Wayne R. Petersen
This issue’s mystery bird is a gull, a representative of a family about which some birders feel that all species are “mystery species.” While there is no question that the family Laridae is complex and often challenging where matters of field identification are concerned, larids also provide a spectrum of research opportunities in the fields of evolution, genetics, speciation, and taxonomy, as well as provide identification challenges to field birders.
A cursory look at the mystery bird reveals a hefty gull with a stout bill, a full-chested look, relatively long, pink legs (visible in the online version), and an overall pale or frosty appearance. This bird is clearly a large gull, not one of the smaller species such as Bonaparte’s Gull. Also of note is the seemingly anomalous presence of dark (blackish or brownish) primaries and what appears to be a dusky tail band visible below the bird’s left wing. Finally, the gull has a neat black tip to an otherwise pale bill. This combination of characters represents an example of the type of conundrum hinted at in the opening paragraph.
So, what are the issues and what are the options surrounding the identification of the mystery gull? First, the frosty appearance, robust size, head shape, and dark-tipped bill pattern of the mystery gull suggest the possibility that it could be a Glaucous Gull, except that it clearly has dark primaries and a seemingly dark tail band. Glaucous Gulls are classic white-winged gulls that typically have immaculately white primaries. Likewise, most Iceland Gulls occurring in Massachusetts and elsewhere on the northeastern coast of North America have pale or white wing tips, or else have at least some gray spotting or charcoal coloration in the primaries. However, there seems to be little question about the degree of darkness on the wing tips of the pictured gull. Where does this leave us?
The reader is now confronted with a classic mystery gull. Given the features at hand, there are two likely options. The bird might be abnormally frosty due to a genetic condition, or else it might be a hybrid. If the gull is a hybrid, the most likely possibility is a Glaucous x Herring cross. Such hybrids are fairly regular in areas where Glaucous Gulls and Herring Gulls coexist in Canada and elsewhere; when they hybridize, they are sometimes called Nelson’s Gulls, a name at one time applied to such hybrids occurring in the Bering Sea, which were then thought to be a distinct species. If the bird were expressing a genetic anomaly, it would most likely show either a form of leucism or schizochromism that would somehow give the bird an abnormally pale appearance to its body plumage, yet not affect the bill or wing tip coloration. Given these choices, hybridism is the more likely possibility, in which case the mystery gull is likely a cross between a Glaucous Gull and a Herring Gull. Regardless of its identity, such individuals are always interesting, even if they can’t always be definitively identified.
The author photographed this gull at Eastern Point, Gloucester, on February 7, 2009.
Wayne R. Petersen