The head shape of Herring Gulls seems to change seasonally

    → Last Updated on

I always enjoy studying Herring Gulls, just looking at variation and trying to categorize the birds into age, sex, and regional groups. Last winter I was watching the gull flock at Turners Falls, MA, paying special attention to when the adults molted into breeding plumage. My thought was that the early-molting birds would be from nearby breeding populations (more southerly so that they would be nesting earlier) and that late-molting birds were more likely to be migrants from the Arctic (nesting later).

Herring Gulls at Turners Falls, MA on 23 Feb 2019. One streak-headed, in full nonbreeding plumage on the left, and one white-headed, finishing molt to breeding plumage on the right. Photo © David Sibley.

The study

This line of investigation proved to be very interesting, as I immediately saw a pattern. Early molting birds with all-white heads in February seemed sleeker and smaller-headed, and gave a subtle impression of a smoother and rounder head profile, with a peak over the eye and a sloping rear crown. Birds that still retained nonbreeding head streaking appeared to show a more angular and more bulky head, with flatter crown and squared-off rear crown. This was the clue I had been hoping to find, and I thought it must indicate a difference between an early nesting population of round-headed birds and a later nesting population of square-headed birds.

Encouraged, I continued to study, looking for other differences that could help distinguish these two “populations”. Over the ensuing weeks, however, I made no headway. Worse, as all of the gulls molted to breeding plumage, I continued to see the same pattern. White-headed birds appeared to have smaller, more rounded heads than the streak-headed birds. I never found a white-headed bird that stood out by having a squared-off, “blocky” head.

sketch of gull head shapes
A sketch, based on photos, showing the head shape of a Herring Gull in nonbreeding plumage (upper) and breeding plumage (lower). © David Sibley

The discovery

Gradually I came to suspect that an individual bird’s head shape might change as it molted to breeding plumage. This made some sense, as birds would need more insulation in winter and therefore might have more and longer feathers, which would produce a more bulky-looking head and neck.

It might have ended there, as an unverifiable hunch, except for the Appledore Island gull study and a dedicated photographer named Dave Adrien. For years, Herring Gulls in the Appledore Island colony have been marked with conspicuous leg bands, and Dave has been photographing these birds whenever he sees them, building up a valuable trove of photos of the same individual gulls at different ages and seasons. He was able to provide me with photos of several known individual Herring Gulls in both summer and winter. The head shapes in these photos match my impressions from the field, and reinforce my suspicion that head shape changes from summer to winter.

Two known individual Herring Gulls photographed in summer and in winter. Photos © Dave Adrien

What does this mean for gull identification?

Head shape is often stressed as an important field mark for gull species. I generally downplay it, because it is so variable and there is lots of overlap between species, but I still look for it as an important component of the overall gestalt of a gull. Discovering that head shape varies seasonally might seem like a setback in the ongoing struggle to identify gulls, but it’s actually a step forward. It can help by refining our impressions of head shape and allowing us to categorize and explain some of the variation.

It also raises more questions: Is this a real difference? Is it produced by different lengths of head feathers and can that be measured? Do other species of gulls show similar variation? It also suggests that the head shape of immature gulls could differ from adults (I’ve long assumed that but never gave it serious study). Does the head shape of immature gulls vary seasonally like adults?

Thanks to Dave Adrien for taking the photos and allowing me to use them. And thanks to the Appledore Island Gull Research team for their work.

Leave a Comment

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.