Slaty-backed Gull 3rd Winter Plumage [Jim Wilson Photo]
Recently in early January, a Slaty-backed Gull (Larus schistisagus) was seen near Saint John at the municipal waste disposal facility by Jim Wilson and others. That marked another very rare species reported from New Brunswick this year! This marks the first time this species has been seen in New Brunswick. This individual is far, far from its home range. We have had many rare species show up here in the past few months and this is another remarkable one. We have had the Steller's Sea Eagle, the Western Tanager, the Townsend's Solitaire, the Black-bellied Whistling Ducks, and Black Vultures which are all remarkable. The Slaty-backed Gull is a very remarkable sighting, on par with the Steller's Sea Eagle.
Please note that most information on this post came from the following: Gulls of the Americas, Howell, Steve N.G. and Dunn, Jon, Houghton Mifflin Company, 2007; from the CD, The Large Gulls of North America, Jon L. Dunn, 1997; and from a 4-day Wings Trip for Gulls led by Jon Dunn in 2007 where I saw and photographed an adult Slaty-backed Gull under tutelage of Jon Dunn and Bruce MacTavish.
The Slaty-backed Gull is an Asiatic gull of the northwestern Pacific. Its normal breeding range is coastal areas of northeastern Siberia including the Kamchatka peninsula and areas around the Bering Sea south to northern Japan where it nests on cliffs and rocky islands. It winters in southern Japan and as far south as Hong Kong. It sporadically wanders to the Aleutian Islands and occasionally to coastal Alaska. Vagrants have been recorded from the Pacific northwest coast of North America, Texas, and in the east from Ontario, Quebec and Newfoundland.
The Slaty-backed Gull is a large robust gull (64 cm long with a wingspan of 147 cm), bigger than most subspecies of the Herring Gull but smaller than the Great Black-backed Gull. Compared to the Herring Gull it appears thick chested and longer necked. One characteristic feature that is shown on all stages is the broad wing base. This species exhibits sexual dimorphism with the males sometimes being much larger than the females. The male has a larger, thicker bill.
The individual seen in Saint John is a 3W bird. In order to make the plumage discussion clearer I will begin with the adult plumage and then move to the 3W plumage. As advanced birders know, there is great variability in gulls and this species is no exception. Some individuals are slow moving into the expected plumage stage and others attain it early. Given that there are over 5 plumage stages with intermediate forms, you can see that gull identification is difficult.
Slaty-backed Gull in Adult Plumage [Bev Schneider Photo]
The most striking feature of the adult gull on first view is its dark mantle (back above the wings). Adults vary from dark gray to almost black. For those who know colours, this individual appeared a perfect deep Payne's Gray. One has to be careful of the lighting conditions when assessing the mantle colour. It varies in different conditions. The status of the paler-mantle adults is currently being hotly debated. Some believe they may be hybrids. Secondly, one notices the dark pink legs often described as 'bubble gum pink' by gull watchers. We don't have any gull species with these bright pink legs. The photo above does not show how pink those legs really are. Next, the winter plumage adult shows heavy streaking on the head, nape and throat which extends down onto the breast. This streaking is gray and often coalesces around the eye giving the bird almost a blurred dark eyeline. The dark around the eye on the bird shown above is not distinct. The adult also shows wide white primary/tertial and scapular crescents. These show up as distinctly wider than on Herring or Great Black-backed Gulls. The photo above shows the primary/tertial crescent clearly but only a hint of the scapular crescent. In the resting position one can often see the 'string of pearls' on the primary wing feathers which will be discussed in more detail later. The eyes are a clear pale yellow with a red orbital ring. The bill is a good identification feature. It is large, pale yellow, uniformly broad along its length, shows a very slight gonydial angle and red terminal spot. In flight the adult shows gray under the wings with indistinct wing tips. The wings show white on the dorsal leading edge and a broad white band on the posterior edges with the band being wider over the secondaries.
Slaty-backed Gull in Adult Plumage [Bev Schneider Photo]
The adult in the photo above shows the darker smudging around the eye, the even width of the bill along its length, the pink legs and the string of pearls. Also note the scapular and primary/tertial crescents.
The wing of the adult is very interesting. See the photo below. The gull wing has 10 primary feathers. The primary feathers are designated P10 to P1 starting in the outer aspect of the wing (see below).
Dorsal Wing Tip Feather Pattern [Photo from The Large Gulls of North America]
The feathers of the Slaty-backed Gull on the dorsal aspect are black on the leading edge and gray on the trailing edge. This makes the wing appear black on the dorsal aspect and gray underneath. The primaries show a small white spot on the tip, followed by a black sub-terminal band and then a white spot called a 'mirror'. Characteristically for this species, the mirrors on P10 and P9 are large. On some individuals the mirror on P9 is not large, as shown here. This feature makes the bird in flight easy to spot. That large P10 mirror is an identification feature. The mirror on P8 is often missing and the mirrors on P7, 6 and 5 are smaller. Also note that the way the mirrors are placed makes the black line on the wingtip crooked. These mirrors make the 'string of pearls' characteristic of this species.
Slaty-backed Gull 3rd Winter Plumage [Jim Wilson Photo]
See the photo at the beginning of this post and immediately above for a view of the 3W plumage. The 3W Slaty-backed Gull was seen at Saint John December 30, 2021 and then again the next day. There were about 1500 gulls present that day at the waste site on the ground and in the air. The Slaty-backed Gull was picked out on the ground and looked suspiciously different based on its darker mantle colour and shape. Studying the photos later the deep pink legs and the mirrors on the primary wing tips were noted. The photos were sent to the experts who confirmed the identification.
So what makes this a 3W Slaty-backed Gull? As noted above, the dark gray mantle, the deep pink legs, the large size and the overall shape (larger than a Herring Gull, long neck, thick chest). The 3W gull is developing into an adult plumage so one can see the beginnings of that. The gray streaking on the head is visible and this specimen shows the dark streaking coalesced around the eye. This looks to me like someone who is just recovering from the 'flu (or COVID). The 3W plumage shows light brown streaking on the breast and sometimes belly which is a holdover from the 2nd year plumage. The eye has usually turned a pale yellow which this bird shows. The bill is usually like the adult with a black subterminal spot or it may be pinkish with a black subterminal band. This bird seems to be in between. But note the bill is uniform in diameter along its length. Characteristically, the upper wing coverts are mixed slaty gray with brown. The scapular and tertial crescents are wide and very visible. The primary wing projection is visible but shows only a hint of the smaller (than adult) P10 mirror and the absent P9 mirror which would be expected for this plumage. The tail in this plumage shows a broad black terminal band which can be broken in the middle. The photos show that the dark band is present. The wing linings (seen from underneath) should be white and lightly marked with brown. This checks out as seen on the photo above. Also, note the broad white terminal band on the secondaries. This is seen both from the top and underneath the wing. The angle of this photo does not show the broad wing base.
Slaty-backed Gull Adult Winter Plumage (right) [Bev Schneider Photo]
The photo above on the right shows an adult winter plumage Slaty-backed Gull with a Glaucous Gull. Note the deep pink legs, heavy streaking on the head, nape, throat and upper breast, yellow eye, light yellow bill and dark gray mantle.
The Saint John individual has not been found in the last few days so it may turn up elsewhere. We need to study its key identification features so we will be prepared to spot and identify it when it shows up next. Good luck and happy winter gulling!
