The Great Auk (Pinguinus impennis) had two brood patches, not one: confirmation and implications

Since the late 1600s it has been assumed that the Great Auk Pinguinus impennis was similar to the Common Guillemot Uria aalge and Brünnich's Guillemot Uria lomvia in having a single, central brood patch. Through the examination of eight mounted museum specimens, we show that this is incorrect and that, like its closest relative the Razorbill Alca torda, the Great Auk had two lateral brood patches. We discuss how such misinformation persisted for so long. We also review the relationship between the number of brood patches and clutch size in the Alcidae. One implication of two brood patches is that the Great Auk would have incubated in a horizontal posture like the Razorbill, rather than in a semi‐upright posture like the Uria guillemots. Assuming that the Great Auk incubated like the Razorbill, it would probably have done so horizontally with its single egg pressed against one of the two lateral brood patches, positioned against the inside of one tarsus (and partially on the web of one foot), and with the wing on that side drooped to provide additional protection for the egg. Incubating in this way may have meant that the Great Auk's pyriform egg would have enabled it to use both level and sloping terrain, as in the Uria guillemots (but unlike the Razorbill). A horizontal incubation also has implications for estimates of their breeding density, which we estimate to have been around four pairs per square metre and, hence numbers on its largest known colony, Funk Island, Newfoundland (maximum 250 000 pairs).