In 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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Many 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.
In my opinion this species is polymorphic and these pale birds are not really leucistic (failure to metabolise melanin). These pale morph birds are still reasonably common in the UK and have been reported throughout their range. They typically all look about the same with a pale cream color replacing the darker gray color dorsally. If this were leucism we would expect more individual variation related to random genetic mutations. As for founder effects, my understanding is that usually is related to the sampling error from a small founder population. In the case of California, the founder population from adjacent states was already so large that any founder effects would likely have been removed long before they ever invaded California. Simple polymorphism is a more parsimonious explanation in my view.
Joe, Thanks for commenting. What do you make of the fact that the pale birds tend to have irregular pale patches? They are not uniformly pale.
In reality, I suspect most of these birds being Oca4/Slc45a2 albinos. In a normal society one those albinos occurs in 1 out of every 700,000 individuals but it appears to be more much more common in Barbary Doves, the domesticated form of the African Collared-Dove and likely Eurasian Collared influence. With a species that tends to have a high population of albinos, I see no reason why the Founder Effect may increase the presence of albino Eurasian Collared-Doves.
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Seancozart is correct, the pale colour of these doves is the result of a mutation of the Slc45a2 gene. To answer Stephen’s question; “What do you make of the fact that the pale birds tend to have irregular pale patches? They are not uniformly pale”. Incomplete synthesized melanin, as a result of the mutation, is very light sensitive and bleaches rapidly further in the sun light. The lighter patches are older feather. And as for the founder effect; many of the founder birds were hybrids with Barbary doves (domesticated Ringneck doves), so the whole of the US population of Eurasian Collared Doves is, in fact’ ‘polluted’ with interspecific genes. About to submit a paper about this to the Bulletin of the British Ornithologists’ Club.
As mentioned earlier, as well as Eurasian Collared Dove and African Collared dove, another species Ring necked dove from Africa Streptopelia roseogrisea is very simiilar, and often bred selectively to be a pale bird, and is widely kept as a captive bird or free flying flock domesticated. Could it be these are contributing to the gene pool situation herewith.