The Olympic Gull

Gulls of one particular species will, if given the opportunity, hybridize with gulls from a different species. The literature is full of examples of intermediate forms between gull species and many of these intermediates occur regularly enough that they have been given their own vernacular names. Most of these hybrid forms occur at the boundaries between the ranges of two parent species. Overlaps between breeding boundaries are called zones of sympatry. One of the best known and best studied of these zones occurs in the Pacific Northwest where the breeding ranges of Western Gull and Glaucous-winged Gull overlap. The resulting hybrid is called the Olympic Gull.

A study done in 1996 by Douglas Bell showed that, in the region between the Columbia River and the tip of the Olympic Peninsula, birds showing some degree of introgression outnumbered birds of clean Western or Glaucous-wing phenotypes during the breeding season.

These hybrid individuals are just as fertile as their parents. Hybrids mate with Western Gulls. Hybrids mate with Glaucous-winged Gulls. Hybrids mate with hybrids. This creates a continuum of variation from dark to light. There is no one single phenotype that constitutes "Olympic Gull" and, from a purely statistical point of view, the average gull one sees during the breeding season on the Washington Coast is likely to be a hybrid - even the ones that are Western looking or Glaucous-winged looking. When I'm asked to identify these birds by folks from out of town I'll often say "that's a mostly Western Gull" or "that's a mostly Glaucous-winged Gull." On eBird I lump them as Western/Glaucous-winged.

Things change a bit in the winter when gulls disperse away from their breeding grounds. One can find strikingly obvious Glaucous-winged Gulls, presumably down from Alaska, or crisply dark Western Gulls, up from the south.

So, how should we sort them here? I have guidelines which you all are under no obligation to follow:
Glaucous-winged Gulls should be pale and the primary tips should be pretty close to the same shade of gray as the back, lighter in younger birds as the feathers wear.

Western Gull 3rd winter birds and adults should show black primaries, not gray, not slaty, just black. 2nd Winter birds should have a dark back and very dark, nearly black primaries. 1st winter birds? Unless they are really dark, I punt to Western/Glaucous-winged....

Posted on February 21, 2017 10:59 PM by mikepatterson