The Wilson Bulletin
Threlfall, William and Mahoney, Shane P.

The Common Murre (Uria aalge) is a widely distributed species that shows substantial geographic variation based on coloration and measurements. The subspecies occurring in Newfoundland is U. aalge aalge. When studying the breeding biology of any species, the ability of investigators to distinguish between adult males and females obviously gives a more complete picture of the roles played by each sex. When a breeding biology and behavioral study of Common Murres was started in 1977 in Newfoundland, efforts were made (using data collected during an earlier study of the helminth parasites of auks conducted there) to determine whether or not the murres could be sexed using only merisitic characters. Birkhead was able to sex some birds after observing copulation and noted that males had larger beaks than females.