Proportions of mammalian‐type and reptilian‐type nephrons in the kidneys of two passerine birds

We compared the proportions of mammalian‐type and reptilian‐type nephrons in the kidneys of two species of passerine birds. The desert house sparrow ( Passer domesticus ) is relatively well adapted for water conservation, whereas the white‐crowned sparrow ( Zonotrichia leucophrys ) is more mesic adapted. The two species do not differ in body mass, but the kidneys of P. domesticus are significantly smaller than those of Z. leucophrys . Associated with its smaller size, the house sparrow kidney has significantly fewer glomeruli (35,700 per kidney) than does the white‐crowned sparrow kidney (53,000 per kidney). The medullary cones, which contain the loops of Henle of the mammalian‐type nephrons, are significantly longer in house sparrows than in white‐crowned sparrows (2.2 vs. 1.9 mm). The number of medullary cones, the number of nephrons per medullary cone, and, hence, the number of mammalian‐type nephrons do not differ between the two species. The smaller number of nephrons in the kidney of the house sparrow therefore represents a smaller number of reptilian‐type nephrons. Desert house sparrows have 18% mammalian‐type nephrons, whereas white‐crowned sparrows have 10% mammaliantype nephrons. The relative reduction of reptilian‐type nephrons in P. domesticus may reduce the flow of dilute urine through the collecting ducts, thereby permitting a greater concentration gradient to be established along the length of the medullary cones.