Bird beaks grow as climate hots up

Birds’ beaks are getting bigger, according to researchers in Australia.

Dr Matthew Symonds, of Deskin’s Centre for Integrative Ecology, examined 410 parrot skins, which had been collected between 1871 and 2008. His team measured the bill-sizes of the specimens and related them to summer temperatures recorded in the areas where the birds had lived during the five years preceding their deaths.

One species, the red-rumped parrot, showed no correlation between bill-size and temperature, but the other four had progressively larger beaks the warmer their environments. Symonds and colleagues argue that the increase in bill size is due to climate change.

That beak sizes might be related to climate isn’t a new idea. In 1877 Joel Asaph Allen, the first president of the American Ornithologists Union, formulated a principle which bears his name. According to ‘Allen’s Rule’, the body proportions of animals vary according to the temperature of their environments. The extremities of our bodies have large surface areas, through which heat escapes; ears hands and feet cool rapidly on cold winter days. Frostbite attacks fingers and toes, not the body’s core. In accordance with Allen’s Rule, therefore, mammals living in cold places should have smaller ears and shorter snouts, on average, than those in warmer climes.

Heat loss is a particular challenge for birds; their temperatures are higher than ours. Tucking beaks into feathers helps conserve energy when sleeping. The arteries supplying blood to the legs of waterfowl run close to the veins carrying blood back to the heart. The arterial blood on its way to the feet warms the returning blood, saving energy.

Herons often stand motionless on one leg. Gerald of Wales, the Norman bishop who visited Ireland in the 12th Century, thought that this was a security measure. A stone, he believed, was held in the raised claw. Should the bird nod off, the stone would fall on a toe and wake the sleeper up. The real reason herons and waders stand on one leg is rather more prosaic; it cuts heat loss from the legs by half. Beaks and feet, being bare, lose heat rapidly to the air, so do birds in cold environments have smaller ones with reduced areas of exposed skin? Allen’s Rule would suggest that they have.

But the rule applies equally to situations where heat must be dumped. Elephants, for example, need to cool down during stifling tropical summers. Their huge flat-surfaced ears serve as giant fans; waving them in the air cools the blood. The human brain, while consuming a fifth of the body’s energy, must not get too hot. To avoid heat-stroke and fever, it’s located at the end of a longish neck. Held high in the air, in a cooling breeze, it’s far from the sun-baked ground. Testicles must be kept a degree or two cooler than other organs, so they are isolated in a sack dangling from the body.

Parrots in Australia, however, need hardly worry about the cold. For them, excessive heat is the problem. Having bills with large cooling surfaces is useful during the blistering heat-waves the country has experienced in recent years. Increases of 4% to 10% were recorded in Symonds’ parrot bills. Such changes may seem insignificant but, according to the researchers, they have a considerable effect on a bird’s ability to cool down under heat-stress conditions.

Although Allen’s Rule has become an established tenet of zoology, little quantitative evidence for it was provided only recently. In 2007, measurements of the exposed parts of seabirds’ legs were shown to be linked to environmental temperatures. A study, published in 2011, found that the lengths of frogs’ legs varied depending on how warm their habitats were. Symonds’ research seems to be the first to record Allen’s Rule ‘in action’ as it were; it shows that bill size actually tracks temperature change.

• Matthew Symonds et al. Climate-related spatial and temporal variation in bill morphology over the last century in Australian parrots. Journal of Biogeography. 2015.