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COSEWIC Assessment and Update Status Report on the Ross’s Gull in Canada

Update
COSEWIC Status Report
on the
Ross’s Gull
Rhodostethia rosea
in Canada
2007

Species Information

Name and Classification

English name:
Ross’s Gull
French name:
Mouette rosee, Mouette de Ross
Scientific name:
Rhodostethia rosea
Other common names:
Rosy Gull (Burger and Gochfeld 1996)
Previous name:
Ross’ Gull
Classification:
   Class
Aves
   Order
Charadriiformes
   Suborder
Lari
   Family
Laridae
   Genus
Rhodostethia
   Species
rosea


This is a monotypic genus, and there are no known subspecies.

Taxonomy: Larus roseus MacGillivray, 1824: Melville Peninsula, Canada. The first specimen of a Ross’s Gull was collected by Sir James Clark Ross in 1823. Ross spent 15 years (1818 – 1833) in search of the Northwest Passage, sailing under Captains William Parry and John Ross. The Ross’s Gull remained an enigma until 1905, when the Russian explorer Sergius Buturlin discovered the nesting grounds along the Kolyma River in Siberia (Blomqvist and Elander 1981, Bechet et al. 2000).

Taxonomic relationships appear to be rather obscure. Ross’s Gull resembles the Little Gull (Larus minimus) in size and plumage sequence, but with a very different adult plumage. Morphometric analysis suggests that it is an early derivative of a primitive or a hooded larid (Burger and Gochfeld 1995).


Morphological Description

Ross’s Gull is a tern-like gull with a buoyant tern-like flight. Compared with other similar gulls, it has a unique combination of wedge-shaped tail, grey underwing and a black nuchal collar. Sexes are alike in size and plumage. In breeding plumage, much of the head and body takes on a rose colour strongest on the breast and belly. The rose colour fades with wear, but is still evident on adult birds into October. The narrow black collar completely encircles the rather dove-like head. In flight, the dark grey of the underwing coverts contrasts with a broad white trailing edge to the wing. Feet are red and the bill black. Non-breeding adults lack the black collar and the rosy hue, and develop a light grey crown, dark flecks around the eye, and a small black auricular spot. The immature plumage has black outer primaries and a broad black diagonal band across the inner wing, forming a broad white triangle on the rear wing, and a black tail band.

Birds in juvenile plumage are dusky brown overall, with white belly and light edges to dark back feathers, giving a scaly appearance. The cap is dark, throat and eyeline white, and tail white with black tips to elongated middle feathers. There is a conspicuous black bar across wings, making an "M" across the back visible in flight. A dark spot occurs behind the eye, and legs are flesh-brown. In first Winter (Basic I), the back is pale grey, head and underparts white, with a black spot behind and dark areas around the eye. The tail, upper wing and leg colour is similar to the juvenile plumage. First summer (Alternate I) plumagelooks like an adult with immature wings.

Ross’s Gulls measure 29 – 32 cm in length, 120 – 250 g in mass, and have a wingspan of 82 – 92 cm. In dimensions this species is most similar to Sabine’s Gull (Xema sabinii) and Bonaparte’s Gull (L. philadelphia).


Genetic Description

Royston et al. (2006) are using mitochondrial DNA sequences from ND4, ND4L, 12S and 16S to investigate the extent of gene flow amongst colonies in both Ross’s and Ivory (Pagophila eburnea) Gulls. Preliminary results indicate that Ross’s Gull genetic diversity appears to be higher than that of Ivory Gull.