Pigeons are an ever-present sight in today's cities, and people tend to either love them or hate them entirely. But these are semi-domestic pigeons, highly-adapted to life with and close to humans. New research from Oxford University's Department of Biology reports on the discovery of a rare colony of their wild counterparts -- wild Rock Dove (Columba livia) -- on secluded Scottish and Irish islands.
The discovery of these colonies is of major interest to researchers and conservationists. The species is virtually extinct in England and Wales today. Feral pigeons, meanwhile, carry an important genetic component from domestic doves. Together, these two factors make it almost impossible for researchers to obtain genetic material from true, un-hybridized wild doves, in order to study the specie's evolutionary history.
For other species, this research helps showcase the role of hybridization processes in the extinction of species.
Pigeon in the coal mine
"Wild Rock Doves are particularly important because the species is one of the animals with the closest relationship with humans" explains William J. Smith, a graduate student from the Department of Zoology at the University of Oxford and corresponding author of the paper, for ZME Science. "As one of the first domestic birds, throughout history we have relied on it for food, navigation, sending messages and in science (they [also] feature massively in mythology and religions)."
"In particular – in science – we use domestic pigeons as model organisms to study genetics, behaviour and urban evolution. To be able to study the undomesticated form will help us to establish how the traits we study in a laboratory/domestic setting evolved, and what their purpose is in the wild."
Researchers led by members of Oxford University's Department of Biology have found rare colonies of the wild ancestors of common domestic and feral pigeons.
Feral pigeons today, the ones that fly around our cities, towns, and villages, are descendent from escaped domestic birds. These domestic birds, in turn, were bred from wild Rock Doves, whose habitat includes sea caves and mountain slopes throughout Africa, Asia, and Europe.
Although feral pigeons are highly successful, Rock Doves have been experiencing a steady and significant decline in their ranges around the globe. But, while we do know that the species has been struggling, researchers have had a very difficult time reliably charting this decline because of the extensive level of interbreeding -- and subsequent replacement with -- feral pigeons. Today, they only survive in very small, isolated populations that feral pigeons did not yet colonize. Quite a few ornithologists believe that there may not be any wild Rock Doves at all.
Some potential areas where the species might persist include secluded sites in Europe, the Faroe Islands, some areas of the Mediterranean Sea, as well as sites in Scotland and Ireland.
"At an individual level, the differences between the wild Rock Doves are slight. The wild form has a slimmer, longer bill and a more rounded forehead. Wild Rock Doves have a noticeably smaller ‘cere’ which is a white blob on the top of the bill. The biggest differences are evident when you see a flock of birds", William Smith explains for ZME Science. "A flock of feral pigeons or hybrids will contain a mixture of different plumage colours and patterns (sort of like a group of domestic dogs of different breeds!), whereas a Rock Dove flock consists of identical-looking birds (more like a wolf pack!), which is quite jarring when you are used to seeing a motley crew of ‘normal’ feral pigeons."
In order to determine whether these sites do still harbor wild Rock Doves, the team studied populations of the birds in Scotland and Ireland, in sites including North Uist (Uibhist a Tuath) in the Outer Hebrides, Orkney, and Cape Clear Island. They looked at their genetic makeup to determine whether these were truly wild birds and quantify the extent of feral pigeon genetic influence in their genomes. Samples for the study -- consisting of feathers -- were harvested through a combination of expeditions carried out by the team themselves and a collaboration with British Trust for Ornithology bird ringers
After sequencing the DNA of a large number of birds, the team was able to underline the differences between feral pigeons and wild Rock Doves, and with that, to measure the level of interbreeding between the two in various bird populations.
The results confirmed the existence of birds in UK and Ireland that are descended from an undomesticated lineage, the original line from which domestic and feral birds split. Despite this, different populations showed various levels of interbreeding. Rock Doves in Orkney have experienced extensive interbreeding with feral pigeons and are at risk of getting hybridised to the point of their extinction as a distinct lineage. Those in the Outer Hebrides, on the other hand, remain almost free of feral pigeon influence.
"We identified feral pigeon ancestry in most of the Scottish and Irish Rock Dove populations we sampled, and there have been feral pigeons in Europe for hundreds of years. It was therefore really surprising to discover that the Outer Hebridean Rock Doves showed negligible signs of hybridisation," explained William Smith in a press release.
"It’s important to remember that hybridisation, to an extent, is a part of nature. Gene flow between and within species can generate diversity which fuels evolution," he adds for ZME Science. "On the other hand, human movement of animals and plants around the world has led to an increase in such gene flow which can cause massive ‘homogenisation’ at a genomic level, replacing locally adapted and rare forms with common or invasive relatives. This reduces overall biodiversity at a global level, which will reduce the capacity of ecosystems to cope with changing conditions (e.g. climate change). The replacement of Rock Doves with feral pigeons (which are a superabundant animal) is one example of this process."
The findings have direct value in showing us that wild doves still exist, albeit in limited numbers and areas, in nature. This will have important implications for the conservation efforts of the species. In a wider context, it also teaches us valuable lessons on how to best preserve the genetic stock of wild species in other areas of the world. Finally, it furthers our understanding of how hybridization can help push a 'covert' extinction of certain species, by slowly replacing it with feral or domestic hybrids.
"Whilst vast amounts of money have been spent in Britain to conserve and study the wildcat, which is threatened with extinction by hybridization with feral domestic cats, the Rock Doves have not previously been studied despite experiencing a near-identical issue. This has meant that we have not understood their conservation status and value," William Smith concludes".
"Now, we are aware that the Outer Hebridean population, in particular, has suffered negligible gene flow with its invasive relative, the feral pigeon. We will be able to monitor this population to try and reduce such gene flow in the future and preserve undomesticated Rock Doves. As well as this, more generally, the extinction by hybridization process has caused significant loss of biodiversity across the world. For example, the Lesser Antillean Iguana, Seychelles subspecies of the Malagasy Turtle Dove, and the Scottish wildcat have all experienced dramatic declines following interbreeding with their commoner relatives after humans introduced the latter into their ecosystem. Our Rock Dove study will contribute to the body of knowledge about this phenomenon, which is of interest to conservation biologists around the world."
The paper "Limited domestic introgression in a final refuge of the wild pigeon" has been published in the journal iScience.