MORPHOLOGY OF THE KIDNEY IN A NECTARIVOROUS BIRD, THE ANNAS HUMMINGBIRD CALYPTE ANNA

The kidneys of Anna's hummingbird (Calypte anna) differ in several sig nificant ways from those of other birds that have been examined. The k idneys of this nectarivore contain very little medullary tissue; 90% o f the total volume of the kidneys is cortical tissue, with medulla acc ounting for only an additional 2%. More than 99% of the nephrons are t he so-called 'reptilian type', which lack the loop of Henle. The few l ooped ('mammalian type') nephrons are incorporated into only a few med ullary cones per kidney. The loopless nephrons are similar to those of other birds. However, the looped nephrons differ in that they lack th e thin descending limb of the loop of Henle, which is found in other b irds and is thought to play an important role in the countercurrent mu ltiplier system in the avian kidney. Instead, the cells of the nephron segment following the pars recta of the proximal tubule resemble thos e of the thick ascending limb, with the large populations of mitochond ria that are typical of transporting epithelia and no reduction in cel l height. The absence of a descending thin limb in Anna's hummingbird is not necessarily a correlate of the hummingbird's liquid diet, becau se thin limbs have been documented in the kidneys of two other humming bird species, the rufous hummingbird (Selasphorus rufus) and the broad -tailed hummingbird (Selasphorus platycercus). The functional correlat es of the unique renal morphology in Anna's hummingbird warrant furthe r study.