Thickness of Common Murre (Uria aalge) eggshells in Atlantic Canada

reported values for eggshell thickness in Common Murre (Uria aalge) are few, and even fewer since the decline in use of organochlorine pesticides and other environmental pollutants that caused significant thinning of shells. The eggshells of Common Murres and Thick-billed Murres (Uria lomvia) are among the thickest and heaviest, proportionately, of any bird and this represents a non-trivial maternal investment. We measured the length and breadth of Common Murre eggs collected from Machias Seal island, new Brunswick, in 2006, and Gull island, newfoundland and labrador, in 2012, and we measured the thickness of the eggshells. Shell thickness was not related to egg size or volume, and it varied in individual eggs. The shells of Common Murre eggs from Machias Seal island (mean and standard deviation [SD] (0.767, SD 0.078 mm) and Gull island (0.753, SD 0.057 mm) were significantly thicker than any previously reported value and among the thickest of all birds. Such thickness is likely a result of nesting on rock substrate with no nesting material and, perhaps, high breeding densities.