The structure of the kidneys of sparrows living in wet and arid zones was compared to each other. A stereological study was conducted to quantify the components of the bird’s kidneys. A total of 10 female sparrows living in wet and arid zones were collected. The length of the kidneys was measured before blocking. The transverse serial sections were taken with a thickness of 5 μm. The sections were stained with haematoxylin and eosin (H & E) as well as periodic acid Schiff- alcian blue (PAS- AL). The kidneys of the sparrows consisted mostly of a cortex with only a small portion as medulla. The medulla was arranged in the form of cones with different lengths, which were randomly distributed within the kidney. The medullary nephron tubules were arranged sequentially. Thick and thin limbs of loop of Henle were separated by the collecting ducts. The histological structure and absolute volumes of the kidneys and components of the nephron were quantified. The absolute volumes of the structures within the right and left kidneys were not significantly different. The volume of the whole kidney (p<0.001) in the wet zone was larger than that in the arid zone. Furthermore, the volume proportion of the proximal tubule as well as cortex to whole kidney size was higher than that in wet zone birds. Whereas, the volume proportion of the medulla, distal tubule, cortical collecting tubule, medullary collecting duct and thin limb of loop of Henle to whole kidney size was higher than that in arid zone birds (p≤0.001). This feature indicates that the arid zone bird species had a more high ability to conserve water by producing concentrated urine.

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