Morphology of the kidney in a nectarivorous bird, the Anna's hummingbird Calypte anna

The kidneys of Anna's hummingbird (Calypte anna) differ in several significant ways from those of other birds that have been examined. The kidneys of this nectarivore contain very little medullary tissue; 90% of the total volume of the kidneys is cortical tissue, with medulla accounting for only an additional 2%. More than 99% of the nephrons are the so-called 'reptilian type', which lack the loop of Henle. The few looped ('mammalian type') nephrons are incorporated into only a few medullary cones per kidney. The loopless nephrons are similar to those of other birds. However, the looped nephrons differ in that they lack the thin descending limb of the loop of Henle, which is found in other birds and is thought to play an important role in the countercurrent multiplier system in the avian kidney. Instead, the cells of the nephron segment following the pars recta of the proximal tubule resemble those of the thick ascending limb, with the large populations of mitochondria that are typical of transporting epithelia and no reduction in cell height. The absence of a descending thin limb in Anna's hummingbird is not necessarily a correlate of the hummingbird's liquid diet, because thin limbs have been documented in the kidneys of two other hummingbird species, the rufous hummingbird (Selasphorus rufus) and the broad-tailed hummingbird (Selasphorus platycercus). The functional correlates of the unique renal morphology in Anna's hummingbird warrant further study.