Date: 3/31/17 2:00 pm From: Wayne Hoffman <whoffman...> Subject: [obol] Re: An observation on gulls
Interesting thoughts, Lars -
I'll try to keep these responses brief.
1. you are correct that when 2 populations increase and then overlap (or overlap more) there can be an increase in hybridization. Subsequent history can follow any of 5 general patterns: a) One replaces the other, ala Blue-winged Warbler. This seems to be happening on a smaller and slower scale with Townsend's and Hermit Warblers. b) Hybrids can proliferate and the zone of hybridization spread out into both parental populations, as in Northern Flickers. c) If hybrids are selected against, isolating mechanisms can evolve to reduce mixed matings, as apparently is the case with Baltimore and Bullock's orioles, and new research suggests may be happening with Myrtle and Audubon's warblers. d) an apparently stable hybrid zone can be established with little genetic interchange back into the parental populations The overlap between Tufted and Black-crested titmice in Texas is an example.. e) The size and location of such a hybrid zone, and frequency of hybridization can oscillate through time, in response to changes in climate, food supply, predation rates, etc. Douglas Bell. who studied Western and Glucous-winged gulls in the 1990s concluded this was happening, and that we are in a presumably temporary expansion phase.
2. Western and Glaucous-winged gulls differ a bit in ecology and behavior, including habitat preferences, migratory behavior and food habits. As a result, the relative numbers of them and of "Olympic" Gulls in any one place outside the breeding season are going to be biased samples that do not represent their actual abundances. Western Gulls are more closely tied to the coast. Although a few can be found in the Willamette Valley, a much smaller proportion of the population goes inland than for the other 2 categories. if you go offshore in winter, you can see very large numbers of Glaucous-winged gulls, and a higher percentage appear "pure" than onshore. Western Gulls do range offshore too, but generally not as far and in smaller numbers. "Olympics" tend to be most common onshore and inland, and thus they can seem more abundant gthat they really are overall.
On 3/31/2017 12:30:52 PM, Lars Per Norgren <larspernorgren...> wrote:
DNA studies quite awhile ago showed that Western Gulls are not closely related to Glaucous-winged Gulls. Glaucous-winged Gulls are part of the Herring Gull super-species, along with California Gulls. The over-all trend among taxonimists from 1970 to present has been splitting of white-headed gulls, most of them in the Herring Gull group and often harder for humans to distinguish than the Thayer's. The classic hybrid scenario is for an expanding species to mate with the more abundant sibling species until a critical mass is reached, at which point the pioneer species goes back to its own kind. This was the case with Yellow-legged Gulls that moved into the Netherlands. They mated frequently with Herring Gulls at first. Now they have plenty of other Yellow-legged Gulls to keep company with. In Iceland the Herring Gulls on the east end of the island mated with Glaucous Gulls until now the eastern colonies have only Herring Gulls.
This phenomenon of replacement is also part of the classic sequence in a hybrid zone. One species is more fit than the other and completely displaces it. "Lawrence's" and "Brewster's" Warblers are hybrids between Golden-winged and Blue-winged Warblers. Pretty much all marginal farmland in the northeastern USA was devoted to hay production in 1900. The universal adoption of automobiles removed demand for hay and most of the landscape experienced passive reforestation throughout the 20th century. Early stages of this second growth, with scattered junipers on abandoned pastures, favored Golden-winged Warblers. Eventually a closed canopy of red maple would develop and favor Blue-winged Warblers. Hybrids were most frequent when the plant community was in transition. I attended the founding meeting of the Connecticut Ornithologists' club (I forget the official name) in 1985 where several people present opined that the Golden-winged Warbler would go extinct. I don't know the status o
f the Golden-winged Warbler thirty years later, but surely Brewster's Warblers are a lot scarcer these days.
A similar scenario is playing out with Spotted and Barred Owls. But the Olympic Gull, the overlap of Western and Glaucous-winged Gulls, has proven to be vary stabile. It was first recognized by science about 1900. In the ensuing 100 plus years the Olympic hybrid zone has not changed much in extent. Wayne Hoffman has repeatedly pointed out on this list that most Olympic Gulls are not true hybrids, where a pure Western mates with a pure G-wing. Rather, hybrids prefer to mate with hybrids On Destruction Island, Washington there are pairs of pure Western Gulls and pure G-wings. But when Wayne and colleagues did research there in the 70s, 60 % of the breeding pairs were assignable to neither species. When Darrel examines a winter flock of gulls and finds 60% to be Olympic Gulls, it is what I'd expect. My first participation in the Great Backyard Bird Count I reported 350 Olympic Gulls at one location. At the time, the system of local birders acting as filters on eBird had not been
fully implemented and the report was automatically excluded.
My lifelong experience with Olympic Gulls inevitably prejudices me in my opinions about Kumlien's Gull. Larophiles in Chicago operate with a different perspective. We are witness to the anthropocene warming, but climate change is not new. The Ice Age is a series of scores of glacial advances and retreats. 12 thousand years ago a huge lake occupied northern Nevada, Lake Lahontan, its shoreline reaching into modern day Oregon near McDermitt. Lake Bonneville was similarly extensive in nw Utah. Seven thousand years ago the climate was warmer and drier than today. Megalithic monuments like Stonehenge were built on islands north of Scotland. The Fort Rock Basin in Oregon was dry for two thousand years. Gulls have been expanding and contracting their ranges in response to this throughout.
And many gull species have increased for anthropogenic reasons- Adouin's Gull in the western Mediterrenean due to changed fisheries; Herring Gull on the American east coast in response to garbage dumps, to name a few. Gulls' adaptability exceeds that of most bird groups. Hybrids have traditionally been seen as less fit than their parents. Sometimes they are sterile; and presumably the survival and reproduction rates of fertile individuals are lower . But the Olympic Gull gives me strong circumstantial evidence of being more fit than a Western or Glaucous-winged Gull. Hybridization seems to be adaptive, and I doubt that it routinely leads to the extinction of parent species of gulls.
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