Structural-functional correlations in the kidneys and observations of colon and cloacal morphology in certain Australian birds.

1. Variations in renal microstructure between the zebra finch and Senegal dove were consistent with their relative renal concentrating abilities (urine/plasma ratios of 2-8 and 1-7, respectively). Compared with dove kidneys, those of the finch contained a higher fraction of mammalian-type nephrons (with Henle's loops), and a lower fraction of reptilian-type nephrons (without loops). 2. Singing honeyeaters concentrated their urine almost as well as zebra finches, although honeyeater kidneys were less specialized (fewer mammalian-type nephrons). Such findings emphasize the need to clarify other osmoregulatory parameters. 3. No significant microstructural differences were found in the kidneys of domesticated as compared with those of wild zebra finches. Hence, osmoregulatory differences between tame and wild birds must be related to physiological factors rather than morphological. 4. Thickness of the renal medulla seemed to be directly correlated with urine concentrating ability. However, certain inconsistencies obscure this relationship such that its resolution will require further research. 5. Histological features of the mucosae of the colon and cloaca are described. The galah and kookaburra displayed a mammalian (non-villous) pattern of mucosal organization. Zebra finches, singing honeyeaters, and particularly emus, possessed colonic and cloacal villi and hence an increased surface area per volume in this region of the gut. This raises the possibility that the colon and cloaca are involved in uring concentration and osmoregulatory activities in these species.