It is not only fine feathers that make fine birds.
- Aesop, “The Jay and the Peacock”
In Aesop’s fable “The Jay and the Peacock”, a jealous jay finds a number of moulted peacock feathers lying on the ground. In order to try and look more like the peacocks, he gathers up the feathers and attaches them to his own tail. He then struts towards the peacocks. They rapidly discover his lie, pecking at him and plucking away his stolen feathers. Chastened, the jay returns to his flock, where they tell him: “It is not only fine feathers that make fine birds.”
Leaving aside the obvious inferiority complex of the jays, the point of the fable is that character takes more than acting – the way you appear on the outside doesn’t always indicate what you actually are. It’s a nice proverb. But, when it comes to biology, it’s totally wrong.
Ornithologists and bird-fanciers have long known that male birds are more attractive than female birds. Males tend to be brightly coloured and have elaborate, unwieldy plumage. Females, in comparison, are often drab and dull – brown being the preferred colour. Darwin, in The Descent of Man and Selection in Relation to Sex, hypothesized that the bright colours of male birds are used to attract females. A male capable of growing an impressive set of feathers is advertising his general health and virility to females. The elaborate feathers of the peacock, Darwin said, were an example of this type of trait[i]. But it wasn’t until the 1980s that scientists actually investigated the peacock’s tail feathers.
In the early 1980’s, Dr Marion Petrie of Newcastle University set out to understand what makes peahens tick – what did they find so attractive about peacock feathers? She corralled peacocks that roamed free in a Newcastle Park, and removed the eyespot on some of their feathers. Then she waited. Unfortunately for the newly groomed peacocks, peahens are a fickle bunch. The males who had the number of eyespots reduced were ignored by peahens, who favoured mating with peacocks with many eyespots.
If growing eyespots makes a peacock attractive, why don’t all males just devote their energy to developing feathers? Well, because that big load of feathers is bulky and heavy and makes you great bait for predators. There aren’t many predators roaming the parks of Newcastle, but I bet naughty British children chase the slow peacocks more often.
Growing large, brighter, or more elaborate feathers makes you more attractive to females. But, it also makes you more vulnerable to predators. Around the same time that Petrie was harassing peacocks in Newcastle, the zoologist Malte Anderrson was doing a similar thing to long-tailed widowbirds in Africa.
Long-tailed widowbirds live on the grasslands of eastern Africa. The males are all black, except for a red slash on their shoulders. Their defining characteristic is an absurdly long tail (hence the name) – up to half a metre in length, many times the body-size of the bird. Females, by comparison, are little brown birds with short tails. The males fly slowly and gracefully over their territories, gliding a few metres above the grass. This makes them easy for females to spot. Females observe the males flying, and then build a nest in the territory of the male that they like the most.
Anderrson was curious what features the female was attracted to, so he caught a bunch of male widowbirds and altered their tail lengths. He shortened some, left some untouched, and elongated others. Then he released the males and watched what happened. When he went back to count the number of nests in each males territory, he found that males with elongated tails were far more popular – they had many more nests, and fathered many more offspring, than either of the either groups of males.
So again, why don’t all males just grow long tails? Well, that long tail has consequences. The flight of long-tailed individuals is slow and ungainly, more like the Wright Flyer I than like a stealth bomber. And the obvious tail, which makes them visible to females, also makes them visible to predators. Long-tailed individuals are more vulnerable to being eaten – and being eaten is bad for your reproductive success.
To explain the variation in tail length, and peacock feather eye number, the Israeli zoologist Amot Zahavi introduced the idea of the handicap principle. The handicap principle argues that males of some species have evolved ‘honest’ signals of their worth – some trait or characteristic which is detrimental to survival in an inferior specimen, but is only a minor inconvenience in a strong individual.
The idea is that a small or weak widowbird could grow a very long tail, but it would not be able to survive: it would be caught and eaten before it could mate. On the other hand, a strong or large widowbird can afford to grow a long tail, because it’s strength allows it to overcome the detriments that the tail causes. It is an honest signal because no individual can fake it, the message it gives off is: “I have deliberately handicapped myself, and still managed to be the best player on the field. Imagine what our babies would be like.”
Sorry Aesop, when it comes to birds, you got it wrong. Fine feathers do make fine birds.