Skuas: skull and ageing


Skuas have robust skulls that show similarities to the skulls of larger gulls. They have well developed depressions for the salt glands and bills are short compared with similar sized gulls and strongly hooked. Without identified material to compare with the skulls of the larger Southern Hemisphere species with overlapping distributions may be difficult to distinguish. Hospitaleche et al. (2009) worked out the differences in cranial and skeletal morphology between South Polar Skua Stercorarius mccormicki and Brown Skua Stercorarius lonnbergi. They found that differences are small but that there are several osteological characteristics that allow distinction between the two species. Chilean Skuas are the smallest of all and the northern Great Skua sits in between.

Supra-orbital ridge

In the large Catharacta Skuas fusion of the skull parts of a juvenile completes during the first winter. From then on they develop a supraorbital ridge during the next few years. Small bony outgrops start to grow from the lachrimals and frontal bone, eventually forming a bridge that becomes heavier with age.

October 1st CY. Fontanelles recently closed.  
Lachrimals and maxillary bones not fused yet (glued in this specimen)
Great skua Catharacta skua 2nd CY
March 2nd CY. All parts fused. Small outrcrop on right lachrimal.

1) # 205 043 001 2nd CY. All parts fused. 
Outcrops from lachrimals and frontals growing towards each other,
not yet connected.
Catharacts skua, Great Skua, skull, adult, supraorbital ridge
1) # 204 365 001 adult. Lachrimals and frontal connected by a broad 
bridge of bone, forming a supraorbital ridge.

1) Courtesey of Kees Camphuysen, NIOZ (Royal Netherlands Institute for Sea Research)