I still remember seeing my first one in Woodstock, Oxfordshire when on my way home from working in Oxford. This must have been in about 1964. One of the technicians in our laboratory was also a keen naturalist and had told me excitedly only a few days earlier that he had seen a pair in Charlton-on-Otmoor. Just a few years earlier any recitation of the names of the common British birds would have included just four British wild pigeons - the Wood Pigeon, the Stock Dove, the Turtle Dove and the Rock Dove - in addition to the ubiquitous Town Pigeon which is a feral form of the domestic dove, which is itself a domesticated Rock Dove, so it didn't count as a different species. The Collared Dove was absent from all the books on Birds I had then (and still have). Ironically, a dove normally described as common was the Turtle Dove, a close relative of the Collared Dove, but which I only ever saw once in Oxfordshire. It is now an endangered species in Britain. The last ones I saw were in Crete last Summer.
The phenomenal expansion of the range of the Collared Dove in the twentieth century was nothing short of spectacular. In the nineteenth century it was confined to Turkey, the Middle-East and India. In the 1920s it appeared in the Balkans and began an extraordinary north-westward expansion so that by the 1950s it made it onto the British bird list for the first time as a rare vagrant, then a breeder in 1955 in Norfolk. It is now abundant and a common garden bird throughout the UK and Ireland, has reached the Faeroe Islands and appears occasionally in Iceland, probably as a wind-blown vagrant. It then expanded laterally from this narrow band extending northwest from Turkey to the British Isles, so it is now present throughout Europe and Western Asia, extending even into the Arctic Circle.
It was introduced into the Bahamas in the 1970 and quickly spread to Florida. It is now very quickly extending its range across North America. If its European success is repeated it will soon be a common garden bird there too and may even extend down into South America.
But what caused this sudden expansion? At the time there was speculation that it must have been a genetic change which enabled the doves to survive in the colder north-west but there was also speculation that it could have been due to post-war changes in agricultural practices or other human activity which made their preferred food more readily available. It is also possible that there had been some form of barrier preventing their expansion. A barrier, especially for a relatively sedentary species (despite the species' expansion individual birds do not often move far from the area they were born in) can consist of something as simple as an expanse of water or even land on which its food is absent. If it can't survive there long enough to make it across, it can't establish itself on the other side, so the barrier is as effective as a wall. Was there some change in the Balkans in the 1930s or 1940s? It has even been suggested that the Collared Dove was introduced there as escapees from captivity.
To me the agricultural change hypothesis is less convincing than a genetic change because Collared Doves are not especially birds of the fields and woodlands but more birds of towns, parks and gardens, and parks and gardens have been a feature of European towns since well before the early twentieth century. The fact that the expansion was initially resolutely north-westward until it reached the Atlantic coast of Europe also points towards a genetic change rather than a change in agricultural practice or other human activity.
In a sense though, whether it was genetic change or environmental change which was the main cause of this expansion is semantic since an evolutionary change will only occur if the environment is right for the genetic change. If the genetic change is adverse in the given environment it will be quickly eliminated and nothing more will be heard of it. Whether the Collared Doves, much earlier in their evolution, had evolved the genes then neutral but capable of exploiting a new environment, by genetic drift, which then needed to 'wait' for the right environment to arise, or whether they evolved alleles of genes able immediately to make use of an existing environment is merely academic. Evolution is always environmentally driven since that is what selects for differential success. This is how genetic variance is selected by an environment and why the gene pool moves towards fitness for survival and makes a species appear 'fine tuned' for its environment. Essentially, the argument reduces to one of timing, not about the mechanism itself.
The Collared dove is a pretty little example of how this evolution happened very recently and very rapidly. In North America you can look forward to seeing it soon if you don't see it already. In Europe you should be so used to seeing it now that you take it for granted and maybe assume it was always present. Some of us are old enough to have seen it arrive.
The newly-evolved, range-expanding form of the Eurasian Collared Dove, coming soon to a garden near you. Evolution in our lifetime.