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Do birds pee?

What are those milky white splatters – are they pee or poo?

For fans of superstition, being pooped on by a bird is a sign of good luck – although if you say that to someone covered in bird poo, they understandably may not feel particularly lucky in the moment.

But let’s take a closer look at that hypothetical bird excrement running down their shoulder, or perhaps a fresh globule that just landed in the middle of your car’s windscreen.

What is bird poo made of?

When our bodies need to get rid of excess nitrogen, our liver converts it into a soluble chemical compound called urea. Our kidneys collect this and dilute it, so that it leaves our body as watery urine.

Birds also need to remove excess nitrogen from their systems, but they don’t make urea. Instead, their livers convert nitrogen into uric acid. This is excreted as a white, paste-like substance.

But if you’ve ever looked closely at fresh bird droppings you may have spotted that it isn’t always completely white and has a lumpy liquid consistency. This is because birds also release their more solid faeces at the same time as their uric acid ‘pee’.

Most birds release uric acid and faeces simultaneously. Ostriches are the only birds known to release the two substances separately

Bird poo on a red car, showing the combination of uric acid and faeces.

Birds release their ‘poo’ and ‘pee’ at the same time. © DeawSS/ Shutterstock

Excreting waste as a thick substance means that birds don’t have to fly around carrying a heavy tank of urine. In fact, birds don’t actually have bladders to store pee in. Instead, uric acid moves from the kidneys down into a chamber of the cloaca – the single opening that's used for mating, laying eggs and excreting waste. It’s here that it combines with faeces that has come through the bird’s digestive system before all being released together.

This system allows birds to conserve the small amount of water that's in their bodies.

As for why bird poo seems to cement itself to your car or other surfaces – it comes down to chemical composition. Unlike urea, uric acid doesn’t break down very well in water. So, once it dries, you’re unlikely to have much success if you’re just using water to try and wash it away.

What is an owl pellet?

Not everything a bird eats comes out the other end. Things that can’t be digested may be regurgitated as a pellet, also known as a bolus.

Owl pellets often contain the fur and bones of their meals. You’ll usually find them in areas where the birds have been nesting – for example, at the base of trees, having been spat out by owls on branches higher up. 

A bird pellet filled with fur and the skull of a small animal.

Regurgitated pellets include the undigested parts of a bird’s food, such as bones and fur. © Dr Morley Read/ Shutterstock

Many types of birds produce pellets – it all depends on what they’ve been eating.

Other birds of prey produce pellets similar to owls’, but it is possible to tell them apart by size, shape, contents and location.

Depending on the species of bird that made them, pellets can also contain fish scales, shells, feathers, plant material or bits of insects. Non-edible items such as bird bands have also been found in pellets from where a tagged bird was eaten by a predator.

What do birds use poo for?

Pooping removes waste from a bird’s body. But some species have found additional uses for their leavings.

To prevent overheating, some birds intentionally defecate on themselves. They poo down the backs of their legs and as it evaporates, it has a cooling effect. This is known as urohidrosis. Storks, gannets, boobies and condors are among the birds that do this. 

Discover some of the other techniques birds use to keep themselves cool.

Some birds use their faeces in self-defence. Predators of fieldfares face an unpleasant reception when the little birds gather together to rain volleys of droppings down on them in a communal defence.

Hoopoe nestlings also use waste as a weapon, shooting a jet of poo and smelly liquid at threats to put them off.

Adult hoopoes don’t clear out their offsprings’ droppings, but many other birds are much more rigorous about nest sanitation. These chicks’ parents regularly empty the nest of faeces, helping to keep their young healthy and parasite-free. Some baby birds’ poop is surrounded by a mucous membrane called a faecal sac that allows their parents to pick it up and remove it more easily. 

An adult hoopoe in flight feeding it's young in a tree hole.

Young birds can be very vulnerable to predators. Hoopoe chicks have learned to use their excrement as a weapon. © WildMedia/ Shutterstock

Seabird colonies can be messy places. Penguins are known for their projectile poop, launching it away from themselves – one study found that the minimum ‘safe’ distance from a pooping penguin was 1.34 metres. By jetting out their poop, penguins may help keep themselves and their nest free from faeces, but they aren’t particularly careful about not pooping directly onto their neighbours.

Some other seabirds, such as guanay cormorants from the coasts of South America, build their whole nest from faeces.

Accumulations of seabird poop are known as guano and are really important to coastal and island habitats. Guano provides the ecosystem with vital nutrients that may otherwise be in short supply.

Since the Incan Empire, people have been using seabird guano as a fertiliser for crops. The guano trade boomed in the 1800s and contributed to wars that raged for almost two decades. Mining this material is thought to have contributed in part to the declines of some species, such as the Endangered cape gannets and cape cormorants, which are some of Africa’s main guano producers.

Today, guano mining isn’t the intensive industry it once was, with far more emphasis placed on the welfare of both birds and workers. The invention of chemical fertilisers reduced our reliance on bird poo. But in recent years these have been in limited supply and prices have risen in some places, and attention has been turning back to guano birds

Cormorants and pelicans standing on a rock covered in guano.

Guanay cormorant and Peruvian pelican guano has been used as a fertiliser since the Incan Empire, which began in the fifteenth century. © Jess Kraft/ Shutterstock

Animals masquerading as bird poop

When it’s not on you or your belongings, bird poop can be pretty inconspicuous. In fact, it’s so low profile that some other animals use it for their own ends.

Cyclosa ginnaga spiders

A spiders web with a conspicuous disk of silk at its centre.

The webs of Cyclosa ginnaga spiders have a conspicuous disk at their centre that looks a bit like a bird dropping. © topimages/ Shutterstock

The orb-web spider Cyclosa ginnaga includes a bold disk of white silk at the centre of its web. This, along with the spider’s size, shape and colour, could leave predators mistaking them for nothing but a bird poo. Scientists think this disguise may reduce how often they are attacked.

Bird dung spiders

A yellow, white and brown crab spider on a leaf holding two captured flies.

Bird dung spiders may use their looks and smell to lure in prey. © Pong Wira/ Shutterstock

Some spiders go the extra mile. The bird dung spider Phrynarachne ceylonica looks like a bird dropping and gives off a foul smell too.

These crab spiders may use their cunning camouflage to hide from predators and lure in potential prey. Some species love poop and actively feed on faeces – known as coprophagia – so the smell could be helping to attract the spider’s next meal.

Minixi suffusum wasps

The potter wasps Minixi suffusum from South America fool predators and parasitoids by covering their nests with fresh bird faeces. While adult wasps are armed with a sting, their young are defenceless, so a camouflaged home may help keep trouble away from their door.

Citrus swallowtail

A white and brown citrus swallowtail caterpillar on a leaf.

Did that bird poo just move? Young citrus swallowtail caterpillars look a lot like a bird dropping on a leaf. 

When they are little, the caterpillars of the citrus swallowtail butterfly are easily mistaken for a blob of bird poo. They also emit a foul smell if predators get too close. Citrus swallowtails can get away with their disguise when they’re small but as they get older, they change strategy. The caterpillars fade into a pale green as they get bigger and blend in with the leaves they live on instead.

Several other butterfly and moth caterpillars use similar bird poop-like colours and patterns to hide. Some even curve their bodies into dropping-like shapes to double-down on the ruse.

Find out more

Fascinated by our feathered friends? Book a ticket for our new exhibition Birds: Brilliant and Bizarre – the early bird catches the worm, as they say. Opens 24 May 2024.