Why are so many Eurasian Collared-Doves leucistic?

0V2A6850In 2006, I wrote a paper about the spread of the non-native Eurasian Collared-Dove into the Central Valley of California. At that time, there were about 43 records. Now, of course, the species is widespread and common. Quoting from that paper, here’s the backstory of their spread throughout North America:

The Eurasian Collared-Dove was first observed in Florida in the late 1970s. These birds likely originated from an accidental release in the Bahamas in 1974. Since then, their spread has been well documented by Christmas Bird Count and by state bird record committees. By the mid 1990s, the species had been recorded throughout the southeast United States. By 2000, Arizona, Idaho, Oregon, Utah, and Washington had documented records. On the 106th CBC (December 2005 – January 2006), over 30,000 individual birds were reported nationwide, compared to just 560 fifteen years earlier. Their rate of increase has averaged 34% per year.

Today, it seems that in any large aggregation of Eurasian Collared-Doves, there are one or two that are unusually pale, blotched with white and cream. They seem to be about 1% of the population or more, though it’s yet to be studied. These birds bare a strong resemblance to African Collared-Doves, which is generally this pale.  However, based on the dark outer web of the outermost tail feather (see below), as well as size and vocalizations, these birds are clearly leucistic Eurasian Collared-Doves.

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EUCD diagram

All of the photos on this page involve the same two darker doves and one pale dove. Above, the tail patterns of the dark dove (left) and pale dove (right) both suggest Eurasian Collared-Dove.  Woodland, California, October 2018.

0V2A6835Many birders suspect this is due to the Founder Effect, a phenomenon that occurs when a small population colonizes a large area. Eventually, all of the birds (or other animal species) are descended from few individuals. In this context, certain recessive traits that were once rare may become more common.

See the Wikipedia account of the Founder Effect for examples of this in human populations.

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Here, one of the darker doves is mating with the pale dove.

 

The basics of Fox Sparrow identification

While there are over a dozen subspecies of Fox Sparrows, they fall into four main groups: Sooty, Red, Slate-colored, and Thick-billed. The images and diagrams below illustrate the main differences.

Sooty is the darkest, usually illustrated as dark chocolate in most guides. Even the darkest Sooties, however, show some gray on the face and rusty tones in the wings and tail– especially in bright light. I’ve chosen a photo in sunlight to show how lighting can change a bird from dark brown to bright rust. Regardless, the back is solid brown.

FOSPsooty

Red is the common form that breeds across the taiga to Nome and Bethel, Alaska, and generally winters in the eastern US.

FOSPred

Slate-colored is the Rocky Mountain version, found throughout much of the mountain West. Note that some consider altivagans, from Alberta, an intergrade swarm between Red and Slate-colored.

FOSPsc

Thick-billed is the southern Cascade and Sierra Fox Sparrow, found in the mountains south and west of Slate-colored range.

FOSPtb

A key distinguishing feature is back color and pattern.

FOSPbacks

Less useful due to variation, but still helpful, are tail and bill length.

FOSPtails

FOSPculmens

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Most forms of Fox Sparrow winter in the lower 48, with southern California host to nearly all subspecies. This photograph shows the green chaparral along the Angeles Crest Highway where a wide variety of Fox Sparrows can be found.

For a more detailed analysis, including discussion of Sooty subspecies, see my paper:

Hampton, S.  2016.  Status and Identification of Fox Sparrow Subspecies in the Central Valley of California. Central Valley Bird Club Bulletin 19(2-3): 28-63.