Perhaps surprisingly, at least three species of birds use Rathfarnham Castle as a nesting site; the feral pigeon, the jackdaw and the herring gull. They do so because in other locations they would choose ledges on sea cliffs or mountainsides to build their nests and lay their eggs, and so, to them, Rathfarnham Castle is essentially one big cliff.
Here, we focus on the first of those birds, the very familiar but in many ways unknown and certainly misunderstood, feral pigeon.
Feral Pigeon (Colm Aille)
What’s in a Name?
Feral pigeon; town pigeon; domestic pigeon; rock pigeon; rock dove. These are just some of the English names for this bird. The generally used Irish name or colm aille translates to (cliff pigeon) and of course there are many variants of this too. All these terms refer to one bird, a bird that scientists label simply as columba livia, which means in Latin, a blue/grey (livia) pigeon or dove (columba). Columba itself may come from an older, Greek word meaning “diver”, reflecting the swimming motion that a pigeon can exhibit as it flies through the sky. The Latin word columba can mean in English either pigeon or dove; indeed the English word “pigeon” comes from the Latin pipionem meaning a “chirping” or “peeping” bird. This may relate to the young pigeon, the “squab” as it does not seem a particularly characteristic description of an adult pigeon’s call. Here, it will referred to as feral pigeon. Feral simply means something that was domesticated and is now wild and the familiar feral pigeon is a classic example of that process. The ancestor of the feral pigeon is the rock dove, which look very similar to the one in the photo. These days it is mostly confined in Ireland and Britain, to the north and west coast of Ireland and the west coast of Scotland. These birds were first domesticated between five and ten thousand years ago, initially for their meat. Although they nest in cliffs and caves, they are seed eaters and would have had to travel to a source of seeds, so it would have been important for them to develop strong navigational mechanisms. Perhaps this is one reason why their domestic descendants have such good “homing” instincts that pigeon racers find so useful, although those traits have been selectively bred for and the hippocampus, the part of the brain responsible for spatial learning and memory, is larger in modern racers than in their wild ancestors.
Nesting and Breeding
Feral pigeons usually choose ledges on buildings and inside derelict buildings to nest, mimicking the cliffs and caves of their wild ancestors and cousins. Unlike many birds, pigeons can be highly flexible in the period of the year that they devote to breeding. This is because all pigeons and doves secrete a substance termed “pigeon milk” from their crop, a part of their digestive tract near the top of the oesophagus, meaning that they don’t have to worry about collecting food specifically tailored to their offspring’s needs. This milk is a suspension of proteins and fats and is fed to the chicks for the first week of their approximately four and a half week nestling period. Only two white eggs are laid and this means that if one chick should die then the other will be able to make use of all the milk. While producing this special milk means that the pigeon does not have to actively search for food for it’s youngsters on top of its own needs, it probably limits the numbers of chicks it can raise at any one time to two. Having said that, it also gives it the freedom to breed almost throughout the year, so this more than makes up for that limitation. The nest itself comprises just a few twigs or sometimes a bulkier structure of twigs and other vegetation.
Humans and Pigeons
While it may well have been as a source of food for humans that rock doves were first taken from cliffs and rock faces it was their behavioural characteristics that lead to other uses and associations. Pigeons have symbolic, religious significance going back to the Bronze Age. Sumerian archaeological excavations reveal pigeon images alongside figurines of a mother goddess providing evidence of an early cultural link. This devotion may be linked to the high numbers of offspring pigeons can produce or perhaps to their courtship behaviour, which displays some quite human like characteristics, e.g. “kissing” or perhaps because they mate for life. Other mother goddess representations occur in Greek mythology, where pigeons were sacrificed to Aphrodite in the hope of blessings in nuptial affairs and also Demeter, who is sometimes depicted with doves. In the Old Testament we see pigeons and doves being used as messengers, as in the story of Noah and the flood, as sacrifices, and in the New Testament as symbols for the Holy Spirit. Similarly, in Islam they are believed to have assisted Mohammed. Perhaps allied to their religious usage is that of aesthetics and today we still value pigeons for their beauty. Selective cross breeding has produced many strange and wonderful varieties of pigeons over the years and are highly prized. The first recorded reference to such breeding techniques appears to be in Homer in 950 BC when he refers to silver doves. Over 300 varieties are known to exist.
Their role as messengers is well documented into antiquity, capitalising on their famous homing abilities. Both the Ancient Greeks and Romans used them in their military campaigns. In 12th century Baghdad merchants used them as messengers and in the 16th century, in modern Syria, a pigeon postal service was in operation. In more modern times they continued to be used as a means of relaying messages in times of war and for their ability to infiltrate enemy territory unnoticed. During the First World War, they were released by British intelligence from aeroplanes behind enemy lines, complete with a harness and parachute. This was jettisoned by a clockwork device some time after landing on the ground. The landing unfortunately proved lethal to many pigeons but enough got through to do damage. Likewise, in the Second World War, they were carried on ships, for instance and released in the event of an attack with details of the ship’s position. Indeed, their use had a knock on effect to another bird in Britain; the peregrine falcon, one of the main predators of pigeons, suffered a decline in numbers after it was licenced to be shot and nests destroyed, in order to stop attacks by them on pigeons on active service. This “homing” ability is utilised by sporting bodies and individuals for pigeon races and provides a very lucrative business.
Navigation and Perception
So what mechanisms does a pigeon possess to enable it to complete such feats of navigation? Like any navigator birds need a map and a compass. A bird uses landscape features, wind direction and other weather indicators, position of the sun, stars and the earth’s magnetic field to provide different types of information needed to travel and integrates them to provide both map and compass. In addition to being able to see visual landmarks and other environmental features, birds seem to have the ability to discriminate polarised light.
Visual landmarks and sun and star position is one thing but how do pigeons and other birds tap in to the earth’s magnetic field? There are two candidates at the moment as to how birds do this. One centres on the possibility that they are sensitive to quantum mechanical effects on particles in the eye. This appears to be only possible in short wavelength blue light. Part of the forebrain linked to the eye is probably responsible for this. The other idea is that somewhere in the bird’s body there is a magnetic sensor made of iron oxide crystals, that is sensitive to the magnetic field. A suggested site for this has been the upper beak. Another suggested location is the ear, meaning they would be able to “hear” the magnetic field. How the magnetic field looks or sounds to a bird makes for interesting speculation.
It has been shown that pigeons are also able to construct a “mental map” and the part of the brain in birds involved with processing spatial location is the hippocampus. Birds that have to locate food sources some distance from their roost tend to have well developed hippocampi and homing pigeons have a much larger hippocampus than other varieties. However, experiments have shown this to be related to use, not primarily genetics. It seems that the more often a pigeon has to find it’s way home, the greater the size of it’s hippocampus.
It has also now been demonstrated that pigeons are able to use familiar odours to help them navigate and possibly infrasonic sound signals and can possibly sense atmospheric pressure variation through tiny feathers called filoplumes. Navigation and migration in birds and other animals is a complex and rapidly developing topic of scientific interest and there is still much to learn and understand.
Cognition and Perception
Pigeons might have the brain of a bird but they are certainly not “bird brains.” Indeed, the whole notion of a bird’s supposed lack of intelligence comes from the fact that it’s brain looks very smooth compared to the folded structure of the mammalian version. The folds in mammals increases the surface area allowing for many more connections than the size alone would suggest. However, this does not mean that a pigeon (or any other bird) has fewer connections than a mammal. The architecture of the two are completely different. The mammals brain is one answer to increase surface area inside a skull; a bird uses another method. The neurones are organised in nodal structures rather than folds and in fact can have more connections in the forebrain than a mammal with a larger brain mass. This gives a smooth impression to the look of a bird’s brain as opposed to the folded structure of a mammals and therefore they were deemed to be less intelligent.
This complex brain enables pigeons to perform complex, cognitive feats. In experiments pigeons have been taught to distinguish between a Monet and a Picasso painting; distinguish “bad” artistic style from “good:” recognise all 26 letters of the alphabet and perform “calculations” of relative probability. Similarly, they are able to recognise pictures of other pigeons on a computer screen; recognise human faces, with or without an emotional expression and recognising pictures of people with or without their clothes on! They have also been shown to recognise themselves in a mirror. All this visual processing is carried on at speeds roughly three times faster than a humans. Watching a film at that speed would appear like a slide show for a human.
Interestingly, a feral pigeon’s characteristic “bobbing” head movement is also thought to aid visual perception. In fact the head doesn’t actually “bob” at all but is pushed slightly forward and remains still as the body catches up with it. This helps the bird to stabilise their view of a world in motion. Pigeons have their eyes located at the side in order to have a good all round view of predators, this means they do not have stereoscopic vision, head bobbing might be a way around this problem.
The feral pigeon is very familiar, often overlooked and often despised by it’s human companions. At one time it’s ancestors were minding their own business on sea cliffs or mountainsides before being co-opted for our own needs. That they are simultaneously beautiful, intelligent and perceptive is often regrettably missed.