Brünnich's Guillemot: skull development and ageing

Brünnich's Guillemot Uria lomvia lomvia. Iceland
Culmen:36.3 mm; total: 99.4 mm; unsexed adult.


Skull development in Guillemots follows the same pattern as in Alca and Alle. During the first winter the separate bones of the juvenile skull fuse. This process will be more or less completed in spring of its second calendar year. By that time the development of the supraorbital ridge starts.

A fragile strip of bone is formed by the ossification of a ligament that runs above the eye between the the tip of the lachrimal bone towards the tip of the postorbital process and bony outcrops that are growing from the edge of the eyecase in between. These fragile bony structures meet each other during the second autumn and grow into a thin bony supraorbital ridge (sor) during its third calendar year. Between this ridge and the frontal bone a depression, the fossa glandula nasalis, contains the crescent shaped salt gland. At the distal end of this depression a foramen is left allowing passage to a duct from the salt gland to the nasal cavitiy (Schmidt-Nielsen 1960).
The development of de supraorbital ridge provides an indication of age up to the third calendar year. After that the ridge becomes thicker and heavier with age. The robustness of the ridge might be a measure of age. Further research on this is needed.

Stages of skull development

Uria lomvia RSM 2003.22.06
Unsexed immature

Uria lomvia RSM 1999.16.03
Unsexed adult

Uria lomvia RSM 1999.16.05
Adult female

Uria lomvia RSM 1999.16.01
Unsexed adult

Uria lomvia RSM 1999.16.09
Unsexed adult

Uria lomvia RSM 1999.16.07
Adult female

Specimens collected in Northern Iceland by R. Bjarkan
Courtesy of B. McGowan, Royal Scottish Museum, Edinburgh