Razorbill: subspecies and ageing

Subspecies

Difference in size between islandica and torda/pica

The two presently recognized subspecies of Razorbill differ predominantly in size and to a lesser extent by the structure of the bill e.g. the depth of the bill and the number of grooves at the distal part of the bill.

skull alca torda torda skull alca torda islandica

Razorbill Alca torda (pica) torda. Ameland, The Netherlands
Culmen: 36.0 mm; Gonys: 22.0 mm; W+2.5;
Total 99.0 mm; unsexed adult

Razorbill Alca torda islandica. Ameland, The Netherlands
Culmen: 33.6 mm; Gonys: 20.4 mm; W+2 ;
Total: 90.7 mm; unsexed adult

The northern subspecies torda (and formerly pica) are on average larger than the southern islandica The number of grooves is also depending on the geographic origin. The large birds from the Baltic (A. t. torda) never have more than two grooves, but birds from higher latitudes (North Norway, North Russia, Greenland and Eastern Canada) show more often three grooves, and were seen as a separate subspecies A. t. pica in the past (Salomonsen, 1944). This subspecies is not recognized anymore, and is included in A. t. torda presently. Barret et al. (1997) state that it is virtually impossible to separate the islandica, torda and pica groups, despite the increasing measurements northwards and size differences among the different populations. There is too much overlap in measurements to define a clear separation. The Baltic birds form an exception to the Razorbill cline because this population appears to be larger than would be expected from their geographical position.

Development of the skull and the bill

Skull

Skull development in Razorbills follows the same pattern as in Uria 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.

Bill

During the first years of its life the bill of the Razorbill develops rather slowly to its mature state. In its first winter the bill is still slender and shows not yet the distinctive vertical white stripe. This stripe appears in the summer of its second year and becomes a bit wider with age. During the following years one grove appears at the proximal side of the white stripe and two to three vertical shallow grooves develop at the distal end of the bill, beyond the white band. In the mean time the bill becomes deeper. The number of grooves varies individually and also on the geographic origin (see above).

The white band and the grooves provide an indication of age of the younger birds, but in adult birds it is by no means an accurate measure. Old birds may have up to three grooves, but nine year old birds that were captured at their breeding grounds showed only one groove. (Camphuysen, 1995).
Rarely birds are found with two white grooves (Lavers et al 2005).

Alca torda islandica (ring Reykjavik C3294)
Adult, 10 y +, unsexed adult
* Ringed Iceland, 14-07-1996;  breeding adult
† Shot Faeroer, 10-12-2004
Bill: 2W+1,5  Culmen 31.3 mm, total: 88.2 mm
Courtesy of Jens-Kjeld Jensen, Faeroer



Like Puffins Fratercula Razorbills have heavier bills during the breeding season. They moult the proximal ridge of the ramphoteca, running from the top of the bill to the gape, after the breeding season. Winter-bills are therefore slightly thinner and smaller. (Salomonsen, 1944).

Stages of skull and bill development


2003 Juvenile: ca.4-5 months, unsexed October, incomplete fusion Bill: 0W+0

2003001 Juvenile: ca. 7 months, female, January, incomplete fusion. Bill: 0W+0

2003509 Juvenile: ca. 8 months, female, February, palate not fused. Bill: 0W+0

2003556 Juvenile: ca. 8 months, female, February, complete fusion. Bill: 0W+0

2003575 Immature: ca. 1,6 y, female, February, Bill: 1W+0

2003507 Immature: ca. 1,6 y, female, February. Bill: 1W+0

2003508 Immature: ca. 1,6 y, female, February. Bill: 1W+0

2003560 (ring BTO M88999) Adult: ca. 2,6 y, female, sor : 1,2 mm
R Helmsdale, Scotland, 5-2000; † Texel, The Netherlands, 2-2003. Bill: 1W+1

2003595 Adult: female, February. Bill: 1W+1,5

2003559 Adult, female, February. Bill:  1W+2

RSM 200.08.73 (ring BTO M49296) Adult: 10,5 y, unsexed, sor: 1.4 mm
* Great Saltee, 06-85; † South Beach, Tenby, Wales, 02-1996. Bill: no data

2003566 Adult: female, February. Bill: 1W+2,5

2003561. Adult: female, February. Bill: 1W+2RSM 200.08.01 (ring BTO M47012)
Adult, 17+ y, male, sor: 2.3 mm
R Great Saltee, 07-82 as adult; † Newgale, Wales, 03-1996. Bill: 1W+2

References


The majority of the skulls are from birds collected on the Dutch coast after the Tricolor oil spill in February 2003. Courtesy of Kees Camphuysen, NIOZ (Royal Netherlands Institute for Sea Research)
The RSM specimens are from the Sea Empress disaster in 1996. Courtesy of Bob McGowan, Royal Scottish Museum Edinburg.