Thursday, October 7, 2010

Silver Herring Gulls - ? Visitors from the Arctic

Last weekend at the landfill I came across a couple of juvenile gulls that seemed to mock my assertion that differentiating juvenile Lesser Black-backed Gulls from Herring Gulls was a "no-brainer".

Well clearly its not a challenge to tell this bird.

Juvenile Lesser Black-backed Gull

From this bird.

Juvenile Herring Gull

Things get a bit trickier when you add this bird to the equation.

Juvenile "Silver" Herring Gull

Shortly after taking up position at the Landfill I spotted what I thought was probably a juvenile Lesser Black-backed. The light was very bright and harsh and in certain lights it looked very contrasting with the mantle feathers appearing grayish black. The bird was definitely not brownish with no warm tones. Depending on the light and view I went back and forth multiple times between Lesser Black-backed and Herring. Somethings about the bird just didn't seem right for a juvenile Lesser Black-backed. 

Juvenile "Silver" Herring Gull

First off the jizz (which can be appreciated in the above pic) seemed more Herring Gull like. It just didn't quite seem "black and white" enough and the underparts seemed uniform and Herring gull like. The bird goofed around in the trash for at least 15 minutes before it finally decided to flap its wings. I was ready.

Juvenile "Silver" Herring Gull

Well that cleared up the confusion very quickly. Large inner primary window unequivocally makes this a juvenile Herring Gull. It however is clearly of a different ilk than the standard juveniles I have been studying closely for the last several weeks. First of all the primaries were very black appearing contrasting very sharply with the inner primary window. The scapulars and tertials were narrowly rimmed with white seemingly very to a Lesser Black-backed. Most notably there was no apparent brown or warm colours to be found anywhere very atypical for the standard juvenile Herring. The bird was very much gray to black with a silvery sheen.

Comparison of first winter Herring Gull with "Silver" juvenile Herring Gull

You might argue that the above picture is misleading as the juvenile bird has much fresher plumage and is thus much more contrasting and the first winter bird is a bit sun bleached and thus browner and warmer looking. However most very young fresh juveniles that I have seen are still quite brown with lots of warm tones. Variation is obviously great but in my opinion these "Silver" birds are very distinct. The pictures below illustrate some of the variation in juvenile birds.

Very young juvenile Herring Gull-July 13th Sault Ste. Marie, ON

Juvenile Herring Gull

Juvenile Herring Gull

Juvenile Herring Gull

Juvenile Herring Gull

Juvenile "Silver" Herring Gull

The above photos nicely illustrate some of the variation in plumage in juvenile Herrings. All the birds except the "Silver" bird have distinctly warm and brown tones to there plumage.

I have been watching and photographing the juvenile Herrings this fall to try to understand the variation so it was clear to me that this bird was different. Shortly after my experience with this bird I spotted a very similar appearing juvenile bird. This bird was however very large and bulky and instantly recognizable as a Herring by structure. A third bird very similar to the first was subsequently noted.

It seemed apparent that a small group of these birds had just arrived to join the local gull flock. They all seemed very crisply turned out with quite fresh plumage. I have heard some gull enthusiasts talk about  "Arctic" Herring Gulls and it seems that they may be distinguishable in the field. After viewing these birds I wondered whether these might be of arctic origin. Their relatively young appearance would correlate with later nesting at high latitudes and an isolated population could well develop a distinctive plumage. I am speculating and would be very interested to hear from anyone who has insight into what appears to me to be a fairly distinct form.

Practically I think it would fairly easy to confuse the "Silver" juvenile Herrings with Lesser Black-backed especially in poor light or if open wing views are not obtained.

Juvenile Lesser Black-backed Gull

Juvenile "Silver" Herring Gull

The above photos illustrate how similar these two usually distinct species become in this circumstance. A few things I noted that help differentiate follow. First the Lesser Black-backed seems to have a white ground colour noticed best in the head. The "Silver" Herring seems to have a gray-silver ground colour. The Lesser Black-backed black feathers are a crisper black while the "Silver " Herring's black has a greyer tone. The white edging to the mantle feathers and even more so the tertials is fine and mostly unscalloped (although this trait is variable) in the Lesser Black-backed. The "Silver" Herring has more marbling in the distal tertials than would usually be found in a Lesser Black-backed.

Close up-scapulars,coverts and tertials- juvenile Lesser Black-backed Gull

Close up-scapulars, coverts and tertials-juvenile "Silver" Herring Gull

The top picture nicely illustartes the fine and unscalloped white edging to both the scapulars (the  feathers blown up by the wind) and the tertials as compared to the marbled terials and scalloped edging of the scapulars in the "Silver"Herring. 

The last differentiating point in a resting bird would be that  the Lesser Black-backed has a more streaky look to its head and underparts while the "Silver" Herring has a more uniform look. This is well illustrated below.

Juvenile Lesser Black-backed Gull showing streaky head and underparts

Juvenile "Silver" juvenile Herring Gull showing uniform underparts and head

Clearly these two birds can be separated easily and definitively if you get a good look at the open wing. The impressive inner primary window really jumps out on the "Silver" Herring Gulls.

I have never keyed in on this variation in the juvenile Herring Gulls previously. I suspect they have been present possibly in small numbers and I have just not noted their rather unique appearance. I am very keen to hear from anyone who has some insight into this interesting form of the remarkably complex American Herring Gull.

Kirk Zufelt

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