Abstract

The ratite order Apterygiformes is the only group of birds in which: 1. Paired ovaries occur consistently without known exceptions. 2. The right ovary (as well as the left) is usually functional. 3. Successful ovulation from the right ovary (as well as the left) is a normal occurrence. 4. If more than one egg is laid during the same season, ovulation occurs alternately from the two ovaries. In addition the following points have also been established: 5. In the North Island Kiwi irregular time intervals between egg laying within one church are apparently owing to the widely differing stages of development of the succeeding follicles at the time of laying of the preceding egg. 6. Only the left oviduct is functional in Kiwis and vestigial right oviducts occur only rarely. 7. From the evidence obtained during this study on Kiwis it appears that the reduction of the right oviduct in birds might well have preceded the reduction of the right ovary during their evolutionary history. 8. Despite textbook statements paired ovaries in birds have been recorded in individual birds of at least 86 species belonging to 16 different orders and paired oviducts in birds have now been recorded in individuals of at least 34 species belonging to 13 different orders. 9. Although it has long been known that paired ovaries occurred commonly but irregularly in some members of the carinate order Falconiformes, species of this group form only 52% of the total number of bird species in which paired ovaries have now been recorded. If normal sexing routine in birds included the examination of the right side of the body cavity in addition to the usual examination of the left side, additional species (and additional orders) with paired ovaries might well be found.