Excretory System of Birds: Structure and Elements

He Excretory system of birds Is composed of kidneys, ureters and cloaca. All three are responsible for eliminating waste from the blood of these animals.

The kidneys are responsible for filtering the nitrogen and uric acid residues from the blood. These are sent through the ureters to one of the cloacal chambers, since they are expelled to the outside (EncyclopediaBritannica, 2013).

Excretory system, urinary system of birds. Excretory system of a bird. Photo retrieved from: people.eku.edu

If one of these three organs fails, the bird dies quickly because of blood poisoning by high levels of urea (Melissa Belliwski, 2017).

The main functions of the bird excretory system are: to maintain the electrolyte balance, to maintain the water balance and to eliminate residues of the metabolic process, in particular nitrogenous products such as uric acid.

Structure of the excretory system of birds

1- The Kidneys

The most important organs of the excretory system of birds are the kidneys. These are two reddish brown organs, each generally consisting of three lobes.

They are found behind the lungs and on each side of the spine of the birds. The kidneys have two thin, straight tubes connected in their mid-lateral part known as ureters (PoultryHub, 2017).

A kidney is made up of the renal cortex and the renal medulla. A microscopic examination of a dissected kidney shows how it is composed of a large number of renal tubules or Nephrons , Each of them divided into cortical and medullary parts.

Birds have two types of nephrons, similar to those found in mammals with a Henlewing (Used to help concentrate urine) found in the renal medulla, and other reptilian nephrons located in the renal cortex.

Nephrons have a duty to extract components of the urine from the blood that flows through the kidneys.

A nephron is composed of a complex network of capillaries contained by a capsule, called Bowman's capsule , In which the blood is directly filtered. It also has a spiral segment ranging from the Bowman Capsule to the Henna Asa (in the mammalian nephrons) and finally have a Distal tubule Which directs the urine to the ureters for subsequent removal of the body.

2- The Ureters

The ureters open and connect to the sewer, located adjacent to the vas deferens of the male or female oviduct. The ureters are connected internally to the kidneys through funnel-shaped structures in each of the lobes of the kidney.

They are conduits that are used to transport the urine directly to the sewer. Since birds do not have a bladder, the ureters must deposit the filtrate through the kidneys into the sewer chamber intended for storage (Kalhagen, 2017).

3- The Cloaca

The sewer is an organ located at the bottom of the digestive, excretory and reproductive systems of birds. It is used to expel faeces and lay eggs.

It is located at the back of the body, below the base of the tail of the birds and is covered by feathers at the lower end of the abdomen.

Birds have a single hole to expel faeces, urine and lay eggs. The cloaca is the organ that allows the execution of all these functions to the extent that bird needs it. Within it are multiple folds of skin and muscle that subdivide it into cameras suitable for different uses (Lovette & Fitzpatrick, 2016).

Bird feces are usually stored in one or several chambers of the sewer. Within it, the absorption of nutrients continues and solid and liquid wastes are mixed and excreted simultaneously once the bird's digestion concludes (MAYNTZ, 2017).

4- Urine

Unlike mammals and amphibians, birds generally do not have a bladder. Urine passes directly from the kidneys to the sewer through the ureters, from where it is transported by a Peristaltic movement To the intestine. There the excess water is reabsorbed before the disposal of the waste.

This process of water reabsorption in birds is similar to that in mammals. However, birds lack the ability to concentrate urine as efficiently as mammals can.

The urine of the birds in a thick paste with a low water content and a high content of uric acid, product of the nitrogen metabolism.

After being mixed in the sewer with solid waste, it is expelled from the bird's body in the form of white or creamy paste on the solid stool.

When the kidneys do not function efficiently or normally, and even when the bird has consumed foods rich in protein, uric acid can be concentrated in the blood in such a way that the excretory system is not able to eliminate it.

In these cases, nephrons tend to become inflamed with high concentrations of urea deposits and white lines appear on the surface of the kidneys. The accumulation of urea can lead to damage to kidney cells and to the eventual development of a nephritis .

Likewise, the high concentration of uric acid in the blood can result in the filtration of acid through the capillary walls, resulting in a disease known as visceral gout, characterized by whitish deposits on the surface of the viscera.

Comparison with the excretory system of other animals

The excretory system of the birds retains some similarities with that of the reptiles, in that both have cloaca and the urine is deposited in a semi-solid state creamy. However, the location, shape, and color of the organs composing both systems differs widely.

Apart from mammals, birds are the only vertebrates that can hold water in their bodies through an osmotic process of urine production. However, the ability of birds to concentrate urine is limited compared to that of mammals.


  1. (2013). Internal Organs. In EncyclopediaBritannica, The Nature of Birds (page 15). Sol 90.
  2. Kalhagen, A. (February 22, 2017). The Spruce. Obtained from Avian Anatomy 101: thespruce.com.
  3. Lovette, I.J., & Fitzpatrick, J.W. (2016). Urogenital System. In J. Lovette, & J. W. Fitzpatrick, Handbook of Bird Biology (page 196). Oxford: Wiley.
  4. MAYNTZ, M. (February 22, 2017). The Spruce. Retrieved from What Is a Bird's Cloaca?: thespruce.com.
  5. (2017). Cuteness. Retrieved from the Excretory System of Birds & Reptiles: cuteness.com.
  6. (February 1, 2017). Poultry Hub. Retrieved from Excretory system: poultryhub.org.

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