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I've read that birds have shorter loops of Henle in their kidneys, and their primary strategy of water conservation is expelling nitrogenous waste in the form of uric acid.

However, the nephrons of birds have loops of Henle that extend less far into the medulla than those of mammals. Thus, bird kidneys cannot concentrate urine to the high osmolarities achieved by mammalian kidneys. Although birds can produce hyper-osmotic urine, their main water conservation adaptation is having uric acid as the nitrogenous waste molecule. (Campbell Biology, 11th edition)

However, I still can't quite understand how uric acid could make birds excrete white paste-like urine with less water than mammalian urine. I know that uric acid is not very soluble in water, and birds reabsorb some water in the cloaca, but this still doesn't answer my question.

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    $\begingroup$ I don't know, but perhaps a better question is, "How do birds conserve water?" Their excrement is paste-like because it is both urine and fecal matter, and if you've ever owned fowl, you'll know that their production of excrement is... well, nearly prodigious. $\endgroup$ Dec 7, 2023 at 14:01

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First off, I think it's helpful to go backward a step and figure out why an animal might make and concentrate urine in the first place. There's no usefulness in itself to having concentrated urine, it's just waste. Rather, for mammals (which, being mammals, we have a bit of a bias of thinking about the world from a mammal's perspective), urine concentration is part of the solution to the problem of needing to get rid of nitrogenous wastes: humans, like other mammals, flush soluble nitrogenous wastes into our kidneys with a bunch of water. This strategy creates a new problem of water loss, and the mammalian solution to this new problem is to collect a lot of that excreted water back so we can get the waste flushed out.

I think that the paragraph you quote is taking the mammal-centric position a bit too far and therefore the ordering of the statements in this paragraph is a bit misleading. I'll rewrite it using most of the same words but a different order. Though the meaning is the same I think it'll be easier to understand:

Although birds can produce hyper-osmotic urine, bird kidneys cannot concentrate urine to the high osmolarities achieved by mammalian kidneys. That's because the nephrons of birds have loops of Henle that extend less far into the medulla than those of mammals. Instead, their main water conservation adaptation is having uric acid as the nitrogenous waste molecule which they can excrete as a solid rather than flushing with water.

I think "hyper-osmotic urine" misleadingly suggests "really really high solute concentrations" because of the prefix "hyper", but in this context "hyper" just means "greater than (blood/plasma)", it doesn't actually say anything about the degree.

Orosz, S. E., & Echols, M. S. (2020). The urinary and osmoregulatory systems of birds. Veterinary clinics: Exotic animal practice, 23(1), 1-19. mentions birds concentrating urine to 2-3 times plasma.

In contrast, "high osmolarities achieved by mammalian kidneys" seems to be using "high" relative not just to plasma but also to what birds can achieve.

Nawata, C. M., & Pannabecker, T. L. (2018). Mammalian urine concentration: a review of renal medullary architecture and membrane transporters. Journal of comparative physiology B, 188, 899-918. mentions urine concentrations in mammals relative to plasma in the range of 4-30x depending on species.


Back to worrying about the mammal-centric thinking: there is a causal implication in both the original and my edited version that is not warranted, which is that the anatomy of the loops of Henle is somehow determining the nitrogen excretion strategy that birds use. It's equally reasonable to presume that birds don't need long loops of Henle since they evolved an alternative excretion mechanism that makes it unnecessary to concentrate urine to a great degree.

The "trick" of that alternative excretion mechanism is using uric acid and excreting a paste. Because uric acid crystalizes and comes out of solution, it doesn't "count" toward osmolarity of the urine, it's excreted as a solid. Birds don't need to remove water to crystalize uric acid, and don't need to use water to flush it out at all, it falls out of solution on its own because it's not very soluble. You could add a bunch of extra water and it would still stay mostly solid rather than dissolving.

So back to your original question: how do birds concentrate their urine? Well, pretty much the same way we do as far as the kidney goes. They also pull some water back in through the cloaca before urine is released, similar to how mammals pull water in from stool in the large intestine. The important difference is not how birds concentrate their urine (because urine concentration isn't a goal/need in itself), but rather how birds excrete nitrogen waste. By not flushing soluble nitrogenous waste with water in the first place, they don't have as much urine to concentrate, so it's less important for them to be very efficient in doing so.

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    $\begingroup$ Having just read some papers on this, I agree with all of this. The main takeaway seems to be 'hyperosmotic relative to plasma', which is easy because of excretion of larger amounts of NaCl. Also, it seems there are other water conserving machanisms birds use that mammals don't, including in the cloaca. Interestingly, there are entire books devoted to just this topic of avian osmoregulation. $\endgroup$ Dec 7, 2023 at 14:56
  • $\begingroup$ Thanks for your detailed answer! I would just like to ask how do birds reabsorb most of the water in the filtrate if they have shorter loops of Henle? If uric acid is not very soluble, and it does not count in determining osmolarity, would this mean that water would diffuse out of the nephron quicker, since the filtrate has higher water potential? $\endgroup$
    – Nibelung
    Dec 8, 2023 at 9:09
  • $\begingroup$ @Nibelung Are you looking for an answer for an assignment? Those questions are not bringing you closer to understanding. $\endgroup$
    – Bryan Krause
    Dec 8, 2023 at 13:00
  • $\begingroup$ No, I'm in high school and this is way deeper than what my teacher would give me. However, I would just like to know how bird kidneys reabsorb all that water in the filtrate. But I'm curious why are those questions useless? $\endgroup$
    – Nibelung
    Dec 8, 2023 at 23:59
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    $\begingroup$ @Nibelung Urine concentration isn't done for fun, it's done to avoid water loss specifically when you would otherwise need to use water to get rid of a lot of waste. Birds don't concentrate their urine any special way. They don't need to, because they don't need to flush a bunch of soluble nitrogenous wastes. They get rid of excess nitrogen by using a non-soluble, non-water-requiring method by converting it to a different chemical. $\endgroup$
    – Bryan Krause
    Dec 10, 2023 at 16:52
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Unlike mammals, birds excretory matter is not liquid but a semi-solid matter which is concentrated uric acid (this is not to be confused with feces which is the dark solid matter). Their urinary system (consisting of kidneys, lower gastrointestinal tract and nasal/orbital salt glands) is somewhat different that can conserve water and produce hyperosmotic urine:

Unique aspects in the avian (urinary) system include the presence of loopless and looped nephrons, lack of the thin ascending limb of Henle's loop, a corticomedullary osmotic gradient primarily consisting of NaCl without contribution of urea, and significant postrenal modification of final urine. The countercurrent multiplier mechanism operates between the descending and ascending limbs of Henle via recycling of a single solute (NaCl) with no water accompaniment, forming an osmotic gradient along the medullary cone.

—Urine concentration and avian aquaporin water channels

Birds do not have any urinary bladder, so they don't store water anywhere. Instead they have aquaporins. In the absence of the bladder, the renal output enters the lower gastrointestinal tract, where it is significantly modified. Birds consume prey that is high in salt content or intake fluids high in salinity. So, they have functional salt glands that are capable of eliminating excess ions, usually sodium chloride, and in the process produce free water. Aquaporins come into the picture and facilitate transfer of water between cells, so they get absorbed and not get excluded as waste matter.

You may find more details of the avian urinary system and the osmoregulation process in the references below:

  1. Nishimura H., Urine concentration and avian aquaporin water channels. Pflugers Arch. 2008 Jul;456(4):755-68. doi: 10.1007/s00424-008-0469-6
  2. Yimu Yang, Hiroko Nishimura, Bird aquaporins: Molecular machinery for urine concentration, Biochimica et Biophysica Acta (BBA) - Biomembranes, Volume 1863, Issue 10, 2021, DOI: 10.1016/j.bbamem.2021.183688
  3. Eldon J. Braun, Chapter 12 - Osmoregulatory Systems of Birds, Sturkie's Avian Physiology (Sixth Edition), Academic Press, 2015, DOI: 10.1016/B978-0-12-407160-5.00012-9
  4. https://avesbiology.com/bird_excretion.html
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