Common Guillemot: skull development and ageing

Guillemot or Common Murre Uria aalge aalge,
The Netherlands
Culmen:  51.0 mm; Total:  115,7 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 aalge aalge

Uria a. aalge 2004004 (BTO R03232; GrF37) Juvenile,  ca. 0,65 y, no sor, fusion complete
* Isle of May, Fife, Scotland, 6-2003
† Texel, The Netherlands, 1-2004
Culmen: 45,6 mm, total: 107,1 mm. 1)

Uria a. aalge RSM 2000.07.03 (BTO X41715)
Adult, ca. 1,7 y,  male, sor: 1,1 mm
* Isle of Canna, Highland, Scotland, 6-1994
† Swanlake, Pembrokeshire, Wales, 2-1996
Total: 107.3 mm. 2)

Uria a. aalge 1988001 (BM T12963)
Adult,  ca. 3,7 y, unsexed, sor: 1,3 mm
* Badbea, Scotland, 5-1984
† Vlieland, The Netherlands, 3-1988
Culmen: 47,4 mm, total: 116,3 mm.

Uria a. aalge 2004001 (BTO X54691)
Adult, ca. 4,8 y,  unsexed, sor: 1,2 mm
* Sumburgh, Shetlands, 5-1998
† Skagen, Denmark, 4-2003
Culmen: 47.0 mm, total: 110,8 mm. 3)

Uria a. aalge 1990016
Adult, unsexed
Culmen: 52.2 mm, total: 114,3

Uria a. aalge 1990017
Adult, unsexed
Culmen: 50.2 mm, total: 115.7 mm

Uria a. aalge (BTO T97375)
Adult, ca. 16 y,  unsexed, sor: 1,9 mm
* Isle of Canna, Highland, Scotland, 6-1991 (2y+)
† Vidoy, Faeroer, 10-2005
Culmen: 42.8  mm, total: 102 mm. 4)

Uria a. aalge 1990018
Adult,  unsexed
Culmen: 50.5 mm, total: 117.5 mm

Uria aalge albionis


Uria a. albionis RSM 2000.07.84 (BTO X12346) Immature, ca. 1,8 y,  female, no sor
* Sanda Island, Strathclyde, Scotland, 6-1994
† Manorbier, Pembrokeshire, Wales, 3-1996
Total: 96.7 mm. 2)

Uria a. albionis RSM 2000.07.01 (BM T31111) 2)
Adult, ca. 10,8 y,  female, sor: 2,1 mm
* Skomer Island, Pembrokeshire, Wales, 6-1985
† Freshwater East, Pembrokeshire, Wales, 2-1996
Total: 99.3 mm. 2)

1) Courtesy of C.J. Camphuysen, NIOZ (Royal Netherlands Institute for Sea Research), Texel ,The Netherlands
2) Courtesey of Bob McGowan, Royal Scottish Museum Edinburg
3) Courtesy of J.A. van Franeker, Alterra Research Institute, Texel, The Netherlands
4) Courtesey of Jens Kjeld Jensen, Faeroer