The most comical, fascinating, complex and exhausting courtship rituals among birds captivate researchers and, perhaps, tell us something more about ourselves.
Amotz and Avishag Zahavi, in the splendid book The Handicap Principle, list a myriad of them — courtship rituals of a thousand kinds and species. The title comes from the handicap — a foolish and exhausting, provocative, apparently senseless and risky behaviour — that the male takes on solely to demonstrate to the female that he is strong enough to afford it. Almost an ethology of braggadocio!
Researchers dwell at length on the kingdom of birds.
The bird-of-paradise (family Paradisaeidae), during courtship, hangs upside down from a branch, beating its wings and fanning out its ornamental feathers. Male skylarks (Alauda arvensis) soar into the sky to sing their serenade, then plunge back down in a breathtaking dive, braking only at the last moment. The western capercaillie (Tetrao urogallus Linnaeus), having fought off all the other males (during the mating season it becomes so aggressive that it even attacks humans, mistaking them for love rivals), jumps, flies, spreads its wings and sings in highly contorted postures. Male frigatebirds of the Galápagos (Fregatidae) inflate an impressive red chest pouch. When a female peafowl (Pavo cristatus Linnaeus), while the male is displaying his fan, approaches a male to examine his plumage, he turns around and presents her with his back — that is, the undecorated side of his tail. A female who does not bother to walk around the male is not interested in examining his feathered train, and is therefore probably not interested in mating either. It has been shown that peacocks with longer, fuller tails produce stronger and more alert chicks.
Bowerbirds (passerines, Ptilonorhynchidae Gray) arrange a kind of stage in front of their nest using the most colourful objects they can find: Gerald Borgia, a researcher from Maryland, catalogued hundreds of "stages" featuring bottle caps, fragments of glass and pieces of plastic; females visit the various "homes" and pair off with the most imaginative decorator!
This is not an unusual attitude among bird species. Female Adélie penguins of Antarctica (Pygoscelis adeliae) offer sex in exchange for the pebbles they need to build their nest. Rather than digging into the ice shelf with their beaks or stealing stones from other birds, they leave their mate and go in search of a lone male. If he shows interest, they assume a prone position to invite him to mate, then pick up a pebble in their beak. The shrewder females manage to obtain the stones without giving themselves up, simply by displaying their charms to the male. Using this method, a female penguin can collect around sixty pebbles in a short time.
Females are always attracted by displays of colour. Research carried out about fifteen years ago demonstrates this in a striking way. Researchers commonly fit the specimens they study with identification rings on the legs. During a multi-year study, ornithologist Nancy Burley discovered that the (very lucky) specimens of zebra finch (Taeniopygia guttata Vieillot) that wore a red ring enjoyed greater reproductive success. Moreover, females tended to snub (unlucky) males wearing green or other-coloured rings. Here one can only note that the beak of the male of this species is brilliantly coloured red, and it is already well known that females prefer the strongly marked "red-beak" trait. The hypothesis put forward is that the little ring was stimulating the natural tendency to prefer highly ornamented males…
Many species exchange songs and vocalisations during courtship — as a call or, even more evocatively, to test the synchrony, accord and harmony with a potential partner.
Some female strategies among duetting birds appear similar, discovered by Wolfgang Wickler while studying the songs of the slate-coloured boubou (Laniarius funebris, a passerine living in Africa): "We followed various species for ten years, recording and analysing their songs in search of meaning. We came to the conclusion that they simply repeat the same phrase over and over again. These duets, however, occur only among monogamous birds living in fixed pairs. In our view, this is a form of learning that obliges the male to invest time and energy in winning the female. Indeed, the female does not permit mating until the male has learned to duet with her. A kind of courtship designed to create the conditions for a lasting union." If, after mating, the male were to go off in search of another female, he would have to spend another long period learning a new song before he could mate again.
The dances of birds have always fascinated researchers, artists, musicians and photographers.
The most celebrated and romantic dances are those of cranes, which dance in pairs with wings outstretched, and of swans: male and female face each other, and the male begins rhythmically dipping his head under water. If she welcomes the approach, she makes the same movement, which gradually synchronises with that of her partner. The implied message is: "I am capable of diving to the bottom of the pond to fetch aquatic plants for you to eat." When one of the two resurfaces with real leaves in its beak, it means the test is over and that love between the two can blossom.
But dances are extremely widespread everywhere. Among black grouse, males face off in circular displays while the females look on; among doves and pigeons, males puff themselves up and coo, bowing repeatedly; and the black males of the Victoria's riflebird (Ptiloris victoriae) almost embrace their females — which are a different colour, tending towards orange.
Dances are a display of fitness, a way to impress the female, but perhaps also a way to test synchrony and the predisposition to respond. A recent discovery about the blue-capped cordon-bleu (Uraeginthus cyanocephalus), published in a new study in Nature Scientific Reports, points in this direction. Researchers already knew that male passerines hop and sing during courtship. A new video filmed at extremely high speed (more than 300 frames per second) by Masayo Soma of Hokkaido University reveals a behaviour previously unknown to researchers: both male and female Uraeginthus cyanocephalus court their partner while holding a piece of nesting material in their beak, after which they hop and sing: they perform a dance very similar to tap, invisible to the naked eye.
The function of the frantic foot-beats of grebes in spring, during the mating season, is identical.
The western grebe and Clark's grebe (the two species of the genus Aechmophorus found in North America) perform a complex ritual in which two birds at a time run across the water for up to 20 metres in about seven seconds. They are the largest vertebrates capable of this feat: a study published in the Journal of Experimental Biology reveals that to defy gravity they adopt a gait in which they strike the water 20 steps per second with their feet spread wide, with force and at great speed.
Here, by contrast, we are talking about bond confirmation, another universal mechanism carried out, consciously or not, by the females. Of many species.
A moving example is the female hornbill (Bucerotidae Rafinesque), an Indian bird similar to the toucan, which walls herself inside the nest with mud and grass to lay her eggs and remains there until the chicks are born. For four months the male alone feeds the entire family. He can be seen on the branch, with his head resting against the trunk as if trying to sense what is happening inside… When he passes her a piece of fruit, she refuses it, passes it back, and so it goes. Often he must insist at length before she accepts it. She is "testing the bond", measuring the fidelity of her hapless husband and strengthening the pair relationship (in scientific parlance, this is called "bond testing").
Researchers are drawn to bowerbirds because they clearly demonstrate the importance of sexual selection, the evolutionary mechanism that explains the persistence of showy male traits such as song, vivid colours and horns. Males do not help the female build a nest, nor do they help incubate the eggs or raise the chicks: all they offer are their genes. This is why females are fastidious in choosing a mate.
Bowerbirds are also studied because they are very similar to humans. The biologist Jared Diamond described them as "the most human of birds". They can build shelters that resemble doll's houses, arrange flowers, leaves and fungi in a truly artistic way; some can sing both the male and female parts of another species' duets, others can mimic the call of the kookaburra or the sound of a chainsaw. They can all dance, and they kill beetles solely to display them in front of the nest. Humans are the only other known species to use animals for this purpose.
Given all these talents, some researchers go so far as to attribute to bowerbirds an aesthetic sensibility and a vague cultural sense, traits rarely found in species other than our own. "It is the females who act as judges," ornithologist Brett Benz reminds us… yet again.