Relative thickness of the renal medulla in birds

The intraspecific mean length of medullary cones in avian kidneys is analogous to medullary thickness in the mammalian kidney. Hence, relative medullary thickness (based upon kidney volume) can be calculated for birds as was done in mammals years ago. Comparative figures are given for 26 species from nine avian orders. The organizational pattern of cortex and medulla in the bird kidney is reviewed, and a simplified diagram of this relationship is presented. With some exceptions, urine concentrating ability and relative medullary thickness are directly proportional in mammals. Contrarily, no similar trend was evident in birds when current information on water economy was compared to relative medullary thickness in various species. There are a number of factors (such as the respective functional roles of reptilian and mammalian-type nephrons, interspecific variations in ion transport, etc.) which require study before the significance of relative thickness in the avian medulla can be evaluated more thoroughly.