Does Size Matter - Thayer's Gull Part 2

Before getting down to the size issue I wanted to share some thoughts on the Thayer-Kumlien's-Iceland Gull taxonomy question.

My understanding is that the most prevalent school of thought among Canadian ornithologist (scientists who actually base their theories on evidence rather than casual observations) is that this is one widely variable species.  Since I am not an ornithologist but a birder I am entitled to draw conclusions based on my casual observations and feel no need to support these (however wild they may be) with any data or evidence.

 I just can't believe that Thayer's and Iceland Gull are one species.  I can however and do believe that Thayer's and Iceland Gull are distinct species that hybridize readily and these hybrids form a large swarm which we currently refer to as Kumlien's Gull. This seems very similar to what is evolving now with the Western-Olympic-Glaucous-winged Gull complex. Maybe 1500 years ago the Thayer's-Iceland Gulls were at this same point. The single species concept just doesn't make sense to me. The incredible variability of Kumlien's in all features and the seeming randomness of these features (ie a dark eye in a very petite, finely structured bird) to me seems in keeping with a hybrid swarm more so than a true cline one would expect in a single species. Just my amateur two cents worth.

Adult Thayers/Kulmliens Gull "intergrade"

Thayer's Gull is  thought of by many as a relatively finely structured gull with a rounder head and finer bill than the Herring. Indeed the females are usually structured in this fashion. This however is not a diagnostic field mark at all. The male Thayer's can be quite robust and to my eye are near indistinguishable from the Herring Gull by structure. In fact many male Thayer's are much more robust and "Herring Gull-like" in structure than the female Herrings. In the field I look closely at all the round headed petite look birds with most of them ending up being Herrings. The bottom line is the structure is a good way to narrow down the field but it is far from diagnostic.

Juvenile Thayer's Gull

Juvenile Thayer's Gull

Juvenile Thayer's Gull

Juvenile Thayer's Gull

The above Thayer's Gull are almost certainly males and are quite robust. I think the only real consistent structural difference between these birds and many Herring Gulls would be a proportionately slightly smaller bill. The top two birds are especially dark and stalky. To me these very dark birds are quite distinctive and I wonder if they originate from a specific area of the Arctic or are they just randomly distributed in the population?  Below are several pictures of a fairly typical "dark type" Thayer's Gull note the flat head, relatively robust bill and deep chest -hardly petite. I included an open-winged shot at the end in case anyone was thinking this was a Herring Gull.

Juvenile Thayer's Gull "dark type"

Juvenile Thayer's Gull "dark type"

Juvenile Thayer's Gull "dark type"

Juvenile Thayer's Gull "dark type"

The larger robust real dark Thayer's Gulls are not what I would call rare here on eastern Lake Superior. I would guesstimate they may make up  1:20  Thayer's  Gulls seen, but it seems to vary year to year with more over the last few years.

The female Thayer's Gull certainly leans much more towards Iceland/Kumlien's Gull structurally. The head is often very rounded and dove-like with a finer bill. Some males certainly may share these structural traits. Since this is the sole way (that I am aware of) of differentiating sex in the field - there is really no "gold standard"  to determine the accuracy of this trait in assigning sex.  The bill measurements (in mm) in "Olsen & Larsson" are quite helpful in addressing this issue

Bill

__________________________________________________________

Adult male                                  46.8-55.5     (50.5)                    

Adult female                               44.0-54.2     (47.8)

First-year male                            40.5-50.0     (45.8)

First year female                         38.3- 48.3    (42.8)

Bill Depth at Gonys
__________________________________________________________
Adult male                                  15.6-18.8     (17.5)
Adult female                               14.4-18.5     (15.8)
First-year male                            12.3-16.4     (14.3)
First-year female                           9.8-16.0     (13.5)


Bill Depth at Base
__________________________________________________________
Adult male                                 16.8-20.4       (18.4)
Adult female                              14.1-18.8       (16.6)
First-year male                           13.4-18.5       (15.8)
First year female                        10.9-16.6       (14.1)

Olsen & Larsson, Gulls of North America, Europe and Asia, 2004. p.-234


The most striking thing in these numbers is that first year birds have decidedly smaller bills then adult birds regardless of sex. If we use bill size as a marker for structure it would seem safe to say that robustly structured juvenile birds are probably males while more finely structured birds could be either sex. If we look at the adults the maximum dimensions vary by only .3, 1.3, and 1.6mm. Thus I think more robustly structured adults could be either sex. The variation in minimum bill size measurements between adult sexes are 1.2, 2.7, 2.8mm. Thus it might be reasonable to assume that the more finely structured adults are females.

So my three conclusions are such:

1.  Robustly structured juvenile Thayer's Gulls with larger bills are probably males.

Juvenile Thayer's Gull- male

2. Finely structured adult Thayer's Gulls with smaller bills are probably females.

Adult Thayer's Gull-female

3. Finely structured juvenile and robustly structured adult Thayer's Gulls can not be accurately sexed in the field.

Adult Thayer's Gull - male or female

Juvenile Thayer's Gull- male or female

I realize that given the sample numbers and the lack of any statistical analysis that this is pseudo-science at best (my conclusions not Olsen & Larrson's measurements). Given that I do this for fun and statistics is anything but - I accept the possibility that my conclusions fall a bit short of being "proven".

I have posted below a few more Thayer's gull pictures illustrating the vast variation in structure

Adult Thayer's Gull - robust structure

Second winter Thayer's Gull -robust structure

Juvenile Thayer's- robust structure

Juvenile Thayer's Gull- intermediate structure

Juvenile Thayer's Gull- fine structure

Juvenile Thayer's Gull- fine structure

Adult Thayer's Gull-fine structure

Worn second winter Thayer's Gull -very finely structured

The last example was taken in March near San Diego. I felt relatively happy it wasn't a Kumlien's Gull given the location but I concede it is a remote possibility. Someone mentioned to me that they had been reviewing Thayer's Gulls pictures on the net and that most were from the west coast and most of these appeared to be finely structured. I am curious if they see these robust dark Thayer's on the west coast? It seems quite possible that there are structural differences between the western and eastern wintering birds that may be representative of different populations.

I searched my collection for examples of very finely structured Herring Gulls which are not at all uncommon. I found two relatively good examples but these are certainly only mid way down the Herring Gull structural continuum and more delicate examples are not uncommon.

Adult Herring Gull-fine structure

Adult Herring Gull-fine structure & dark eye

The bird directly above is not only finely structured but it has a dark eye. So superficially it resembles a Thayer's Gull. However it can be definitively differentiated even in this picture by the obvious mirror on the underside of P10. The beginning of the black underwing at the proximal end of the mirror is just barely visible.

This bird is a good example of the importance of a basic understanding of the vast variability of many traits  (size, structure, eye colour, plumage) in large gulls.

Below are pictures of a bird I would call a "classic"adult Thayer's Gull in almost every way. The exception is of course the light eye. I have read that this feature is present in up to 10% of adult Thayer's Gull. My thought would be that in my experience a bright light eye like this is rare. Make sure to note the  all white undersides of P10 with no mirror. This is often quite visible when gulls are at rest as in the first picture. Also note the increased size of the white spots on the upper sides of the primaries a good clue when scanning a large group of gulls.

Adult Thayer's Gull -light eye

Adult Thayer's Gull -light eye

Adult Thayer's Gull -light eye

Adult Thayer's Gull -light eye

One final bird I would like to discuss. I have considered this bird a runt second winter Thayer's Gull up until now. This was based primarily on the fact that there is dark pigment on the tips of the primaries all the way into P5 which in adults has been used as a definitive field mark to differentiate this species from Kumlien's Gull. The dark eye and the relatively darkly pigmented primaries seemed to support Thayer's Gull.

Second winter Thayer's/Kumlien's Gull

Second winter Thayer's/Kumlien's Gull

Second winter Thayer's/Kumlien's Gull-showing dark pigment on 6 primaries (P10-5)

I started to get a bit suspicious that my extrapolation of the "dark pigment to P5" field mark to less than adult birds may not be valid after this bird that I initially called as Thayer's based on this same criteria.

Second winter Thayer's/Kumlien's Gull

Second winter Thayer's/Kumlien's Gull-showing dark pigment on 6 primaries (P10-5)

This bird certainly seems to fit much better into the Kumlien's Gull mould than Thayer's. A light eye already, finely structured and lighter smokey gray primaries all seem more consistent with Kumlien's Gull. I decided to look back at definitive second winter Kumlien's to see if I could validate or refute the 

"dark pigment to P5" field mark.

Second winter Kumlien's Gull-showing dark pigment on 5 primaries (debatably 6) (P10-6)

Second winter Kumlien's Gull-showing dark pigment on 6 primaries (P10-5)

Second winter Kumlien's Gull-showing dark pigment on 6 primaries (P10-5)

Second winter Kumlien's Gull-showing dark pigment on 6 primaries (P10-5)

Second winter Kumlien's Gull-showing dark pigment on 6 primaries (debatably 6)(P10-5)

So at least 4 out of 6  (or 5 out of 7 if you include the bird just prior to this set) of the second winter Kumlien's Gulls definitively have dark pigment on 6 primaries in to P5. So it seems quite certain that one can not extrapolate this adult field mark. 

Below a second winter Thayer's for comparison.

Second winter Thayer's Gull with dark pigment on tips of 7 primaries (P10-4)

Most interesting at least this one second winter Thayer's Gull has dark pigment on 7 primaries (P10-4). Thinking maybe I was on to something  I looked to see if this was consistent. It wasn't. The following bird and at least one other quite definitive Thayer's Gull showed only pigment to P6.

Second winter Thayer's Gull with dark pigment on tips of 6 primaries (P10-5)

So bottom line it looks like either second year Thayer's or Kumlien's can have 6 pigmented primary tips with at least some Thayer's having 7. I am still unsure of the exact identification of the original bird but given the extremely petite structure on would really have to lean towards Kumlien's.

In conclusion size and structure in Thayer's Gulls is quite variable and may be misleading if one is not familiar with the full range of variability. Males are typically more robust at any age and there is a large overlap in these features with Herring Gull and as well with Kumlien's Gull at the other end of the spectrum. More definitive field marks including plumage characteristics are essential for definitive identification. Stay tuned for Part 3 of Does Size Matter? Lesser Black-backed Gull.

Thanks

Comments welcomed and appreciated.

Kirk Zufelt