Why Flightless Birds are Condemned to Lay Eggs

Birds evolved very fast and are the only vertebrate group that never turned to ovoviviparity. While this is explainable as an evolutionary pressure for feathered flight and reduced size, their fast evolution apparently caused the disappearance of a significant proportion of genes. However posterior evolutionary trends of Aves to become flightless and with increased body sizes were not accompanied by an ‘expected’ turn to ovoviviparity, something known for a number of reptile taxa (e.g. some constrictor serpents). This impossibility is particularly noted in marine flightless birds from which penguins are certainly the best extant example having to endure seasonal extreme energy losses when coming ashore to lay eggs. This same limitation was most certainly the major cause for the extinction of the largest marine bird in the Northern Hemisphere – The Great Auk Pinguinus impennis. Here we discuss why ovoviviparism is probably an impossibility for birds while proposing lines of research that could prove, or disprove, our hypothetical and speculative view.