Heermann’s gulls, also known as White-headed gulls (a name preferred by some as being descriptive), have been abundant in the open waters of the Salish Sea since mid- July — until now. On October 12 when I took the photo above, White-headed gulls were gathering in several large flocks offshore, over a hundred in each group, a sign they were mobilizing for departure. On our most recent boat trip a few days ago, along the exact same route, I saw 6 of these gulls total. Except for those few stragglers, our Heermann’s gulls have left the Pacific Northwest, headed south toward their breeding ground in Mexico. Today is a good day to reflect on these very special gulls.
The Daily Bucket is a place to note what you are seeing around you: animals, weather, meteorites, climate, soil, plants, waters. Each note is a record that we can refer to in the future as we try to understand the phenological patterns that are quietly unwinding around us. To have the Daily Bucket in your Activity Stream, visit Backyard Science’s profile page and click on Follow.
Flying north after breeding is not the only unusual feature of these distinctive birds. Their appearance is remarkable too: while the plumage of most gulls is primarily white, like the California gulls in the background below, Heermann’s gulls are gray all over except for white accents on their wings and tails, and seasonally white heads. During the non-breeding half of the year their heads are as gray as their bodies. Despite their sooty plumage you can tell Heermann’s gulls from any other by their bright orange bills from far off.
While they’re away they molt into breeding plumage and still wear that when they first arrive back here in July: heads brilliantly white, a stunning contrast to their vivid orange bills and gray bodies. Unmistakable.
Throughout August they gradually molt out of breeding plumage. If you don’t see them when they first arrive you’d probably wonder why they’re called White-headed gulls!
While these gulls range all along the West Coast from July to February, they are very restricted in where they nest. 95% of all the 500,000 or so Heermann’s gulls nest on one tiny island in the Sea of Cortez, Isla Rasa, which is less than one square kilometer. This limited breeding location is why they are considered “Near Threatened” by the IUCN, and indeed there was some question whether the species would survive after a century of human interference at Isla Rasa. Guano-mining terraformed the island for 40 years until the 1910s, and that was followed by aggressive egg-collecting. Locals on the mainland collected so many bird eggs that the nesting populations of these gulls and Elegant terns were decimated. Mexico granted protection to Isla Rasa in 1964 but birds were still highly vulnerable to predation by rats, originally brought to the island by guano-miners. Since gulls and terns nest on the ground, their eggs and chicks were easy prey. In the early 1990s the rats were eradicated and bird populations rebounded. Enriqueta Velarde, a Mexican biologist who has been monitoring nesting birds on Isla Rasa since 1979, describes the breeding behavior of White-Headed Gulls: they arrive in mid-February, and begin nesting in late March. Eggs hatch after a month and chicks fledge 45 days later. By June the gulls begin dispersing, some south but most north along the coast. The Salish Sea is as far north as they range, taking about a month to fly the 1000+ miles up here.
This short video of the breeding colonies on Isla Rasa was uploaded to Youtube in 2009, so it dates from before that.
With their legal protection (and enforcement: Velarde is present during the breeding season and gently but firmly discourages egg-poachers) you would expect to see a fair number of juvenile gulls every year ranging north, typically arriving later in the summer. But in the last couple of years I’ve seen very few. Juveniles are all gray and their bills are tan rather than orange. They can be distinguished from adults at a distance since they lack white margins on their wings and tail. I saw a fair number of juveniles in 2014 but not many since.
In monitoring gull and tern numbers over the past 35 years Enriqeta Velarde has seen a decline in nesting on Isla Rasa in recent years. Her comments in the video above reflect conditions she is no longer seeing. The Elegant Terns have nearly ceased breeding on Isla Rasa, and have moved to Southern California. Velarde has identified two major factors affecting breeding success at Isla Rasa: purse-seiners have decimated fish in the Sea of Cortez, especially sardines, the primary food these birds need to feed their chicks. The sardine fishermen moved into the Sea of Cortez after the fishery in the Pacific collapsed. The second reason is warming sea temperature. The least successful breeding seasons correlate with El Niño winters. Warm water reduces marine productivity, cutting into the number of fish available.
With global warming relentlessly increasing the temperature of the sea, successful breeding seasons for White-headed gulls are likely to be fewer. This summer I saw my first-of-season gulls in June, which means those birds did not breed, and flew north early. They know there’s fish to be found in the Salish Sea, for now anyway.
The no-longer-white-headed Heermann’s gulls have departed our waters for another winter in Mexico. I will miss these stunning seasonal birds. I hope they have a good breeding season next spring. Two things I’ll be looking for next year are arrival time (did they remain at Isla Rasa for the whole breeding season?) and how many juveniles appear up here later in the summer (they take longer to fly north than adults). I hope the survival of this species has not reached a tipping point.
#####
Time for you to share what you’re seeing in your natural neighborhood . . .
"SPOTLIGHT ON GREEN NEWS & VIEWS" IS POSTED EVERY SATURDAY AT 5:00 PM PACIFIC TIME AND WEDNESDAY AT 3:30 ON THE DAILY KOS FRONT PAGE. IT'S A GREAT WAY TO CATCH UP ON DIARIES YOU MIGHT HAVE MISSED. BE SURE TO RECOMMEND AND COMMENT IN THE DIARY. |