San Mateo County Birding Guide

Gull Hybrids and Subspecies in San Mateo County – Alvaro Jaramillo

In San Mateo County, we do not expect to see hybrid gulls during the breeding season. At that time the local breeders are Western Gull and California Gull. There may be some non-breeding species present, such as Ring-billed Gull on the Bayside. However, in the period between October and late March an abundance of northern breeding gulls visit us. The county is known as one of the best gull watching areas of the country, as it has a wide diversity and a number of rare gull records. For Slaty-backed Gull, Half Moon Bay may be the single spot that has had the most records over the years of any US spot south of Alaska! What confounds gull watching are the hybrid gulls that arrive as well as some rare subspecies. Here is a quick guide of where they come from and their general abundance:

Olympic Gull

"Olympic Gull" is the informal name that is given to the hybrids between Western and Glaucous-winged Gull. The main area where they come from is coastal Washington State, but they extend to northernmost Oregon. Because the two parental species are large bodied, and big billed, this hybrid also shares those characteristics. This is a common hybrid, but it is usually found at lesser numbers than either parental species in the local winter gull flocks. These birds come from a hybrid swarm, so they are not half way between the two parental species, but run the full spectrum from pale winged like a Glaucous-winged to black winged like a Western.

Olympic Gull
Olympic Gull (Western-x-Glaucous-winged Gull), adult, Half Moon Bay ( Alvaro Jaramillo) 1

Olympic Gull
Olympic Gull (Western-x-Glaucous-winged Gull), adult, Half Moon Bay ( Alvaro Jaramillo) 2

Olympic Gull
Olympic Gull (Western-x-Glaucous-winged Gull), adult, Half Moon Bay ( Alvaro Jaramillo) 3

Olympic Gull
Olympic Gull (Western-x-Glaucous-winged Gull), first cycle, Half Moon Bay ( Alvaro Jaramillo) 1

Olympic Gull
Olympic Gull (Western-x-Glaucous-winged Gull), first cycle, Half Moon Bay ( Alvaro Jaramillo) 2

Olympic Gull
Olympic Gull (Western-x-Glaucous-winged Gull), Seal Point Park, December 2018 (Donna Pomeroy)

 

Cook Inlet Gull

"Cook Inlet Gull" is the informal name for the Herring x Glaucous-winged Gull hybrids. They come mainly from Southeast Alaska, Cook Inlet in the Anchorage area is where they are best known. This is a very common hybrid, and it was unappreciated as being so until rather recently. On the coast, Cook Inlet Gulls are usually more common than "Thayer's" Gulls. It is frequent that Cook Inlet Gulls are as common or even more common than pure Herring Gull on our coast. This is the most common hybrid in our county. Again, they are a hybrid swarm, so any hybrid can be anywhere in the continuum between a Glaucous-winged and a Herring. Often they look like a Herring Gull, but show pale underwings, or dark eyes.

Cook Inlet Gull
Cook Inlet Gull (Herring-x-Glaucous-winged Gull), adult, Half Moon Bay (Alvaro Jaramillo) 1

Cook Inlet Gull
Cook Inlet Gull (Herring-x-Glaucous-winged Gull), adult, Half Moon Bay (Alvaro Jaramillo) 2

Cook Inlet Gull
Cook Inlet Gull (Herring-x-Glaucous-winged Gull), adult, Half Moon Bay (Alvaro Jaramillo) 3

Cook Inlet Gull
Cook Inlet Gull (Herring-x-Glaucous-winged Gull), first cycle, Half Moon Bay (Alvaro Jaramillo) 1

Cook Inlet Gull
Cook Inlet Gull (Herring-x-Glaucous-winged Gull), first cycle, Half Moon Bay (Alvaro Jaramillo) 2

 

Nelson's Gull

"Nelson's Gull" is the hybrid between Herring and Glaucous Gull. They are quite rare, and show up but a handful of time in a winter. Most of the individuals seen here are first cycle birds, rather than adults. These are hybrids that occur randomly in the Arctic, but are not a hybrid swarm. As such most look half way between the two parental species.

Nelson's Gull
Nelson's Gull (Herring-x-Glaucous Gull), Half Moon Bay (Alvaro Jaramillo)

Nelson's Gull
Nelson's Gull (Herring-x-Glaucous Gull), second cycle, Half Moon Bay (Alvaro Jaramillo)

 

Seward Gull

"Seward Gull" is the hybrid between Glaucous and Glaucous-winged Gull. This is a rare and little known hybrid. If it often not clear if one is seeing a hybrid, or a variation in one of the parental species. It is thought that these birds come mainly from the Seward Peninsula in Alaska. They are little known, and infrequently identified. It is possible that they are more common than what we currently believe, but still this is a rare hybrid.

Seward Gull
Seward Gull (Glaucous-x-Glaucous-winged Gull), second cycle, Half Moon Bay (Alvaro Jaramillo)

Seward Gull
Seward Gull (Glaucous-x-Glaucous-winged Gull), Gazos Creek Beach Access, Nov 2019 (Malia DeFelice)

 

Vega Gull

"Vega Gull" is not a hybrid. This is the Siberian subspecies of the Herring Gull. Most international taxonomic bodies consider this a different species than the Herring Gull. They are tricky to identify. Adult look like our Herring Gull but are darker above, and tend to have dark eyes. The first year birds usually show a white tail base, contrasting with a dark tail band as well as additional features. We think this species is annual in San Mateo county, but it is very rare. If this species is ever officially split in North America from the Herring Gull, once again Half Moon Bay would become one of the better places to find this rare Asian vagrant.

Vega Gull
Vega Gull (Siberian subspecies of Herring Gull, Larus argentatus vegae), Half Moon Bay (Alvaro Jaramillo)

Vega Gull
Vega Gull (Siberian subspecies of Herring Gull, Larus argentatus vegae), Half Moon Bay, (Alvaro Jaramillo) 2

 

Thayer's Gull

"Thayer's Gull" up until recently considered a species separate from Iceland Gull. The expected Iceland Gull in San Mateo county is the Thayer's Gull. If you are reporting any Iceland other than Thayer's, it should be accompanied by good notes or a photo as any other version is incredibly rare.

Thayer's adult
Iceland Gull (Thayer's; Larus glaucoides thayeri), adult, Half Moon Bay (Alvaro Jaramillo) 1

Thayer's adult
Iceland Gull (Thayer's; Larus glaucoides thayeri), adult, Half Moon Bay (Alvaro Jaramillo) 2

Thayer's adult
Iceland Gull (Thayer's; Larus glaucoides thayeri), adult, Half Moon Bay (Alvaro Jaramillo) 3

Thayer's first cycle
Iceland Gull (Thayer's; Larus glaucoides thayeri), first cycle, Half Moon Bay (Alvaro Jaramillo) 1

Thayer's first cycle
Iceland Gull (Thayer's; Larus glaucoides thayeri), first cycle, Half Moon Bay (Alvaro Jaramillo) 2

first-year Thayer's Gull
Iceland Gull (Thayer's; Larus glaucoides thayeri), second cycle, Half Moon Bay, February 2017 (Donna Pomeroy)

adult Thayer's Gull
Iceland Gull (Thayer's; Larus glaucoides thayeri), adult, Pilarcitos Creek Mouth, February 2013 (Donna Pomeroy)

first-year Kumlien's Gull
Iceland Gull (Thayer's; Larus glaucoides thayeri), first cycle, Pilarcitos Creek Mouth, January 2019 (Alvaro Jaramillo)

 

March 24, 2021