The all-conquering Collared Dove
When Collared Doves reached Britain and then Ireland, they found an ecological niche not filled by any other bird. In the absence of competition, they demonstrated the potential for geometric population increase that is probably inherent in all species. But what, for Collared Doves, was the spark that lit the fuse?
Some birds, no matter how often you see them, never lose their appeal. I am not talking about species that are incontestably sublime or exciting to encounter, such as the sparkle of a Kingfisher or the blitzkrieg ability of a Peregrine. No, the one I have in mind is Collared Dove. To most readers, Collared Doves are just one step above House Sparrow. The bird is a boring commoner of gardens and other habitats — suburbs, public parks and farmyards — where it rubs shoulders with Homo sapiens. That, of course, is the key to its success. By moving in around us, its tribe has prospered.
My first memory of Collared Doves harks back to the time when the species was headline news, a rara avis. This is why I am still fond of them. Funnily enough, the first to reach Ireland was discovered where I live — in Bangor, Co. Down. It was noticed in Ward Park on 8th July 1960, feeding near pens where various ‘ornamental’ species were kept [1]. Collared Doves eat a range of food, most of which can be found around human settlements. Loitering at hen runs is, therefore, smart. Alas, the discommoding arrival of an unusual small dove among poultry is often interpreted to mean that the newcomer is anything but wild.
The Ward Park individual was not pooh-poohed as a cage-jumper. Why not? Eight years earlier, in May 1952, Britain’s first was discovered by Mr and Mrs Hampshire near their cottage in Lincolnshire. It fed on their chicken-run. Ornithologists of the day, instead of acknowledging a pioneer in a movement that conquered Europe from the Balkans to Holland in less than thirty years, were cynical. It was damned by faint praise: ‘if [it] crossed to this country by the Straits of Dover, then it might be wild.’ [2] Worse, because it had the savvy to feed among poultry, its credibility was irreparably damaged: ‘we feel bound to conclude that no adequate evidence has so far been produced for adding Streptopelia decaocto [Collared Dove’s scientific name] to the British List.’ An early example of Brexit?
Tantamount to a two-fingered salute to the ornithological establishment, more Collared Doves quickly followed. By 1955, at least one pair was breeding in Norfolk. Soon, settlers were everywhere. By the time spearhead battalions starting popping over the Irish Sea, attempts at belittling the species’ ability to establish bridgeheads by crossing wide stretches of water had been forgotten. Because Collared Doves are distinctive and occur in areas frequented by people, earlier reports surfaced of pairs breeding in Dublin and Galway in 1959. Therefore, Bangor’s Collared Dove was not the first.
By 1965, breeders were nationwide, as were flocks — even in the far west. Part of an account in 1965’s IRISH BIRD REPORT states: ‘now well established in Tralee, Kerry, where a flock of twenty on December 8th contained many juveniles … [by 28th December] a flock of 51 in Tralee.’ Another facet of Ireland’s colonisation was the scatter of early arrivals. This was in keeping with the same pattern noticed in Europe by James Fisher in 1953 [3]: ‘the only two ringing recoveries [an adult that travelled 716 km SSE from Saxony in Germany and was found six months later in Italy; a nestling that flew 580 km SW from Saxony to the Ardennes, where it was found six months later] show very clearly that some pioneering doves do not come from the nearest place colonised but from a considerable distance.’
Ireland’s conquest followed similar lines. Forget establishing a toehold in the east. Among the vanguard, several did not stop until they reached the shores of the Atlantic. A handful descended on treeless islands. Published examples include ‘one, Inishtrahull, Donegal, July 18th 1961.’ Human nature is apt to conclude that a Collared Dove on a ‘last chance saloon’ island off the Irish west coast had paused there because, if it kept going, it faced a watery grave. But how would it know? While some may have baulked at the sight of the ocean, others may well have flown out to sea on exploratory flights and, when their mission to find land failed, turned back and settled on the first terra firma available.
The history of the Collared Dove in Europe — up to the time of the plantation of Britain and Ireland — is brilliantly documented by James Fisher [3]. The bird’s march from Turkey to the shores of the North Sea is fascinating. But the story has many peculiarities and has, I feel, been misrepresented. Looking back from present time, it is easy to swallow the popular view of a species that swept across mainland Europe on a northwest bearing until it colonised everywhere in its path. Yet there is no cogent explanation for why it expanded in the first place or, once underway, how continent-wide saturation was achieved in less than fifty years.
James Fisher hit upon the clever idea of mapping the spread by ‘joining points where the Collared Dove has been observed to the nearest point where it had previously been observed.’ This created, not one standard direction, but several. Nonetheless, the initial phase of expansion was northwest: ‘the main spread gathered its momentum primarily in the great plains and valleys of the Danube north of Belgrade.’ In a nutshell, the birds followed the wide corridor of optimum habitat that led them along the full length of the Danube to Germany. Once they accessed the plains stretching north from the German province of Hanover (the establishment of breeding pairs at the Hanover town of Celle in 1948 established an important nucleus) a route was clear to the shores of the Baltic, the Kattegat (the sea between the Baltic and the North Sea) and the remainder of northwest Europe adjoining the North Sea.
Although that expansion occurred, it is obvious that the geographic arc from Hanover was not just northwest — it was also north and northeast. Hence, in reality, colonisation spread into regions where habitat was ripe for settlement, irrespective of compass direction. Although the initial momentum was from the Balkans to the North Sea, later movements headed northeast and ultimately east from Europe to Russia’s Ural Mountains. Meanwhile, pushing out from strongholds in the eastern Mediterranean, others went west along the north Egyptian coast. Nonetheless, as noted by James Fisher in 1953, some individuals bucked the standard direction of peers and made flights in any direction — such as east from Ireland to Holland. Was the bird bonkers or did it have a plan? Was successful colonisation of Europe achieved randomly through sheer weight of numbers that exploited an unoccupied niche, bolstered by an ability to breed like rabbits?
Once Collared Doves were on a roll they certainly had lots of virgin territory to exploit. Because they feed their young on crop milk, they are emancipated from the need to find specific chick-rearing food and, provided breeding adults can feed themselves, are able to produce several broods in a year. So, like locusts, they can ‘breed as they go’. Although some pioneers exploded more than 1,000km, the majority covered intermediate distances, right down to a handful of kilometres. Perhaps of greater significance, those going in any northerly direction would derive the added benefit of daylight lengthening rapidly in early spring. For all nature, lengthening daylight is important because it triggers the growth of sex organs and brings organisms — in this case Collared Doves — into breeding condition. This is called ‘photo period’. Maybe, for Collared Doves expanding from ancestral areas where day length was less marked between seasons, the experience of a novel photo period served as an extra breeding stimulus? Not only had the species found new habitat, it possessed the means to populate it quickly.
Unlike people, birds are born with their mental faculties virtually fully developed. They have to be — because young become independent as soon as they can fly. As well as already knowing what food to eat, where to find it and what predators look like, the offspring of migrant birds know where to go to spend the winter. All birds instinctively know this stuff because the information is hard-wired into their genes — which they acquired from their parents. However, not every member of every new generation inherits a carbon copy of its parents’ genetic template. Evolution occasionally scrambles a small bit of the information. We call this a mutation. The word has a sinister connotation but mutations can be good or bad. A better way to classify them would be trial and error. Failures achieve nothing — although they may doom the recipient. On the other hand, a successful mutation can produce a winner. Probably, this is what nature intended and why the process keeps happening. In a way, it is evolution’s lottery.
Imagine a Collared Dove, a species with a fairly sedentary disposition living in sunny climes, being born with an urge to fly off and explore pastures new. This phenomenon is part and parcel of most bird lives. Because birds have wings, they can do such things. But the adventure usually achieves nothing and, who knows, the individual may simply return home at the end of it — like an avian Bilbo Baggins. Not surprisingly, the age class involved in wanderlust is usually young.
The reason for involving young rather than old is because a gene mutation that says ‘go west young man’ will manifest itself in a member of a new generation — and not surface in an old-hand. The upshot is the occasional out-of-range appearance of an unusual bird — generally a singleton and generally a youngster. Let’s speculate that, eventually, youthful adventure led to romance and a pair of adventurers bred outside the species’ traditional range. In so doing, they struck lucky. More importantly, their luck might endure because their chicks will inherit the genes that led the parents to strike off in a novel direction.
What began in one bird as a mutation — in just one of the parents — and caused it to break with convention, now becomes a ‘fashionable’ gene that, because it led to a successful outcome, will be copied. In other words, a trial-and-error process changes the instinct of subsequent generations that inherit the genetic information. Nature, therefore, loves a winner. Such a scenario would account for the dramatic gear-change that swept through the Collared Dove population. Probably the bulk of the population within the core range played no part in expanding the global range. That process was kick-started by a tiny number of Young Turks — possibly just one individual that was able to breed successfully and passed on its genes.
I am not a scientist. I struggle to read scientific articles. I blame that on those scientists that write in a language purporting to be English but which often seems to be deliberately obtuse and impenetrable to preserve some kind of ‘scientific community elitism.’ Ironically, some of the best English in scientific papers is written by authors for who English is not a mother tongue.
Although Peter Berthold is German and his research is extremely detailed, I have been able to follow his every word. Back in the early 1990s, he made a momentous discovery about bird migration. He proved that birds’ migratory instincts are encoded in their genes. This means that, for example, if your parents went in a certain direction for a certain time and were able to breed and produce you, that you will do the same thing — because their genes tell you to. In this scenario, the assumption is that both adults covered a fairly similar itinerary; otherwise they might not have met and bred. But Berthold also proved that, by captive breeding, migratory distance and direction can be altered by cross-breeding say, a bird that migrates west, with one that migrates east. The offspring of that pairing will follow an intermediate direction — in this case, south.
The same applies to genetic information relating to distance. In captive breeding experiments using a migratory warbler called Blackcap, Berthold was able to produce offspring that, in effect, ‘split the difference’ between the migration criteria held in their parents’ genes. So, if one parent was programmed to travel a long distance and the other was from a resident population, young would be born with a programme telling them to migrate an intermediate distance.
Here is how Peter Berthold described the process: ‘there is growing evidence, for a variety of species, that all important migratory traits like the urge to move, the direction and distance of migration and the morphological and physiological attributes required to migrate, may be under fairly strong genetic control in numerous species. Because the genetic control of these traits appears to be so widespread in birds, it seems reasonable to propose that the observed changes in avian migration patterns have a heritable basis [that is, are inherited by young from the genes of parents] and are therefore caused by micro-evolutionary processes.’
Berthold proved that the process operated in the real world. He undertook a major piece of research on Blackcaps and showed that the newfound wintering population in northwest Europe — especially in Ireland and Britain — came about through ‘micro-evolution’.
Here is his article: https://www.tandfonline.com/doi/pdf/10.1080/00063659509477155?needAccess=true&
Let’s get back to Collared Doves. It seems as though the bird’s expansion may have been triggered by a mutation that was successful and was incorporated in successive generations. Although the direction of spread that reached Ireland was initially northwest, the bird’s expansionist behaviour switched direction when colonists started to become successful in other compass points. When Peter Berthold’s micro-evolution principles are applied, it becomes easier to understand why offspring in ‘new lands’ would themselves be programmed to expand further. Given that successful pairings were likely to generate youngsters with a variety of pre-programmed genetic preferences vis-à-vis distance and direction, the burgeoning population had the seeds to spread in an almost geometric pattern. Furthermore, as if a genetic fair wind was not enough, photo period could have been an extra driving force that kept production lines busy.
Has the process ceased or slowed? I do not know. Once Ireland and all of Scotland was colonised, outlying islands came next. Collared Doves started breeding on the Faeroe Islands from the early 1970s. As far as I can ascertain, just over 40 made it to Iceland between the 1970s and 2006. Iceland may be unsuitable as a breeding area. One pair bred in 1971 but not since [4]. Further afield, Collared Doves have been found in northeast Canada, including Newfoundland. Sadly, because some were released in the Bahamas and Florida, North America is being colonised by founders that were not wild. Although individuals have reached Newfoundland and other extremities way beyond the nearest edge of the currently expanding population, their provenance may never be known — but surely a caught bird’s feathers ought to provide isotopes pointing to the area of origin?
It is ironic to think that, among the Collared Doves feeding in our gardens at Christmas, some may be destined to fly northwest to replicate the great leaps made by forebears that brought the species here in the first place. Because no suitable terra incognita awaits their arrival, owners of the kamikaze gene that prompted such long-distance explorations will, over time, perish. Ultimately the population will once again become stay-at-homes, controlled by genes that, for now, say ‘enough’.
REFERENCES
1 This and subsequent references in the text to Irish records of Collared Dove come from the relevant IRISH BIRD REPORT for the years in question.
2 May, R. & Fisher, J. 1953. British Birds, vol.46, pp. 51–55.
3 Fisher, J. 1953. British Birds, vol.46, pp. 153–181.
4 The Birds of the Western Palearctic, volume 4, pp. 343.
