Black-headed Gull size variation

(Update 11/27: Added more pictures below)

One noticeable feature on this weekend's Black-headed Gull was the fact that it was barely larger than the surrounding Bonaparte's Gulls, and thus was hard to pick out when it had its head tucked while sleeping. This prompted me to look into the size variation in the species.

Sibley lists the dimensions of the two species as follows:
Bonaparte's - L 13.5" WS 33" WT 190g
Black-headed - L 16" WS 40" WT 270g

The size difference is noticeable as stated and drawn, but Sibley adds the following caveat:

Variation in structure and plumage in these species, as in all other gulls, is dramatic. Careful study of flocks will reveal exceptionally large, small, light, and dark individuals that may cause confusion when seen singly.

This was readily apparent in the Bonaparte's flocks that we studied at length in close to shore. There were noticeably larger Bonaparte's that had me hoping for Black-headed or Kittiwake when they woke up. The Black-headed was just slightly larger than the large Bonaparte's, and the difference was only readily noticeable when the bird was active preening and feeding.

But just how much overlap, if any, is there between the two? For this I consulted the two gull guides: Grant, and Olsen & Larsson.

Dimensions from Grant:
Wing (W), Tail (T), Bill (B), Tarsus (Tr), in mm

Bonaparte's - W 246-271 T 99-108 B 27-32 Tr 33-37
Black-headed - W 280-315 T 104-124 B 30-37 Tr 42-47

According to these dimensions, there is minor overlap between the biggest Bonaparte's and the smallest Black-headed in bill and tail lengths, but no overlap in tarsus or wing length.

Olsen & Larsson provide a much more detailed analysis of size and weight:

Black-headed M/F adult
W 284-335 / 280-310
B 30.1-37.3 / 28.2-35.2
Tr 40.0-48.9 / 38.0-46.6

Wt 186-400g / 166-350g

Bonapartes M/F adult
W 250-278 / 246-275
B 26.9-33.2 / 23.7-31.6
Tr 32-37.9 / 32-37.9
Wt mean wintering adult weight - 176.8g
breeding adult male weight - 182-227g

From these data, we can tell the size extremes come vary close in the two species in wing length and tarsus, and overlap in bill length and weight. How does this analysis of size variation hold up in the real world? I submit to you two photos for comparison:

Here is a direct, side-by-side comparison of our Black-headed Gull: the Black-headed Gull (right, if you couldn't tell) appears hardly any bigger than the accompanying Bonaparte's. Indeed, because it is hunched up it almost appears to be the smaller bird. Compare to this image pulled from the web:

(Source: Icelandic Birding Pages)

I don't know the circumstances of this photo, but the birds appear very close together. Even given the distortion of the closer Bonaparte's appearing relatively larger, the Black-headed Gull is clearly the much larger bird!

Jim Pawlicki just sent me two more photos for comparison, of a Black-headed Gull that showed up on the Niagara River last summer. A larger bird in every dimension!

(Previous two photos by Jim Pawlicki)

We definitely observed a Black-headed Gull that was very much on the small end of the size ranges given for Black-headed Gull. It just goes to show that size is a finicky thing in identification, and analysis of all marks available for study is essential!



References:

David Allen Sibley. 2000. The Sibley Guide to Birds.
P J Grant. 1986. Gulls: A Guide to Identification. Second Edition.
Klaus Malling Olsen and Hans Larsson. 2004. Gulls of North America, Europe, and Asia.