Spreading north and west

By Bill Chaisson

Of a Feather
Of a Feather

On our recent trip to Oregon, I decided to download Merlin. I know my eastern bird songs pretty well, but I have not been out west often enough to be familiar with those birds. Plus, I was curious about the application, which was developed by the Cornell Laboratory of Ornithology and added its “Sound ID” function a couple of years ago. Essentially you just hold your phone up in the field and a list of birds appears on the screen.

I was in a tract of woodland called WildWoods in Lincoln City on the coast of Oregon when I gave it a try for the first time. A thrush I did not know suddenly became the Swainson’s thrush, a bird I have seen only in migration in the east but here in Oregon was singing in every corner of this forest. Similarly, a Wilson’s warbler was defending its territory at nearly every bend in the trail, another species I have seen only passing through in the east.

A less pleasant discovery was that I am indeed losing the high end of my hearing as I age. When the microphone picked up the chestnut-backed chickadee I could barely hear it and would have missed it entirely had Merlin not identified its call. I never did get a good look at them.

But perhaps the most surprising name to pop up on the screen was the Eurasian collared dove. In this case, I was looking right at it, sitting on a telephone line in a residential neighborhood in Lincoln, and I thought it was a ringed turtle-dove. I was aware the latter species was precariously established in some western cities, but had forgotten about the collared dove, which is an entirely different story.

Streptopelia decaocto is native to India and nearby China but has been spreading throughout the world for reasons that have not been explained. The oddest feature of its pattern of dispersion is that after it has a beachhead in a new territory, it then begins to spread north and west. Fifty birds escaped from captivity in the Bahamas in the 1970s and it is now found nearly throughout North America, Mexico, and the Caribbean, but the initial direction of colonization was diagonally across the continent toward British Columbia. The eBird map shown at this species’ page on birdsoftheworld.org indicates I was lucky to see it in coastal Oregon, where it is not particularly common.

Its call is a deep, resonant hoo-HOOO-hoo, which could be mistaken for an owl or some sort of odd baritone mourning dove. Overall, it is a light gray bird, the color of dry sand. It is the size of a pigeon but much more slender with a longer tail. The tail, however, is squared off, distinguishing it from the native mourning dove. The terminal edge of the tail is white underneath, which can be seen when it is flying.

But perhaps the most distinct field mark is the black half-collar, which is what caused me to think it was a ringed turtle-dove. Unlike the collared dove, the ringed turtle-dove is a domestic “species.” The last word is in quotation marks because it freely interbreeds with other doves when it encounters them. It is thought to have been derived from the African collared dove (S. roseogrisea) and may have been imported into Europe as early as the 16th century. Unlike S. decaocto, this domestic bird, which is also called the Barbary dove or ringneck dove does not thrive in the wild and does not spread beyond the urban or suburban locations where it is introduced. It has been given the scientific name S. risoria, but that too is often presented in quotation marks.

The Eurasian collared dove began to spread out of India in the 10th century. It was in Istanbul by the 16th century, where it was protected by the Ottomans, which helped it spread through that empire into the Balkans. With the end of the Ottoman control of southeastern Europe after World War I, the collared dove declined without protection. However, after the 1930s it began a rapid expansion, at first to the northwest and then turning westward. By the mid-1950s it was in Great Britain. By 1969 it had reached the Faeroe Islands, halfway to Iceland. The westward spread was slower; it did not reach the Balearic Islands off Spain until the 1990s.

The similarity among the various collared and turtle-dove species has caused their initial appearances in some locations to be hard to pin down in North America. S. decaocto was not positively identified in Florida until 1986, although it is thought to have been there earlier and been confused with S. roseogrisea, another escapee. These early colonizers were descended from a few birds released during a burglary on New Providence Island in the Bahamas in the 1970s.

By 1988 it had spread beyond Florida and by 1999 it was established in Oregon. It has now been recorded as far north as Fairbanks, Alaska. Other New World releases have resulted in the bird now being present from Belize to New York.

The species disperses by what is called “jump migration.” A group of birds will fly hundreds of miles from where they were born and establish a new population. These jumps can be as far as 600 miles from where the birds were hatched. The region in between is then filled in more slowly. The dispersal flights take place in North America and Europe between March and May and are generally toward the northwest.

Although collared doves have been observed to be aggressive and dominant toward other dove species, where this has been studied in Florida their population growth does not seem to have affected the numbers of native dove species. The non-natives have so far remained in built landscapes (like pigeons). It remains to be seen whether they will spread into more natural settings.