Bird Sense (30 page)

Studies of chickens, however, provide rather convincing evidence that birds can experience the
feeling
of pain. Chickens kept commercially at high densities often resort to pecking each others’ feathers, and sometimes to cannibalism. In an effort to prevent this, the poultry industry amputates the tips of the birds’ beaks. On the basis of our earlier discussion of the sense of touch, you may be able to anticipate some of what’s coming next.

Beak trimming is a rapid procedure performed with a heated blade that simultaneously cuts and cauterises the beak. It appears that trimming results in a period of initial pain lasting two to forty-eight seconds, followed by a pain-free period of several hours, which is then followed by a second, more prolonged, period of pain. This is similar to what we experience following a burn injury. The initial period of pain in chickens was demonstrated by measuring the discharge from two types of nerve fibres, simply referred to as A and C fibres, from the pain receptors. The A fibres are responsible for the rapid, reflex-like pain response; the C fibres for the later, longer-lasting pain sensation. Young fowl seem to experience less pain and to recover more rapidly from beak trimming than adult fowl. The older birds also seemed to experience more discomfort and fifty-six weeks after the operation they still avoided using their beaks, preening less and performing less exploratory pecking than birds whose beaks had not been trimmed.
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The important point here is that, apart from some head-shaking immediately after the operation which presumably reflects the initial period of pain, the birds did not show any
obvious
outward signs of discomfort. Only by measuring subtle differences in their behaviour and physiology was it possible to demonstrate the longer-lasting
feeling
of pain.

On a more positive note, I am sometimes asked what my favourite bird is. For a long time I felt that question to be futile, but my experience with one species in
2009
changed the way I think about this. If I was asked now, I would have no hesitation in saying the sylph hummingbird, a South American beauty. In fact, there are two sylph species, the long-tailed and the violet-tailed. As their name indicates, these are tiny, elegant hummers of the most exquisite proportions and the most extraordinary colour: iridescent metallic green on the crown, and, depending on which species, metallic green or blue under the chin, and brilliant cobalt blue or violet along the entire length of their elongated tails.

Encountering a long-tailed sylph in Ecuador for the first time gave me the most extraordinary buzz, which lasted several days. The sylph was so exquisite that I wanted to possess it, to capture and hold on to its beauty. A photograph isn’t enough because it cannot do the bird justice, but also because a single image isn’t adequate to capture the full essence of the bird. I understand now why Victorians wanted to fill cabinets with the still sparkling but lifeless bodies of hummingbirds – it goes some way to providing the multiple images necessary to make these birds so vibrantly captivating.

For an ardent birder, seeing a rare or beautiful bird is a little bit like falling in love. When people say they love birds it is because, on seeing a particular bird, they get a special buzz in their brain.

Love was once believed to be impervious to scientific investigation, but recent technological advances mean that neurobiologists now feel that they have a window through which they can view human love. Using fMRI scanning technology, researchers can literally see into the brain while a subject says what emotions he or she is experiencing. When someone in a scanner looks at a photograph of a person they are passionately in love with, very specific parts of their brain ‘light up’. These are areas of increased blood flow, and hence increased brain activity, and lie in the cerebral cortex and in the subcortical regions which together are referred to as the ‘emotional brain’. Significantly, they are also known to be part of the ‘reward system’ of the brain. Looking at a photograph of a much-loved partner or lover, the hypothalamus region of the brain releases substances known collectively as neurohormones, which, by providing a link between the nervous system and the endocrine system, stimulates the reward centres.
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These neurohormones therefore play a vital role in the formation of relationships. There are other effects when people fall in love: another neurohormone, called serotonin, falls to levels similar to those found in people suffering from obsessive-compulsive disorders, which may explain why lovers sometimes become single-minded and obsessive. Two other neurohormones, oxytocin and vasopressin, also produced by the hypothalamus (especially during orgasm), increase on falling in love, and also seem to play a vital role in bonding.

These findings had their origin not in a bird, but in a mammal, the prairie vole, one of just a handful of mammal species with a long-term pair bond and with shared paternal care. During sex, oxytocin and vasopressin are released from the vole’s brain, facilitating and reinforcing their pair bond: oxytocin in the female, vasopressin in the male. If, however, the secretion of these two chemicals is experimentally blocked, the voles fail to form a relationship. Conversely, even without copulation, an injection of the two chemicals will result in a relationship being formed. Even more remarkably, when researchers introduced a gene that stimulates vasopressin secretion into males of a different, non-monogamous vole species, the meadow vole, it showed a significant increase in the tendency to form a pair bond with a female, suggesting that bonding may depend on a single gene. The researchers who conducted this work are keen to emphasise its preliminary nature, and that we should be careful of extrapolating to other species, but their results do suggest a mechanism linking pair bonding behaviour and the reward system in the brain.
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We do not yet know whether similar processes occur in birds. Currently, there are two research groups investigating this, both using the monogamous zebra finch as their study species. Although they have detected neurohormone activity in the appropriate parts of the brain, so far it is not clear that the same processes that occur in the prairie vole also occur in the zebra finch. The research is ongoing so we should soon know.
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The reward system is central to everything we as humans do. It is what keeps us going: it is why we eat, why we have sex and why some of us watch birds. The greatest pleasures that (most) humans can experience, however, are the emotional experiences associated with love and lust. Love can be both romantic and parental, and both forms involve ‘attachment’ or bonds: between partners, and between parents and offspring. Romantic love, of course, usually leads to physical desire and lust. It is easy to propose an adaptive explanation for love: a pair of individuals working together is more effective and successful in rearing offspring than individuals with any other breeding system – at least under certain ecological circumstances.
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Birds, too, are famously monogamous, by which I mean they are unusual among animals by breeding as pairs – a male and a female joining forces to rear offspring together. In a survey conducted in the
1960
s, David Lack estimated that over
90
per cent of the
10
,
000
known species of birds reproduce in this way. The rest are either polygamous (a breeding system subdivided into
polygyny
comprising one male with several females, and, more rarely,
polyandry
, comprising one female and several males), or promiscuous, with no kind of bond whatsoever between male and female. Later, the notion of almost ubiquitous monogamy among birds had to be modified, when molecular paternity studies revealed just how widespread extra-pair paternity is. Even though Lack was right about the majority of birds breeding as pairs, monogamy does not imply an exclusive sexual relationship. Copulations outside the pair bond and extra-pair offspring are common, and ornithologists now distinguish between what they call social monogamy (breeding as a pair) and sexual monogamy. The latter is an exclusive mating arrangement in which there is no infidelity, and is exemplified by the mute swan and a relatively small number of other species.
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I am not going to speculate about the emotions that might be involved in avian infidelity. However, it is worth thinking about the emotions associated with pair bonds, especially the enduring bonds of long-lived birds, and the bonds that occur among different members of co-operatively breeding groups like white-winged choughs, bee-eaters and long-tailed tits. In all cases there is likely to be an emotional dimension to bonding. The problem is that, so far at least, we have no way of unambiguously demonstrating such an effect.
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Here is how it might work. There are several things that birds do that we know are tightly associated with social relationships, both with a partner, and, in co-operatively breeding species, with other group members as well. These include greeting ceremonies, certain vocal displays and, as we’ve seen, allopreening.

Whether the goose whose partner was shot near Resolute in northern Canada experienced any emotional response to its loss is something we do not know. Geese are normally long-lived with long-term pair bonds and strong family ties – the young remain with the parents for several months and the family even migrates together. When pair members are temporarily separated, they typically perform a greeting display or ‘ceremony’ on being reunited. Such displays are widespread among long-lived birds and are particularly protracted when pair members are reunited after a winter’s separation, in birds such as penguins, gannets and guillemots. Throughout the breeding season, pair members greet each other, even after a relatively short absence, when one bird returns after a foraging trip. Strikingly, the duration and intensity of these greeting displays is closely tied to the length of time the pair members have been apart.
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Bryan Nelson, who has studied gannets and boobies throughout his career, described the North Atlantic gannet’s meeting ceremony as ‘one of the finest displays in the bird world’. If you visit a gannet colony, such as the Bass Rock in Scotland, you can see this display very easily. As one member of the pair returns to its partner at the nest, the two birds stand upright, breast to breast with outstretched wings, and skyward-pointing beaks. In a frenzy of excitement they clash their bills together, intermittently sweeping their head down over the neck of their partner, calling raucously all the time.

Under normal circumstances this greeting display lasts a minute or two, but Sarah Wanless, who studied gannets at Bempton Cliffs in northern England, observed a particularly prolonged instance. At one of the nests she was regularly checking, the female of the pair disappeared, leaving the male to care for the tiny chick alone, which, against all the odds, he did. One evening, the female returned after a remarkable five-week absence and luckily Sarah was there to witness it. To her amazement, the two birds performed an intense greeting ceremony that lasted a full seventeen minutes! Because the greeting ceremonies of humans (kissing, hugging, etc.) are also more elaborate the longer participants have been apart, it is tempting to assume that birds experience similar pleasurable emotions on being reunited.
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In many species, like the Eurasian bullfinch, pair members keep in contact while they are foraging in dense vegetation by their constant piping contact calls. In other species, including African shrikes, robin-chats, and certain tropical wrens, pair members perform antiphonal singing – an alternating duet so beautifully synchronised that it sounds like a single bird. The function of such duetting is not fully understood, but may serve in territory defence.
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One of the most remarkable of all such displays is the ‘carolling’ by Australian magpies, which, like the white-winged chough, is a co-operative breeder. Carolling consists of the entire magpie group – some six to eight birds – standing on the ground, often around a bush or a fence post, and together uttering their hauntingly melodic song (which fans of the TV series
Neighbours
will be familiar with since it is often on the sound track). As Ellie Brown, who studied magpie carolling, said: ‘Communal songs, like motets and madrigals, are made up of the combined melodies of all the singers.’ In terms of function, Ellie likened it to a human war chant, creating and reinforcing the group cohesion necessary to maintain and defend their territory.
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