Bird of the Day: Larus schistisagus!

Since Tuesday I've been enjoying looks at a life gull species right here in Ithaca: Slaty-backed Gull!

Tom Johnson and others have diligently documented this bird, first found by fellow Cornell student Jay McGowan (thanks again Jay!) right near campus. See photos with comparisons to the nearby Herring, Ring-billed, and Lesser Black-backed Gulls:

Photos by Tom Johnson, Mike Harvey, Kevin McGowan, and Tim Lenz

Photo by Tom Johnson

Interestingly enough, this bird has been seen so well that Tom has linked it to a bird in both Pennsylvania and Ottawa as very likely the same individual. There is this bird from Nov. 20, 2007 in Ottawa, Ontario. Tom reports on the Pennsylvania bird in an email to cayugabirds-l:

So... what follows may seem a bit bizarre.

I observed and poorly photographed a subadult Slaty-backed Gull in Pennsylvania over winter break that other observers and I thought appeared pale-mantled at the time (21 Dec 2007). I saw this bird again last Wednesday (16 Jan 2008) at the same location (Tullytown Landfill, Bucks County, PA), though during that observation I felt the mantle was a bit darker than at first. Anyway, I became somewhat suspicious yesterday looking at the Slaty-backed Gull Jay McGowan found at the Cornell compost piles (22 Jan 2008), and began comparing the photos taken of the two birds last night. While I am really shaking my head at the odds, I think that they are actually the same individual (it seems to me that the same individual vagrant gull was photographed in Ontario, Pennsylvania, and New York).

Here are some side-by-side composite images of the PA and Ithaca birds:
http://picasaweb.google.com/jaegermaster/SlatyBackedGullIthacaNY/photo#5158882077363298274
http://picasaweb.google.com/jaegermaster/SlatyBackedGullIthacaNY/photo#5158863445795168178

The quality of the PA photos is definitely not great (and there aren't any good spread wing photos), but if you compare the overall shape, structure, general shade, facial pattern, bill pattern, and pale markings on the wings, both birds appear quite similar. So the Ithaca and Ontario birds are pretty clearly the same individual, and the Pennsylvania bird also seems to be the same, though I'm not absolutely sure of that.

Tom has also carefully studied this bird with respect to identification and potential hybridization:

Photo by Tom Johnson

Everything is great for a subadult (likely 3rd cycle) Slaty-backed Gull with the possible exception of the relatively pale upperparts. I was able to directly compare the bird to an adult graellsii Lesser Black-backed Gull, and noted that the SBGU generally appeared lighter than the LBBG, though this varied. Occasionally the Lesser Black-backed Gull and Slaty-backed Gull appeared to have identically shaded mantles, but this varied with the angle at which the birds were standing relative to the light and me. Variation in upperparts shading has been a subject of debate and study for a while now, and two articles are particularly relevant (citations below).

The first is a 1994 article in Birding by Gustafson and Peterjohn that suggests wide variability in upperparts coloration in Slaty-backed Gull. However, in 1999, King and Carey suggested in Birder's Journal that the Gustafson/ Peterjohn assessment of variability was not accurate, and that overall, Slaty-backed Gull upperparts are somewhat uniform in coloration, with a small degree of variation ranging between the coloration of graellsii (lighter end) and intermedius (darker end) Lesser Black-backed Gull (Larus fuscus). They suggest that Slaty-backed Gulls with upperparts paler than graellsii should be examined for signs of hybridity and that the paler specimens examined for the Gustafson/ Peterjohn article likely represented hybrids. Slaty-backed Gulls have been found to hybridize with Glaucous-winged and Vega Gulls in northeastern Asia. Anyway, aside from pale upperparts, I found nothing on the Ithaca bird today consistent with a hybrid. The small apical spots on the outer primaries combined with dull, somewhat brownish greater and primary coverts and the dark bill suggest a third (or possibly) fourth cycle gull.

Here are the citations for those articles:

Gustafson, Mary E. and Peterjohn, Bruce G. 1994. Adult Slaty-backed Gulls: Variability in Mantle Color and Comments on Identification. Birding 26(4):243-249.

King, Jon R. and Carey, Geoff. 1999. Slaty-backed Gull hybridization and variation in adult upperparts colour. Birder's Journal 8(2):88-93.

Lastly, Tom has reported on this season's banner year for Slaty-backed Gull reports in the US. Here is his report to the North American Bird Distribution listserve:

Reports of Slaty-backed Gulls continue to trickle in from around the lower 48 US states and southern Canada this winter. I've collected as many reports/ photos as I could find and added them to a Google map here:

http://maps.google.com/maps/ms?ie=UTF8&hl=en&msa=0&msid=110169529319749975869.000442310e1bcdead2daa&ll=52.696361,-113.730469&spn=51.330654,164.53125&z=3&om=1

It is clearly an unprecedented year for SB Gull in the northeastern US - it seems interesting that in several cases in the east (last winter in New Hampshire, this winter in Massachusetts and Newfoundland), multiple SBGUs have shown up at the same time/ place. I imagine that this stems from the reports coming from the largest concentrations of gulls within the respective areas, but is still noteworthy as the states/ province where this has happened have very few records of the species (indeed, Massachusetts scored its first Slaty-backed Gulls only a few weeks ago, and 2 individuals were confirmed to be around Gloucester at the same time).

Very interesting... in terms of an "invasion", has there been any considerable growth/ expansion in Slaty-backed Gull populations recently? Several people have characterized Slaty-backed Gulls as turning into the new Lesser Black-backed Gulls (in respect to their expanding status in North America). The invasion of wintering Lesser Black-backed Gulls in North America coincided with a large population increase and expansion of breeding territory toward North America in that species; can anyone offer an explanation for the appearance of numbers of Slaty-backed Gulls in the lower 48 and Canada?

Also, while I think I've added all of the eastern US/ Canada sightings from this winter to the above map, I may have missed some from the west (especially California). Is there any indication of a repeat of last winter's Slaty-backed boom in California this year?

It has been wonderful to observe at close range this life gull species! It sure beats staring down into the gorge at swirling gulls at Niagara.

Big thanks to Tom for all his work in researching this bird and for letting me repost his work here.