seconds), which if it endures for minutes or longer is often already seen as a coma. Still curious what bird could drop something that heavy though. $\endgroup$
    – Trioxidane
    Apr 29 at 14:12
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    $\begingroup$ Just how many swallows (european or african) are needed to carry a coconut aloft? Because you would need something of that size to knock someone out by impact. $\endgroup$
    – PcMan
    Apr 29 at 14:51
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The ‘knockout’ is really caused by a skin-contact neurotoxin.

This bird has quite an effective self defence strategy. When startled it flies away while simultaneously doing it’s business (as many bird species do). It’s faeces, however, contains a neurotoxin which can cause a range of effects from sudden death (in small predators) up to mere unconsciousness or partial paralysis (in humans and the like). The bird’s cloaca is designed such that a panicked flight will spray this paralytic agent behind the bird as they flee.

The effects are generally temporary, but an unwary explorer that wipes away a blob of bird droppings will find themselves waking up sometime later with a numb hand and a powerful headache.

Explorers unlucky enough to startle a flock of these birds usually die when their diaphragm stops moving.

I’d expect these birds (certainly the males) to exhibit aposematism and have highly specialised predators.

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    $\begingroup$ The definition of a bad day: being seated in a bench park after getting out of a failed job interview, when suddenly a bird poops on your head from above. Boom, you're frustrated, dirty AND paralyzed on the ground. $\endgroup$ Apr 29 at 13:06
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    $\begingroup$ Record scratch “So you’re probably wondering why I’m lying in the park, wearing a suit and covered in bird... poo. Lemme tell ya: Today has not been my day” $\endgroup$
    – Joe Bloggs
    Apr 29 at 13:13
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    $\begingroup$ Maybe not knockout, but a Gimpie Gimpie bird might do the trick. It's normally a tree. Although not described in the following article, it is reported to feel like you've been short and set on fire with an acid at the same time. People even commit suicide to escape the pain. en.m.wikipedia.org/wiki/Dendrocnide_moroides $\endgroup$
    – Trioxidane
    Apr 29 at 14:09
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    $\begingroup$ There is an existing bird that does this, the Foo Bird of the Maldives, but the neurotoxin only has activity when the affected skin is subsequently exposed to air. The afflicted are therefore advised to leave the feces in place until they reach a clinic. Hence the local saying, "If the Foo sh*ts, wear it." $\endgroup$
    – jdunlop
    Apr 30 at 17:55
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    $\begingroup$ @jdunlop You need to be careful in the water, too, as any seal can plainly foo. $\endgroup$
    – PcMan
    May 1 at 17:41
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Aeschylus (c. 525/524 BC-456/455 BC) was a famous Greek playwright with an allegedly bizzare death.

In 458 BC, he returned to Sicily for the last time, visiting the city of Gela, where he died in 456 or 455 BC. Valerius Maximus wrote that he was killed outside the city by a tortoise dropped by an eagle (possibly a lammergeier or Cinereous vulture, which do open tortoises for eating by dropping them on hard objects[24]) which had mistaken his head for a rock suitable for shattering the shell.[25] Pliny, in his Naturalis Historiæ, adds that Aeschylus had been staying outdoors to avoid a prophecy that he would be killed by a falling object.[25] But this story may be legendary and due to a misunderstanding of the iconography on Aeschylus's tomb.[26]

https://en.wikipedia.org/wiki/Aeschylus#Death[1]

So if the fictional birds fly high, especially a lot higher than 10 meters, anything heavy and hard it drops could kill someone. I note there is not much difference between the force necessary to knock someone out and the forece necessary to kill someone.

I remember a grove of black walnut trees over 100 feet tall. I imagine that any walnuts which fell from the top during walnut season, or any branches which broke off from the top when it was windy, would hurt a lot if they hit anyone.

So manybe these birds eat some sort of large fruits which have large, indigestable seeds the size of black walnuts, and don't digest the seeds but pass them out in their poop. And maybe the birds have a way to aim and expell the seeds with great force.

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    $\begingroup$ “ And then the eagle lets go. And almost always the tortoise plunges to its death. Everyone knows why the tortoise does this. Gravity is a habit that is hard to shake off. No one knows why the eagle does this. There’s good eating on a tortoise but, considering the effort involved, there’s much better eating on practically anything else. It’s simply the delight of eagles to torment tortoises. But of course, what the eagle does not realize is that it is participating in a very crude form of natural selection. One day a tortoise will learn how to fly.” - Small Gods, Terry Pratchett $\endgroup$
    – Joe Bloggs
    Apr 29 at 19:03
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Unconsciousness process

Unfortunately this is too long to be a comment on the answer of M. A. Golding.

Impacts to the head, concussion and unconsciousness: As Trioxidane noted in a comment on the question, Hollywood gives a very misleading view of how blows to the head turn out. The first concept to look at is how a concussion is caused. In non-technical terms, it is the brain getting bounced around inside the skull. Due to the spine not compressing very well, this does not happen much as a result of blows from above that strike the top of the head, as PcMan related anecdotally. Impacts from this angle are most likely to have either minimal effect or fracture the skull and cause a serious and possibly fatal brain injury (but not necessarily immediate unconsciousness). (Fortunately PcMan suffered the former and not the latter.)

In order to maximise the chance of bouncing the brain around (ie causing a concussion), the head needs to be accelerated forwards, backwards or to the left or right. This article suggests that impacts from the side are more likely to cause a concussion than impacts from the front or back, although the data at the time was suggestive rather than conclusive. What this means is that if a bird's biowaste projectile is going to concuss rather than either annoy or kill someone, the bird needs to be flying very low and very quickly so that sideways rather than downwards force is delivered to the head.

(Note that even with ideal concussion-producing conditions, only 1 in 10 concussions results in noticeable loss of consciousness.)

The next part is looking at how much force is required and how it is delivered. Googling "speed of MMA punch" throws up a few results - I'm going to cherry pick a conversation in which an unnamed study apparently found 9.14 m/s and round up to 10 m/s. Let's say that the arm throwing the punch masses 4 kg (ignoring the body behind it), so we're looking at kinetic energy of 200 J.

10 m/s is a pretty puny airspeed but 4 kg is a massive amount of bird doo doo. Let's double the speed - giving 4x the kinetic energy, which brings the payload mass down to 1 kg. For a really big bird this is vaguely believable - the ostrich loses only 2.3 L/day of water through evaporation, urine and feces under harsh conditions, but that is enough for 1 kg fecal bombs. Assuming that the fecal matter has the same density of water and is spherical in shape it is roughly the same size as a fist, so there are no concerns about overpenetration.

There it is - an at-least-ostrich-sized bird with a decent horizontal airspeed (72 km/hr) that flies across rather than along paths (if any) to release big droppings. Without vast numbers of really big birds the risk of being struck would be very low, so presumably this is deliberate behaviour used to hunt or drive off intruders in their territory. There would need to be a reason for evolution to favour horizontal bombing runs over the more easily aimed dive bombing technique. Perhaps it is used to strike intruders who have taken cover under trees or rock overhangs.

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Your bird is simply huge.

The upper estimates for the Quetzalcoatlus's weight are around 250 kg. A beast that big might feasibly drop a pound of dropping at you from above. At terminal speed, and depending on what the beast ate, that would be like taking a coconut to the noggin'.

Mythical fauna has larger birds yet, such as the roc. The opening image in the wiki shows a roc, which looks like an eagle, carrying an elephant in flight. If such a bird decided to drop on your head, you'd be dead.

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  • $\begingroup$ That's precisely why I am asking if it can fly... $\endgroup$
    – L.Dutch
    Apr 30 at 17:40
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    $\begingroup$ @L.Dutch-ReinstateMonica the Quetzalcoatlus did fly. $\endgroup$ Apr 30 at 18:00
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Perhaps the bird eats minerals from cliff faces to aid in digestion(or maybe to get salt). The small rocks it eats get lumped in with the rest of its waste, and when it defecates it drops a part-rock part-waste bomb. Additionally, maybe the bird values water, so in its digestion it removes most of the water from its waste, further hardening and turning it into a rocky mortar perfect for breaking necks.

Finally, the bird wouldn’t have to be that large to have deadly poops. A bird a hundred pounds in weight, well under the historical record, could produce 2-3 pound poops. From 100 feet up, a rock-sludge ball of that weight could easily kill or maim.

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